im ready to build my motor. im building a k20a with wiseco 9.8cr pistons. i got my block machined at a shop and they measured their piston to wall clearance. they wrote on my box that the piston to wall clearance is 5/5.5. anybody know what that means? everytime i see piston to wall measurements its like .00035, .00045. i never seen 5/5.5. anybody help?

 

whoever "they" are is who you need to call and ask

 

they are probibly saying .005" to .0055" which seems ok. I don't think you mean hundreths of a thousand. as in .00055" , it's most likely "5 thousands to 5.5 thousands" piston to wall. which will be "ok"

 

^^ thats what i was figuring. i see when people say 4.5 its .0045. im planning to run 25-30psi at the track which i told the machine shop. so they prob figured all that heat will expand them a decent amount

 

but anybody experience or use .005 with 10-12 psi? worst gon happen is i have a little piston slap during cold start?

 

That sounds like a lot. What kind of pistons are you going with?? What type of alloy?? And what did they recommend?? I'm actually getting my block bored right now for 4032 pistons, and the manufacturer recommended about .003 PTWC for those boost levels (these pistons evidently don't expand much).

 

check threads on this site.

im using wiseco pistons. wiseco recommends .0025. a lot of people tried using them at .0035 at high boost and ended up messing up the whole side of their pistons FAST by wall contact. so .0045 would be the minimum they recommend to set your piston to wall at (they refering to the members with expierance)

i guess its always better to build them slightly loose. but if they too loose can that cause problems too? .005 too loose? i know we are talking a hair here from .0045 and .005 but just trying to figure if i should junk this block and start again or use the .005

 

you will be fine...ive gone as much as 7 on an arias piston and it wasnt bad...you may have some cold start noise though...fwiw wisecos seem to like a little tighter ptw...i usually run em at .0045 or .0050

 

what would be considered cold start noise?

 

I have my Wisecos that are in the motor right now setup at .0045", and I'm shocked to say they are quieter than the stock cast pistons at startup. Makes me think .005-.0055 would be perfectly fine.

 

cool, good info to know. does the psi effect anything on p2w? if i run 12psi or 25psi? i know the more psi, more heat, more expansion. but will it effect anything at low psi that doesnt produce as much heat?

 

i think im going to go with .0045. seems like people are successful with it.

now my question is, i know wiseco has a coating on the side of their pistons. should i tell the machine shop to spec it to .004? that way when the coating wears off it will open to .0045?

 

hmmmm???

 

I did mine at .0045 with the coating on the pistons, so the clearance may easily grow. No concerns from it though. Just cold started it and drove it directly to work as is normal for me. I LOVE this setup.

 

ok so .0045 it is. thanks guys

one more question. do i use the ring gap specs on the sheet still? or are those gaps for the .0025 p2w spec wiseco provides?

 

ring gap has nothing to do with p2w.

 

hows that? say if the block is honed to a .0025 p2w, the rings will have a closer gap compressed in there compared to a block honed to a .005. am i missing something?

 

you set the gap when the motor is final honed...ptw will be a non factor....the only way it would matter is if you had the cylinder bored to spec out at .0025,set your ring gaps and then decided to hone it out to .0050...then your gaps would be thrown off due to increased bore diameter..

 

ring gap is about bore size, not p2w.

 

i see, so just use the gaps on the spec sheet given?

 

0.0045

 

If the block is already bored/honed, and someone wrote that, then 5/5.5 is what ur gonna have. You can't "go" with less clearance now. It is done by the guy on the hone carefully measuring/ stopping the machine, to achieve the proper bore size (within a half thousandth) which when the pistons' outer diameter is subtracted, equals the p2w clearance. Which, unless ur damn lucky, or had a really good shop, they didn't do anyway. Wonder what they were shooting for? I wouldn't take their word for it, measure it if u can.
my old sig....


But asking around, you'll find 4.5~5.5 is recommended, so no worries.

 

Since I wanted to have the ability to abuse the motor without worrying about butting ring ends from a bit of detonation or whatever, I went with the ring gap specified by Wiseco for boost plus nitrous. Motor is very happy!

 

just drop them in the respective bores one at a time, make sure square to bore, measure with feelers, and if needs be file to fit. Go with there specs, but to the tight side for oil control IMO. You may find some are already at the max of spec etc.. so u cant do owt with them, but can swap into next cylinder and so forth to see if any tighter in there. When ur happy with all, assemble.

 

If you leave it to 0.0055 you will be burning some oil all the time. the ring gaps will be wide.

Thats is normal on a car looking to get 500hps, but if you want a street car, stay on 0.0045.