Don Terrill’s Speed-Talk

I don't have my pistons to measure them right now but I took a dial bore gauge to my cylinders after a hone for a re-ring and some of the bores have grown .0015". These are Silvolite hyper pistons and they don't give listings for piston to wall clearance, only to bore the block to the proper oversize and get the correct pistons (example 4.030 pistons should be bored to a 4.030 bore and the pistons will be a little smaller than that. They don't publish a range but I'll call them.

But it got me to wondering, is the loose end of piston to wall clearance good or bad for a race motor? Will it not seal as good and if so will it be offset by reduced friction? This engine has about 30 nights on it and a ridge was developing. Not one you could hold your fingernail against but one you could definitely tell was developing. The pistons did not get held up by this on their way out but were definitely a challenge to get out, from what I see it's the rings that push out against the walls holding the rings int.

I have found that more piston to wall clearance lets you make more power, but it will not last as long in the long run. One thou extra seems to be a very good compromise for more power without any long term issues. Sometimes you will get more noise on cold engine startup so if this is a concern, take noter


JOE SHERMAN RACING ENGINES

As Joe said, a bit more noise is all you will probably notice. Hypereutectic pistons don't grow as much as old-school forgings. I've found an extra thou with hypers is existentially greater than would be an extra thou with a forging. Having said that, Silvolite says hypers require 20-40% more end gap and severe use requires extra clearance. Bottom line, you'll probably be fine with the extra P-T-W clearance as long as you have the .022" correct ring end gap.

jack vines

about 8 years ago I ordered a set of .060 over pistons.
Got em, built it and ran it for years.

A couple of years ago a rod bolt let go and kicked a rod out.

I was recently looking through papers and noticed they sent me pistons for .055 over so I went out and measured them.
Sure enough they were 4.370 instead of 4.375.
From now on I'll look closer and measure for myself but after all those years running an extra .005 of P to B clearance and there were no noises nor was there any odd looking wear on the skirts, rings or bores.
Maybe it was because I was using methanol or because I was using top lube in the methanol or 50w oil or who knows what.

Like the last poster said some pistons need more or less clearance, some applications need a little more or less.

But as far as .0015 over, I wouldn't loose a minute of sleep over it.

Good luck

I'm runnin .006 on my sbc.Little noisy til warm then it's sewin machine city! When I axed my engine builder if that was to much clerance,he says"Yer talkin to a guy that runs .010 piston to wall clearance!!! " (He built and raced many Super Stock chevy engines) Rest in Peace Jeff!

redliner wrote:I'm runnin .006 on my sbc.Little noisy til warm then it's sewin machine city! When I axed my engine builder if that was to much clerance,he says"Yer talkin to a guy that runs .010 piston to wall clearance!!! " (He built and raced many Super Stock chevy engines) Rest in Peace Jeff!

I miss Jeff. When my wife and he were going thru cancer he would call up and ask "how's Joan doing?" One of my favorite customers.
.

DCal wrote:

redliner wrote:I'm runnin .006 on my sbc.Little noisy til warm then it's sewin machine city! When I axed my engine builder if that was to much clerance,he says"Yer talkin to a guy that runs .010 piston to wall clearance!!! " (He built and raced many Super Stock chevy engines) Rest in Peace Jeff!

I miss Jeff. When my wife and he were going thru cancer he would call up and ask "how's Joan doing?" One of my favorite customers.
.

Yes he was a very good man and always ready to help! I miss him greatly! I only knew him a short time but I have been changing my combo and his advise is sorely missed! You knew you could take his word to the bank!

I have discovered that my last build on a 350 chev was done at extremely tight ptw clearance. Was told it was 30 over, pistons are marked as 30 over, but with the verniers and bore guages I am seeing 4.026 on the skirt and 4.028 on the bore. It did crack the lands on number 3 piston and there was some scuffing around the top edge of number 5, but the bores are clean and clear after 25,000 miles in 2 years.

I've taken a lot of engines down and have put them together as well. I do this daily. I really have never seen an engine not perform or any signs of problems when the pistons where over the manufactures spec for piston to wall clearance. A matter of fact they (the skirts) seem to look great when loose.

Here is the problem with setting up real loose. If any part of the engine is slightly questioned by the customer, they will put the manufactures spec recommendation first and you second.

If you where to run two identical engines, with the exception of piston to wall clearance, on the dyno for two days and then took them apart for inspection. The manufactures spec pistons would not look remotely as good as the engine with more piston to wall clearance.