Well I was talking with my machinist yesterday and he said that when ordering the rebuild kit for my engine i'll need to identify what piston to wall clearance I need?? I hope i'm making some sense because i'm not sure quite how to describe this. redface.gif Basically he's concerned that if I run nitrous on my engine and don't have the proper "piston to wall clearance" :confused: then the piston may expand too much for the cylinder. It's going to be a .040 327 with forged pistons that'll be receiving somewhere around a 100-150 shot of nitrous. Can anyone give any recommendations or explain to me what he's talking about? If i'm making any sense? sorry. I'll ask him asap also.

Also, I was reading a post that was put up by Malibuman400 regarding summit rebuild kits. I plan on using one of these....I think it's $386.95 or something....with the forged TRW pistons. Unclepennybags stated that he had problems with his high volume oil pump....loosing oil pressure at 4750rpm. Is there something wrong with high volume oil pumps? Should I use something else? I only ask because it's part of the rebuild kit. I have a 5qt pan.

Sorry this post is so long.....my engine will be being assembled soon and i'm just trying to make sure I cover all issues. :D

Thanks a bunch in advance guys

you guys always seem to know the answer graemlins/thumbsup.gif

peg, it is up to your machinist to bore/hone your block for the proper piston to wall clearance. Pistons are manufactured to specific sizes and there are no variations. For example a .030" over piston will probably measure about .0285" at the skirt, depending upon the piston manufacturer. The machinist will need to have your pistons before machining the block, so he will be able to measure the net skirt diameter.

The piston skirt clearance should not be affected by nitrous. Sorry, but I would be skeptical of this "machinist".

I don't think you will have a problem with piston to wall clearances at 100-150 HP. Guys are running that much N2O on stock cast & hyperutectic pistons without problems.

Are you sure he wasn't talking about ring end gap? It would be nice to set your ring gaps wider with a healthy dose of N20, but have never heard of piston to wall changes.

you don't need the HV/HP oil pump. Chevy's over-oil the top end anyway and an HV/HP pump can pump all the oil out of the pan into the valve covers and valley area, leaving the pan empty. There's a lot of opinions and controversy about this, but I don't mind saying I haven't used one in nearly 25 years now except for one deal where the guy brought me all the parts he has already bought and would not take back the HV/HP pump. The engine turned out to be an oil-burner and he eventually pulled it out and brought it back to have the pump changed, which fixed the problem. Typical deal IMO.

Tom

Engineguy - He's actually really good, he's built a number of really good engines. I probably just messed up on what I was trying to describe.

Eric68 - It probably was ring end gap that he was talking about actually. Sorry about that, I must have described it wrong. What would you recommend I have the rig gap set at if i'm going to be running around a 150 shot on it every so often?

Tom Mobley - When I order the rebuild kit (it comes with a high volume oil pump) do you think I should ask them to swap the HV pump out with something stock? Wouldn't it be good to have a HV oil pump though so that you know you're getting good oil pressure and circulation? :confused:

Thanks again guys

i run that h/v pump no problems here
50 psi @ 2500 rpm.if there bad i dont know why they sell so many.i dont think they would engineer something that pulls all oil from pan and put it up on the top end,they dont say "must use aftermarket 6/7 quart pan"but thats just my 2 cents worth.
shane

Ovelle - That's what I was thinking. I had a HV oil pump in the 327 that i'm now rebuilding and the gauge never read below 35 or so. However my engine had horrible oil circulation problems, and ate up two camshafts. I don't know if there is a correlation between the two. Is there much of a chance that an oil pressure gauge would read between 35 and 65 while driving, but the oil pressure was actually almost nonexistent? This was on a digital dash panel. Probably better off going with a mechanical gauge for the oil pressure right?

Thanks

I run a stock 502 with a 150 shot and never had any problems.

Originally posted by ovelle:
i run that h/v pump no problems here
50 psi @ 2500 rpm..... shane Shane,

I had 52-54 psi @ 2500 also. Check out what you get when you hang that big 1-2 shift @5500 rpm. I hope you don't get a shock like I did!

Unclepennybags - Could you explain the problem to me/us? You're saying that your oil pressure would drop during the 1-2 shift? Why would a HV oil pump do this and not a stock one? Shouldn't a high volume one be better? Just trying to understand the problem...don't mean to criticize.

http://www.chevelles.com/cgi-bin/forum/ultimatebb.cgi?ubb=get_topic;f=9;t=012800

Didn't need to be shifting. Happened whenever I crossed 4750.

EXCELLENT explanation unclepennybags....thank you very much. graemlins/thumbsup.gif I'll definetly make sure that's addressed when my engine is being assembled.

the clearance is built into the piston, but Some machinists like to run an extra thou on drag race applications.
this all should be in the directions that came with the pistons.

My concern, as will your machinist's concern is that the rings be gapped correctly. Nitrous is going to get that piston top HOT!

