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Discussion Starter · #1 ·
Thanks to everyone on this site that takes the time to help others with their problems.

I've had a 1970 Chevelle about 2 months now, which came originally with a 350, 2BBL, TH350. I know very little about the history of the car.

I removed the heads and found damage (oversized valves with worn valve guides) that warranted replacement. I am interested in basically obtaining stock performance, and purchased heads which were equivelant to stock with 76cc chambers and 1.50 exhaust, 1.94 intake valves.

While cleaning up the block (still installed in car) I noted the engine data stamps, including the block cast number (3914660). Much to my surprise, it seems I've got what was originally a 1968 327, from Camaro, Corvette or maybe Nova...no way to know.

I checked with the machine shop regarding the heads, and they verified that they will work with a 327--no problem.

While preparing to install heads, I thought to measure the piston stroke, just for information. I rotated the engine to bottom and measured distance to top of cylinder, then rotated to TDC and measured. I'm just using a ruler, not a caliper, so my measurement isn't PRECISE. However, I found that my piston has about 3.5" of travel.

Now looking in my manual, the main difference between a 327 and a 350 is the stroke...both have 4" bore. The (manual) stroke for a 350 is 3.48" For a 327 it is 3.25". I know that despite using a ruler, I am not off by 1.4"--which makes me wonder....

Is it possible someone put a 350 crankshaft into this 327 block, thereby increasing displacement to 350 cu in?? If that is possible, then it seems that is what they've done.

So this leads to my question--sorry for all the preliminaries--
I'm looking for a camshaft, something close to stock (which for a 350 in my car was valve lift of about .390" for intake, .41" for exhaust).

I am concerned about valve/piston clearance. It seems that when my pistons are at TDC they are about 1/32" from the top of the cylinder. Is this normal?

I'm thinking that changing the crankshaft from a 327 to a 350 this will increase travel at both the top and bottom of the cylinder bore about 1/8". If the blocks are really the same, it would mean that a 327 would have 1/8" more clearance between piston and valves due to shorter stroke.

This would perhaps account for lower compression ratio in 327? I'm not sure about that.

I just want to be sure that piston/valve clearance will be adequate with my pistons coming, basically, right to the top of the cylinder bores. (and I AM referring to the highest part of the piston--of course due to piston design shape it's not totally flat on top). My pistons do have the small crecsent grooves in them which I assume is meant to provide additional clearance.

The truth is I know very little about this, but want to be sure that a "stock" hydraulic cam, with these "stock" heads will be safe with pistons that high up in the cylinders.

Thanks VERY MUCH for any input!!
 

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I'm thinking that changing the crankshaft from a 327 to a 350 this will increase travel at both the top and bottom of the cylinder bore about 1/8". If the blocks are really the same, it would mean that a 327 would have 1/8" more clearance between piston and valves due to shorter stroke.
No expert here but if you change the crank for a longer stroke then you need to change the rods to compensate so you don't drive the piston into the head. A 327 crank needs 327 rods, a 350 crank needs 350 rods. The stroke is the combination of the crank and rod length, not just the crank.

EDIT: Stroke is the crank, but needs a matched rod length to keep the piston in the cylinder. To short or long a rod, well you can figure out what will happen.
 

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Discussion Starter · #3 ·
No expert here but if you change the crank for a longer stroke then you need to change the rods to compensate so you don't drive the piston into the head. A 327 crank needs 327 rods, a 350 crank needs 350 rods. The stroke is the combination of the crank and rod length, not just the crank.

EDIT: Stroke is the crank, but needs a matched rod length to keep the piston in the cylinder. To short or long a rod, well you can figure out what will happen.

Thanks for the reply. It sounds logical. And the fact is I don't know if they changed this crankshaft or not, and if they did whether they changed the connecting rods as well...all I know for sure is that it seems I have a 327 block that is getting 3.48" of stroke, which implies a 350 crankshaft.

So my main question remains--if my piston is coming to within 1/32" (at it's highest point) of the top of the cylinder, will this provide enough clearance between piston and valves? I haven't seen any other engines to know if the piston actually comes that close to the top...or if this is abnormal due to a crankshaft change--350 crank with 327 rods--or if it is normal to see a 350 piston come that close to the top at TDC.

Again, I don't know, I'm just trying to use logic and picture this in my brain...sometimes theory and reality are quite different!

Thank you all!
 

