arnie

After talking to SRP on 2 different occasions, months apart, I'm somewhat puzzled as to why they recommend making no allowance/adjustment on piston to wall clearance, when a thermo barrier coating has been applied to the piston crown. They obviously figure it does not alter the heat that the skirt has been subjected to. I could understand not altering the ring end gap, but the skirt clearance? I would think that a major source of the heat that the piston skirt expansion is in response to, originates from the heat of combustion. Apparently, I am wrong. Apparently, the piston skirt expansion is due to the coolant on the other side of the cylinder wall. I find it difficult to accept this. Enlighten me.

81ZMouse

I also find this hard to accept. I spoke with Dan Swain at Swain Technologies, www.swaintech.com He said there should be some adjustment in piston clearances. Usually, a piston manufacturer gives a range for the clearance to shoot for. Dan told me to get the clearances at the low end of this range. For instance, my BME pistons needed 0.0070-0.0090 clearance, and I set them up at 0.0070. My advice would be to talk to Dan, he can help you determine your needs. Don't assume piston people understand the impact of coatings (even though I agree that they should).

One thing I do know, is when I had Swain coat my pistons, I also had them coat the sides of the piston. On that coating, Dan said to make NO adjustment to the clearances. He said it doesn't add much (maybe a 1/2 of a thousandth), and the extra lubricity would offset any need for additional clearance. He also said the side coating bonds itself to the metal, and even though it may look like it has worn off, the material of the piston is still coated.

Good luck.

Shane

Z28tt

I've spoken to both Swaintech and HPC about the thermal barrier coating on the top of the pistons and the skirt coatings. They've both said *not* to change the piston dimensions to account for the coating. Their reasoning is that the thermal barrier coating will prevent the pistons from getting as hot, and therefore not expanding as much. I ordered a set of custom JE's, and the phone salesman said the engineers specify the clearance on the pistons based on the application. I'm expecting around a .0065 clearance for my 4.1215 cylinder bore, and specified a .039 quench (which will be .037 after TBC). The TBC is about .002 thick, while the skirt coating is much thinner - .0002 to .0005 IIRC. To keep you thinking, aluminum has 2.167 more thermal expansion as iron (.00013 vs .000006 inches per deg F). If your block is 4.000 at 70 degrees, and your pistons are 3.994 (.006 clearance), figure at 250 degrees your block is now 4.0043 and pistons are 4.0033 - now .001 clearance. I would assume the absolute minimum temp of an engine part is the coolant temp, but the top of the piston would obviously be much hotter due to combustion. Just more rambling from me...

ToddR

What about doing Both the combustion chamber and the pistons. Would this create a higher chance of detonation by trapping more heat. Or just crank back the timing in the now more efficient chambers?

Thomash

I agree with Z28tt. The coating keeps the piston from getting hotter so I see no need to increase piston to wall clearance.

Todd (sorry) it makes no sense to coat the pistons and combuston chambers to hold in heat to create more HP and then back off the timing to cool combustion to prevent detonation.

arnie

Maybe the point was missed. The idea is to decrease not increase clearance.

ToddR

Unless the combustion chamber is so efficient that it requires less timing BTDC to prevent detonation but still make more cylinder pressure for more power. That would be getting to a closer to a perfect world of having the timing set so it's not pushing down on the piston while it's still trying to come up.

arnie

It hardly pays to bring this up now, considering the time elapsed.
Anyway, did the math. It doesn't matter how I twist the numbers above, or add an extra '0', they don't jive. Would you like to recheck/repost? If it matters to you. I will agree with the iron expansion rate of .000006" though, (5 '0's) the block bores should expand about a thou in 180*.

Read somewhere the expansion rate of aluminum is about 4 times that of iron. But it really is mute, when you consider pistons expand at varying amounts, based on design and alloy composition.

Z28tt

Sorry, AL should have been .000012. I forgot an extra zero. Ductile Iron at .000006. Numbers were from Machinery's Handbook. I just did it as an off-the-cuff calc to see... It's not accurate since the piston is cam-ground and not a consistant cross section, but should be in the ballpark.

BTW - My pistons arrived from JE today. They made them w/ only .00045 clearance. I'm thinking it's a bit tight for 25 psi boost & 7000 rpms, at 1 hour at a time road racing :/

A.