SBC block bore and taper query ?? - Chevelle Tech
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post #1 of 5 (permalink) Old May 13th, 08, 9:09 AM Thread Starter
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SBC block bore and taper query ??

I am about to assemble my first 383 stroker short block

the block was bored 30 over already - I bought it as a short 355ci block - took out the assembly - then took it to a Florida drag racing & boats engine building machine shop to have it decked and torque plate honed for race conditions - to fit my SRP piston(1404...348?? part#) 1.125 pin height hi-comp forged pistons /6inch rod/ 3.75 stroke assembly

We are trial fitting for notching the block for the rods -

the first thing I noticed the piston skirts of the SRP's would not go down into the bore without scuffing/scratching the piston/cylinder

essentially you had to have the piston skirts lined up dead on and give it a tap to get the first part of the skirts in - if the piston even slightly went in off centered - it would not go unless straightened up and centered

If I turned the piston upside down and into the bore - it went in without any drama -

Once the skirts passed the start of the bore, when you put it in the correct way up, and hooked up to the crank (without rings) they went up and down ok - you could hear a slight sound of scuffing but it was expected with just piston in the bore with only a little oil and no restriction on the movement

So I got my local machinist guy to come and do some measurements for me

I am listing measurements by cylinders per side, which is how we measured it, so I can have your comments on what I should be doing, if anything, with this block


1 4.038 4.040 4.040

3 4.038 4.040 4.040

5 4.038 4.040 4.040

7 4.038 4.040 4.040
__________________________________________________ ______

2 4.033 4.038 4.042

4 4.033 4.038 4.039

6 4.033 4.038 4.039

8 4.033 4.038 4.044

All Pistons were measured at .500 above the bottom of the skirt as stated by SRP and they are all 4.024 diameter

So my machinist thoughts are - his best estimate is that the bores at the top of the cylinder all seem to be round - so its not an oval issue as I might have thought - but all cylinders appear to have a taper -

He admits he doesn't know about all the theory behind racing etc.
but his inclination would be for the bore not to have any or any significant taper

He also said there is nothing that honing would do to correct a taper or to "straighten" the bore as it would only exaggerate the taper and that I would have to find out

We tried a feeler gauge with the pistons in the bore just to cross reference our measurements and could get a .010 and a .012 gauge into the bore freely when the piston was fully in and just around the skirt where we were directed to take measurements of the diameter of the piston on two sample pistons on either side of the block

SRP recommends a minimum of .0035 to .0045 clearance - so my machinist says the clearance is there well above the minimum - but the entry into the cylinder bore and the taper should be checked to see if it can work for what I want to do

He doesn't know if I could try perhaps an emery cloth on the skirts of the pistons to smooth them off some - but certainly he doesn't think a honer would do much to help with the bore -

My feeling is that the start of the bore perhaps has too "sharp" an edge - like the block was decked after the honing - rather than before - so no "relief" was there at the start of the bore - there is definitely a good looking cross hatch in the bore

Just for comparison we measured a TRW L2304 .030 piston that was part of the original assembly in the block before the decking and honing and it measured 4.021 around the same area as we measured the SRP's

So there is a 3 thousandths difference in the skirt diameter

Well sorry for the earful - but given the above - what would you recommend as far as proceeding with this block and about fitting the rings if we are to proceed with it

85 Camaro
383ci/AFR 210 CNC-65cc/SRP dome/ProSystem 850DP/Motown intake/Comp. sld. roller/Big Shot plate/TH-350 manual/Protorque 8in./4.71 Ford 9in/29.5x10.5x15

NEW PROJECT-1980 Berlinetta coupe - basic budget resto - 460ci with mahle pistons/Brodix RR's/700R4 -coming (not too) soon !!
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post #2 of 5 (permalink) Old May 13th, 08, 9:28 AM
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Bill R
Join Date: Aug 2002
Location: Gold Canyon, AZ
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Re: SBC block bore and taper query ??

You are correct, the tops of the cylinders should have been chamfered after being bored, especially if the decks were surfaced. Your measurements are all over the board and don't make much sense to me. When a cylinder develops a taper shape, the bore is always larger at the top than at the bottom. This is due to the fact that there is much more heat and significantly less lubrication at the tops of the cylinders, when the engine is running.
Your machinist is not totally correct in his assertion that a hone would not correct a very minor taper. A rigid head honing machine, such as a Sunnen CK-10 will indeed straighten a tapered bore.

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post #3 of 5 (permalink) Old May 13th, 08, 9:42 AM
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Re: SBC block bore and taper query ??

It sound like a very bad hone job for sure and if it was plate honed it should be checked with a plate or a cylinder head bolted on the block and measured from the bottom of the bore.

You may want to take it back and have your machinist there and go over it with the owner to get some compensation as it soulds like they may owe you a block or a set of pistons and a balance job it thats been done.

Keep us posted on the events to follow.
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post #4 of 5 (permalink) Old May 13th, 08, 11:30 AM
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Re: SBC block bore and taper query ??

In my machine shop in college, we always put a .0015" taper larger at the bottom due to the issues of the top of the cylinder opening up more due to heat at running temperatures. But thats a whole lot more than .0015".

9 years and FINALLY she's drivable, just in time for my first born son.


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post #5 of 5 (permalink) Old May 13th, 08, 1:10 PM Thread Starter
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Unhappy Re: SBC block bore and taper query ??

Just as an update the machine shop guy called when he got my e-mail today containing my query above to confirm that the block is junk as it is

He says he would not have had the piston I was intending to use - and so would not have anything to measure the bore against

and so all he did was hone it and the most it would have taken out on a plate hone was 1 thousanths -

if it was bored badly or near the end of its service limit - I guess he is saying he would not have been responsible to check that or been able to check that

He confirms however that he would not go over 4.035 on any bore that he is doing

If he knows this, then why hone the block if its that far out already on some cylinders

he says I might have already had old pistons in use for the block or a certain type of piston which he would not be aware of

I asked him about the apparent 5 thou difference between the two banks of cylinders - and he says some shops will bore to the pistons they have and may have used a set - maybe not even a matching set of pistons - and so alternate the bores to suit

he says in his opinion the finish is right but not for the size pistons I have

he wants to make it right but I have already shipped the block out of the US to Antigua where I live

For now I think I will put this one on hold - I honestly think that the bore could and should have been checked - we are not strangers to the shop for that matter

I accept that I said not to bore the block as it was already 30 over - but honing it should at least require some measuring - and this could have been easily picked up and done and even reported to me before sending it out

ahh well - we don't have access to "race" experienced machine shops here - and I really wanted a decked and honed block for this build and thought I had cleared all my hurdles

I guess this is one more machine shop horror story for the files

85 Camaro
383ci/AFR 210 CNC-65cc/SRP dome/ProSystem 850DP/Motown intake/Comp. sld. roller/Big Shot plate/TH-350 manual/Protorque 8in./4.71 Ford 9in/29.5x10.5x15

NEW PROJECT-1980 Berlinetta coupe - basic budget resto - 460ci with mahle pistons/Brodix RR's/700R4 -coming (not too) soon !!
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