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The 327 and 350 both share 4.00 bores and use the same 5.7 rod from the factory. The 327 pistons have a different dimension from the wrist pin to the top of the piston that accomodates the difference in stroke. In all actuality, 327's usually had a higher compression ratio from the factory due to manufacture in the non-smog era. On a small block, you usually do not run into valve to piston clearance problems until .560 valve lift or so. this leaves the camshaft choice wide open. It all will depend on the rest of your combo, how you plan on using the car, etc. I'll be the first to tell you that we will need more info to make a proper recommendation. The spec that you are referring to is deck height. It is normal for the piston to sit "in the hole" from the factory. A performance rebuild could include "decking the block": this would be machining the block to establish a predetermined deck height (usually zero) and it also makes sure that the cyl heads have a good, true, flat surface to mate to.
 

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I read somewhere that if the piston is going to go too high in the cylinder you have to use different pistons, ones which have a the connecting pin higher in the piston head--effectively a thinner piston--so that you can keep the increased stroke without the piston hitting the head.
I read the same post today (may have even been Bill K. though not 100% sure) and I believe the theory was that a theoretical zero clearance is ideal, though not necessarily achieveable. That said I believe the same post would imply your 1/32" clearance between the highest point of the piston at the top of its stroke and the top of the cylinder wall would be sufficient to prevent damage.
 

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Discussion Starter · #6 · (Edited)
Thanks for the information, SpdRacer! That helps a lot.

Thanks also Tom--for answering the question about the clearance at top of piston travel.

Is it a safe assumption that my 327 block has a 350 crank since I'm getting approximately 3.48" stroke? And as you say, the rods would be the same, so that's good to know. What I don't know is if they changed the pistons when they did this conversion--

If the engine had stock 327 pistons and you put a 350 crank in it, would that cause the pistons to extend beyond the top of the cylinder? And if so--because you point out that a 350 piston would have a pin higher up inside the piston body--since my pistons only reach within 1/32" from the top of the cylinder bore that they must be the correct pistons for a 350? Does that amount of clearance sound normal for a 350?

And if I can safely assume, then, that I've got 350 cubic inches of displacement, it should be okay for me to refer to my repair manual (which is for the original engine, a 350) when tuning the engine, etc, since it effectively IS a 350...nevermind that it's in a 327 block. Right?

I plan to use this as a daily driver--no interest in racing--I don't want to sacrifice fuel economy for additional power, so stock cam values sound good to me. I don't want to assume that my current push rods are the right length either, since I had some valve problems before and now have new heads--if I can get a stock cam/lifter set, then stock push rods (if there is a stock length) that will make things a whole lot simpler.

My manual says stock intake valve lift is .390, exhaust .410, plus or minus .003. I don't know if a cam with exactly those numbers is available, but if it's not then going a little higher will be okay, according to what you're saying--and I'll definitely keep it below 0.50"
From what I've read, it sounds like a short duration cam with relatively high lobe separation would be suitable.

Thanks for any input or answers to questions!
 

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68 and 69 327's have the larger 350 crank journal size. pre 68 327's have the small crank journal. I understand why you would want to go back "stock" with the cam in order to not sacrifice mileage and street manners, however, I will suggest just checking around at what is available ou there. There is so much power AND mileage to be gained by picking an aftermarket cam. It is all in the combination. Do some research on this site and others on the web before you sink money into a "stock" combo. As far as pushrods go, yes there is a standard length although it escapes me as to what it is. Check this article out about pushrod length.... www.carcraft.com/techarticles/116_0401_setting_pushrod_length/index.html
 

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Discussion Starter · #10 ·
A piston sitting .030" in the hole is a bit much if you want to make power, need regular gas? could work with a turbo, got cast pistons?

Add that .030" to a .040 head gasket, now you have plenty of space up there.

Thanks 2cool...although I'm not sure I understand your post.

Are you saying my 1/32" is alot?? You think it should be less, or that less is better? But then you say that when added to the head gasket thickness there is plenty of space...meaning....better? Or worse?

Also, what about the regular gas comment?
I don't know what kind of pistons I have, and as I mentioned in the title of this thread, I am a rookie, so I don't understand all the technical implications of these variables.

Thanks for offering your help.
 