Motor Martyr - What would be the correct gap size for my engine if i'm going to be running a 100-150 shot? Would going with a gap size for nitrous affect it's performance naturally aspirated. I don't quite understand the specifics of this whole deal to be honest. :confused: I can understand that the piston would expand more while running nitrous, so you'd want to make sure it's not to tight for the bore...if that makes sense....but I have no idea what would be ideal gapping? What is standard? :confused:

.005-.006 per inch of bore for N20

The pump is not was causes pressure. Its the bearing clearances that causes the oil pressure. The oil pump is there to move the oil to where its supposed to be. Thats why they use the volume term. It just moves the oil from one place to another.

bigjimzlll - Thanks. Would that be kinda "loose" so to speak when not running the nitrous though? Is standard .003 -.004? :confused:

This is from speedomotives web site.


The chart below is a general guideline for cylinders with a 4.00" bore, adjust the figures to match your engine's cylinder diameter:

Top Rings (ductile iron, 4" bore)

Supercharged

Nitromethane .022 - .024"

Alcohol .018 - .020"

Gasoline .022 - .024"

Normally Aspirated - Gasoline

Street, Moderate Performance .016 - .018"

Drag Racing, Oval Track .018 - .020"

Nitrous Oxide - Street .024 - .026"

Nitrous Oxide - Drag .032 - .034"

2nd Rings (plain iron, 4" bore)

Supercharged

Nitromethane .014 - .016"

Alcohol .012 - .014"

Gasoline .012 - .014"

Normally Aspirated - Gasoline

Street, Moderate Performance .010 - .012"

Oval Track .012 - .014"

Pro Stock, Comp. .012 - .014"

Nitrous Oxide - Street .018 - .020"

Nitrous Oxide - Drag .024 - .026"

One more thing...I broke a piston(hyper) with a .030 over 350. They were gapped at .017. It was with a 150 shot(with too much timing) I since have built 3 other engines. I gapped them at .024..no noticable extra blow by and the perform great...you might be safe at .016 or so...but once you bust a ring land..i rather err on the loose side

bigjimzlll - The gapping is in reference to the outside diameter of the ring and the outside diameter of the piston right? I'm still trying to figure out exactly what we're talking about here. Sorry....this is new to me. The bore of this 327 is .040" over so 4.040". Do I need to take that into consideration when selecting a ring gap size? Or does that matter since naturally the block is bored out .040" and the pistons are .040" oversized? :confused: Would you mind giving me a quick explanation of ring gapping and the importance? Sorry about all the questions....I just want to make sure i'm looking at this the correct way.

If i'm understanding this right then i'd want to go with...

.024" gapping on all the rings? I only plan on running a 150 shot every so often.

I already ordered the rebuilt kit for this engine....did I just screw myself? I mean is this something I should have specified for the order, or is the gapping done when the engine is being assembled? :confused: They will be forged pistons if it matters.

Thanks a million

Go with what the ring/piston manufacturer recommend on the ring gap.

I ran .016-.018" top ring gap, .010-.012" second ring. Piston to wall clearance of .005" per manufacturers spec sheet.

the ring gap is the distance between the two ends of the ring while half way down the bore...no piston..just the ring. What happens is when the ring is on the piston..under normal operating temps. the gap allows for expansion. When N20 is used, the temps go up a lot. and if there is not enuff room for the rings to expand, it will either break the ring or bust the ring land

I see......ok, thanks for the description bigjimzlll. When I get the rebuild kit will it say what the ring/piston manufacturer recommends it be set at? Just wondering. Thanks again guys.

most of the times the rings will have the gap measurements. They are usually .004 per inch of bore...so at 4.04 bore.. the gap will be .01616. But this with out N20. I would go a mininum of .018-.020 if nitrous is to be used

your pistons come with instructions, piston manufacturers have different guidelines on what they want you to run for ring gap, depending on the aluminum and manufacturing process.

A good example is KB, they require more top ring gap, and many dont add it.

I'm not an N2O guru, but I will say that no real mods are required for a 100-150 shot of N20. Guys hit stock engines with that all the time.

If it were my motor, out of paranoia, I would take the recommended top ring gap (usually .004" per inch of bore as previously mentioned) and make double sure that each top ring is gapped at least that plus an extra thou or two. I might even go out to .020" but no wider than that. Just red your piston manufacturer's instructions to be certain.

The thing is though, if you go too wide on the gap you will be giving up some HP due to blowby. The gap is mostly to take up expansion due to heat in the ring but there is already a safety margin built into that typical .016" number.

Just my opinion.

PS. KB pistons are strange animals - I broke one in an NA motor with a .024" end gap. Should have been closer to .030".

Alright, sounds good. I'll look over the recommendations from the manufacturer when I get the rebuild kit. I'll probably go a .001 or so over....but i'll ask the guy who's building the engine for me also. What do you guys think about speedpro power forged pistons? The kit I got was $459....comes with forged pistons (10.5:1), rings, HV oil pump, all gaskets (all felpro)and bearings, brass freeze plugs, Plastigage, and the assembly lube. Does this sound expensive? Thanks again.

speed pro...sealed power...TRW..all the same..a very strong piston..but real heavy

Originally posted by bigjimzlll:
speed pro...sealed power...TRW..all the same..a very strong piston..but real heavy Heavy, yes, but lighter than the cast pistons that came stock in a 327!

Pretty good price. What are you going to do for heads and cam?

Mike