I was talking to this guy at the SJ festival of speed. (kind of a small show at the Shark Arena) and he was going on and on about race engines. when i talked about the ones i had built, he seemed unsure that we could have such tight tollerances in our Alusil engines. in fact , he was doubtful that we could do it without sleeves. anyway, long story short, he finally believe me , but not that we could have near .001 of clearance between piston and cylinder.

going off the spec, there is about .035mm, which is just over .001". (for the 2 valve engines) So, why is the spec just under .001 for the S4????? (spec is .020mm, which is under .001") i know this is measured from the 90degree from the piston skirt position, so , what is the clearance at the skirt??

Last question: If we nicasil a block, and the coating is real hard as they say, how does thermal expansion work if alusil expands a lot with heat, and the pistons are not forged and are susposed to expand at near the same rate? does the coating on the cylinders expand at the same rate as the block material? are the piston vs cylinder dimensions the same stock, or does a nicasil block with JE pistons , for example, require difference clearances?

mk

 

0.001 is tight for piston clearance. Typical values I believe are in the 0.0015 - 0.003 range. The pistons I had Diamond make me spec'd 0.005, which is loose. If they weren't in an inline engine, it would piston slap like crazy on startup.

 

The spec for our engines are .00078" (.020mm) for the 5 liter and .0013" (.035mm).
So, why would you want to make the pistons smaller than the spec?

anyone know what the diameter difference is for our pistons as measured at the skirt vs the front and back of the piston?

mk

 

I hope this isn't off topic, But I have a set of 85-86 5.0 pistons showing up some time this week. I am going to have a 4.7 block bored for these pistons. Should I have the shop use the S4 specs like Mark posted, or use the 16 valve specs?

 

great question! Im wondering what the shop did for scots engine. thats the one spec I didnt know about. Im sure they go off the manual or little white book.

I should have measured the cylinders before i assembled.

mk

 

I would go with the S4 spec, the tolerance would be piston specific not block specific. When you order aftermarket pistons different manufactures may call for more or less clearance in the same application depending the piston material used.

 

Thanks for that Brian. That makes sense. In your case Mark if you used an experienced 928 machinist he likely used the S4 spec. In my case, the shop I'm going to approch has no 928 experience. I'm going to have to provide them with the method and specs. So this post was timely.

It's possible Porsche learned how iron coated pistons reacted during heat cycles and figured out they could tighten up the clearance based on the stability of the alusil. My guess...

Nickasil is used in all kinds of bores these days. But most often they seem to be used with forged pistons. Refering back to Brians post, the bore clearance may be subject to the piston material and growth rate not the block.

Am I even close?

 

sounds like a Greg Brown question. (or maybe its more simple than that)
Maybe it is as simple as the bock material (same for S4 or early 4.7) so, it boils back to the piston.
What happens when the coating is nicasil, does the clearance have to be increased? (and if the pistons are JE's, does that mean they are forged?)

mk

 

This doesn't sound good. If they haven't done 944 or 968 blocks before don't walk, run.

If they are familiar with Alusil and have done four banger blocks before then they should know correct specs for 5L 928 also.

 

Not that I have any specif experience with the Nicasil but if you use a high silicon content piston, you should run the gap close as it will not close up as the engine warms. In my two modified engines, I have bought 4032 type aluminium, it is very close to what Porsche use which is Mahle 124S. My bore size is between 103 to 104 mm for both engines and I will run 1 thou clearance max.

Engines that slap will wear out quicker than engines that don't, you don't need to run 2618a pistons unless you are running lots of boost pressure. Even some F1 engines recently experimented with 4032. The strength is not that far behind and it is only certain characteristics of 2618a otherwise known as Rolls Royce 58 that are superior, the link will provide a bit of history. http://www.accralite.com/accralite/rr58.htm

In short the Nicasil will expand with the alusil as it is bonded to it and the coating is very thin, it should not affect the clearance, just go and check what the current GT3 piston clearance is or even the Carrera GT, that will provide some insight I think.

Greg

 

Interesting. So, why the difference between the S4 piston (or 85 4 valve) and the older 4.7 pistons?
.00078" vs .0013" and about 10x that for chevy steel cylinders!