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Discussion Starter · #11 ·
68 and 69 327's have the larger 350 crank journal size. pre 68 327's have the small crank journal. I understand why you would want to go back "stock" with the cam in order to not sacrifice mileage and street manners, however, I will suggest just checking around at what is available ou there. There is so much power AND mileage to be gained by picking an aftermarket cam. It is all in the combination. Do some research on this site and others on the web before you sink money into a "stock" combo. As far as pushrods go, yes there is a standard length although it escapes me as to what it is. Check this article out about pushrod length.... www.carcraft.com/techarticles/116_0401_setting_pushrod_length/index.html
Thanks again for the help. I've spent half the day researching all this stuff, including reading about cam lift, duration, separation, overlap.
If I had as much time as I'd like to study this, I think I could gain a much better understanding about how each of these variables will affect how the engine eventually runs. Unfortunately, as time is pressing, I need to commit to something soon, and unless I can find someone willing to offer some specific alternatives, I'm probably going to feel safest going with something as close to stock as possible. Maybe I'll call a Crane Cams tech rep and get their advice about the subject...

After doing some study about pushrod length and rocker geometry yesterday I ended up ordering a test push rod and some test valve springs, figuring this was worth being sure about. It seems like a process that could become rather time consuming, though, that's why going towards stock sounds like it would simplify the process.

I definitely appreciate everyone's input on this forum.
 

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What do you intend to use the car for? for everyday use it should be fine and it'll run regular gas. For performance the smaller the space the better, your 1/32 is about .030" if it were .005" about the size of a hair that would be great for performance but you might be buying premium gas do you want that?
 

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2 cool is saying, if I may interpret, that .030 thousanths deck height is std for a factory piece, but less than optimal for a performance application. When you factor in the deck height of .030 with the heasd gasket thickness of say .040, you have plenty of room piston to valve. However, this space also lowers your compression ratio. In order to run on pump gas (87 right since we're on a budget and mileage conscious?) you need the comp ratio to be 9:1 - 9.5:1. To calculate this, you need the deck height (.030), the gasket thickness (.040), the vol of dome or valve relief in your piston, and the cc of your heads. You can figure out whast cc heads you have by looking up their casting number and can guestimate you pistons by looking up flat tops at Summit racing for example. Cam companies are going to ask you for the compression ratio, the intake and carb, the stall of the torque converter, the weight of the car with you in it, the rear end ratio and the tire height that you plan on running to make the best cam reccomendation. That's why I said its all about combination. Try playing around with this website:
www.csgnetwork.com/compcalc.html
Using it I come up with a c/r of about 8-8.2:1 depending on the head gasket used. If your new heads had smaller chambers, 64 cc for example, you'd be looking at a nice 9.5:1
 

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Discussion Starter · #14 ·
2 cool is saying, if I may interpret, that .030 thousanths deck height is std for a factory piece, but less than optimal for a performance application. When you factor in the deck height of .030 with the heasd gasket thickness of say .040, you have plenty of room piston to valve. However, this space also lowers your compression ratio. In order to run on pump gas (87 right since we're on a budget and mileage conscious?) you need the comp ratio to be 9:1 - 9.5:1. To calculate this, you need the deck height (.030), the gasket thickness (.040), the vol of dome or valve relief in your piston, and the cc of your heads. You can figure out whast cc heads you have by looking up their casting number and can guestimate you pistons by looking up flat tops at Summit racing for example. Cam companies are going to ask you for the compression ratio, the intake and carb, the stall of the torque converter, the weight of the car with you in it, the rear end ratio and the tire height that you plan on running to make the best cam reccomendation. That's why I said its all about combination. Try playing around with this website:
www.csgnetwork.com/compcalc.html
Using it I come up with a c/r of about 8-8.2:1 depending on the head gasket used. If your new heads had smaller chambers, 64 cc for example, you'd be looking at a nice 9.5:1

Thanks for all the help Warren. I tried to get a more accurate deck height using a straight edge and feeler gauge, it seems I've got about 0.055", more then my earlier estimate. Also, I found that my cylinders must have been bored out, they now are 4.060". Stroke is 3.48". My heads have 76cc chambers and my gasket thickness is about 0.040". Head gasket bore diameter is 4.125". I have dished pistons with 4 valve relief grooves. I just estimated my piston dome volume at 10cc. With those numbers I come up with compression ratio of 7.9:1. I know my deck height is too high, but I don't think there's anything I can do about that.

Is that compression ratio too low? Yes, I want to be able to run regular 87 octane gas, and I understand that it will be fine for that. Are there any other performance problems that may result from such low compression ratio?

I read someone mention the potential of using a .020" head gasket. When I put that number in the calculator my compression ratio increases to 8.2:1. Would that be a smart choice?

I am still leaning towards a stock cam, only because I don't know enough about the whole world of camshafts to make a wise choice otherwise. From what I've learned, I think that a cam with lift of 0.39 intake, 0.41 exhaust, 204 deg duration, 112 degree lobe separation would work.

I don't know what kind of rear end I've got, never been down there yet to check it out...

Does anything about this combination raise any red flags?

I have a stock intake with 2 bbl rochester.

Do you think the 7.9:1 compression ratio will work with a cam like that?

Thanks again for your time and advice.
 

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With a compression ratio that low your kinda stuck with a stock cam since a performance cam bleeds off compression, thats one reason to raise the compression when going with a bigger cam.

It's going to be sluggish even with a stock cam, I think I would have those heads millied .050" if it hasn't been done already and use a thin head gasket.
 

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Welcome to Team Chevelle Cokonuthead!!!
7 posts!!!!!

Dished pistons with valve reliefs?
.055" down the hole?
.060-over?
327 Block and 350 guts?
76cc heads?
This sounds like the "The Engine From Hell!"

Let me get a clarification from our guy in Hawaii.
Is there a shallow bowl shape all the way across the top of the piston? If so, this is a 'dished' piston. If the face of the piston is flat and there are the crescent-shaped valve reliefs...then it is a 'flattop.' A flattop with 76 cc is about a 9:1 engine. A dished piston with 76cc head is 8:1.

If it is already .060 over....this is often the limit, though not always, for a SBC.
How are the bores for wear and taper? How are the piston skirts? Fixing this thing may not be cost effective. Instead of all the fixes....may be best to start with something else like a running engine, a take-out from a wrecked car.....
Could you post some pictures? (I sent you a private message on this)

If the block and pistons on this beast are viable for further service....
we need to know, if flattop or dished.
If flattop, a stock cam will work great.
If you have dished pistons...a RV type cam is ground in such a way to make the engine develop low speed cylinder pressure....this will wake-up a low compression 350 for daily driving.
If you have dished pistons, and since your 76cc heads need guides..think you mentioned that post #1, why not put some other heads on it with smaller chamber to raise the compression?
More on this when you send pics or exact piston type....
And for peace of mind, new valvesprings whatever way you go.....
 

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Discussion Starter · #17 · (Edited)
Welcome to Team Chevelle Cokonuthead!!!
7 posts!!!!!

Dished pistons with valve reliefs?
.055" down the hole?
.060-over?
327 Block and 350 guts?
76cc heads?
This sounds like the "The Engine From Hell!"

Let me get a clarification from our guy in Hawaii.
Is there a shallow bowl shape all the way across the top of the piston? If so, this is a 'dished' piston. If the face of the piston is flat and there are the crescent-shaped valve reliefs...then it is a 'flattop.' A flattop with 76 cc is about a 9:1 engine. A dished piston with 76cc head is 8:1.

If it is already .060 over....this is often the limit, though not always, for a SBC.
How are the bores for wear and taper? How are the piston skirts? Fixing this thing may not be cost effective. Instead of all the fixes....may be best to start with something else like a running engine, a take-out from a wrecked car.....
Could you post some pictures? (I sent you a private message on this)

If the block and pistons on this beast are viable for further service....
we need to know, if flattop or dished.
If flattop, a stock cam will work great.
If you have dished pistons...a RV type cam is ground in such a way to make the engine develop low speed cylinder pressure....this will wake-up a low compression 350 for daily driving.
If you have dished pistons, and since your 76cc heads need guides..think you mentioned that post #1, why not put some other heads on it with smaller chamber to raise the compression?
More on this when you send pics or exact piston type....
And for peace of mind, new valvesprings whatever way you go.....
Thanks for the post and message George.
I sent you and email with some pictures, I can't figure out how to post them here.

I'm sorry the message is kind of long, I gave you ALL the background info I have on this engine.

Since I've already got new heads, the valve guides and springs should be all good.

Are there any disadvantages to using a thinner (like 0.020") head gasket? Since the "stock" gasket is nearly twice that thick, does a thinner one pose any risks or can it cause any problems? Or is it simply a matter of using the thickness you want to, perhaps to influence compression ratio (like in my case) with no down side?

Thanks again!
Scott
 

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OK here are the pics of Scott's piston and engine...white 70 Chevelle 4-door
Very clean-looking car I might add!

The shortblock looks pretty fresh...thought we were were talking about something on its last legs......
Looks like a cast dished Chevy 350 piston to me. With your NEW 76cc heads, yes, the springs should be new or serviceable/old and should take any cam you seem to be considering. With the generous clearances of stock-type stuff, no need to worry about piston to valve clearance or even need a pushrod checker.

You have replacement style pistons. They have less compression height to reduce compression GAINED by an overbore (more swept area being compressed into the same size combustion chamber). Not uncommon for these slugs (pistons) to sit down a bit in the bore.

Too late in the game and not enough time to deck the block, etc so a thinner head gasket would help though not essential....
Used cam, used lifters must go back into their original holes as they wear-in together. New lifter on one old cam lobe.....not a recipe for success here.
Bite the bullet, new cam, lifters, and a quality timing set. CHEAP Rebuilder-style timing sets usually retard the cam for emission purposes. You could benefit from an RV or 4x4 cam, very short duration and above stock lift.....this will make your engine think it is a big dog.....and lobe separation around 112 as you mentioned in your email. With you in a hurry and in Oahu, you may be limited to what cams are on the island there...you may be limited to the stock .390/.410" cam. I would install it 4* adv to bump up the low speed cyl pressure...

Anyone else have comments?
 

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Discussion Starter · #19 ·
OK here are the pics of Scott's piston and engine...white 70 Chevelle 4-door
Very clean-looking car I might add!

The shortblock looks pretty fresh...thought we were were talking about something on its last legs......
Looks like a cast dished Chevy 350 piston to me. With your NEW 76cc heads, yes, the springs should be new or serviceable/old and should take any cam you seem to be considering. With the generous clearances of stock-type stuff, no need to worry about piston to valve clearance or even need a pushrod checker.

You have replacement style pistons. They have less compression height to reduce compression GAINED by an overbore (more swept area being compressed into the same size combustion chamber). Not uncommon for these slugs (pistons) to sit down a bit in the bore.

Too late in the game and not enough time to deck the block, etc so a thinner head gasket would help though not essential....
Used cam, used lifters must go back into their original holes as they wear-in together. New lifter on one old cam lobe.....not a recipe for success here.
Bite the bullet, new cam, lifters, and a quality timing set. CHEAP Rebuilder-style timing sets usually retard the cam for emission purposes. You could benefit from an RV or 4x4 cam, very short duration and above stock lift.....this will make your engine think it is a big dog.....and lobe separation around 112 as you mentioned in your email. With you in a hurry and in Oahu, you may be limited to what cams are on the island there...you may be limited to the stock .390/.410" cam. I would install it 4* adv to bump up the low speed cyl pressure...

Anyone else have comments?
Thanks for the suggestions George.
When you say "timing set", what are you referring to? I fear that there is yet another element to this engine building that I have no experience with and no time to learn...I've heard the term "degree the cam" but have no idea what that entails. Is a "timing set" got something to do with that?

The 4 deg advance you're just referring to ignition timing, set later on with a timing light, right? Or are you referring to something else.

I'm sorry I am so ignorant about this...I've mostly always been a bolt on and off mechanic, when it comes to the technical stuff I tend to get timid.

As far as RV or 4x4 cams, do I just need to specify that with a parts store? If looking at those cams I'll find that they have higher lift with the same duration/separation as an auto/street car?

Thank you again for your help.
Scott
 

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Timing set = gear for the crank, timing chain, gear for the cam; they come together in a set. This timing set resides under that metal timing cover which the water pump sits in front of. You must also remove the harmonic balancer from the front of the crank.
The timing set keeps the cam 'in time' or phase with the crank.
Most of your auto part stores, book stores, etc have a "How-To" book on the SBC. Some come with a handy CD/DVD on stuff like cam install and how to degree a cam. Not a put-down here but you are looking at some work that is not for a novice. With some reading, patience, etc, you can get through this.

4* adv means installing the cam 4* ahead of the intake centerline. A cam degreeing kit may be available as a rental from an auto parts store..... Typically two ways to change the cam's installation degree...offset bushings or using a better timing set that offers 4* adv, 0* or 4* retarded positions.

Most project cars/DIY use the dot to dot method.

Get some other opinions here before dashing off to your local parts store!
Sad to say that the parts store people sometimes lack car knowledge. When you go shopping for a cam and the guy asks you if you have Halogen Headlights so he can progress through his parts look-up screen, RUN!!! If you have a smart parts guy and tell him/her you have a low compression 350 in a daily driver and you want some more responsiveness....

Others from the site will chime in here with some cam ideas...
Also call or email Crane, Lunati, Isky, Comp, tell them what you have and what they recommend.
Probably want weight: 4000#
your rear differential gearing: 3.08:1?
transmission type: th350?
use street/cruise car not a hotrod
compression: 'some'

Again, if you are gonna try all this, get a How To book on the SBC right away.
You have been doing your homework, looking things up, asking questions.....
 
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