Thread: Turbo piston/wall clearance
04-04-2008, 05:02 PM #1
Turbo piston/wall clearance
For those setting up motors
you all abiding by the manual for specs? which seem to be rather tight?
or are you fitting them a little bit looser to start-off given the increase heat and boost?
i'm swapping pistons in my turbo motor from cast OEM pistons to the lower compression forged pistons... however the forged pistons already set me up a little looser... hardly anything at all really. the pistons i'm putting in are .0008" smaller than the pistons that were in there.. (yes that's the correct number of zeros. 1st size OEM cast pistons were 99.96mm, i'm installing forged 99.94mm pistons
on NEW engines seadoo has them setup at less than .002"... in car world and most turbo applications they are starting off wayyyy looser than that... i'm debating how loose i should go inside mine
04-04-2008, 05:09 PM #2
- Join Date
- Dec 2006
- Crest Hill, IL
Shibby, does open loop cooling factor in? Isn't there an minimum operating temp? Block temp for proper swelling of parts? Or does that only apply to CAST pistons...
04-04-2008, 05:19 PM #3
Rule of thumb!
Cast piston (very close clearance) .0006-.0012
Forged piston .001 per bore Diameter. Ex 3inch bore .003 clearance.
Add for power adders.. say like .001 in your case.
.003-.004 clearance should be a good start. and you could re-hone up to .005 total after that. Being round (staying round) is far more important than a tight clearance, remember that!
after about .005-.006 you should look into re-sleeving the bore.
--all numbers above are in inches---
04-04-2008, 05:21 PM #4
- Join Date
- Jun 2007
In all the honda turbo motors i have built i have went with .0045. These are 15-25psi turbo motors making 500-700 whp. I am assuming you are using JE forged pistons. This would be my suggestion. Forged pistons will heat up and expand. You MAY have some slight piston noise/slap until they are heated up. What does JE say to do?
Some guys say .0030-.0040. Some say .0050-.0060. This is on 500-700hp honda motors. I would go with .0035-.0045. You will be just fine with that!
Shawns #s look good to me!
04-04-2008, 05:36 PM #5
Danny... yes the open loop will be a factor, but really mainly only at initial startup in cold months... if i setup the motor on the very tight side, which from talking to a few people seadoo seems to be setting them up a little bit tight at less than .002" and if i ran my ski right now in 40 degree water temps and start it and start riding right away that piston may expand a little too fast compared to the bore and result in a cold seizure i would think.. so proper warm-up is going to be critical
Shaun, thanks for the response, i appreciate it.
right now i'm at .0027"
it's a slightly seasoned block, ok maybe not very seasoned... about 7 hours on the motor, i broke it down and the cylinders are mint but i haven't put a hone to it yet until i researched the piston size variance and the suggested turbo specs on the bore...
so i'm not too far off from a .003-.004 start... i'll hone it out a little more give myself a little extra room
Last edited by Shibby1485; 04-04-2008 at 05:39 PM.
04-04-2008, 05:38 PM #6
Cody... these are not JE's.
just the stock RXP Mahle forged pistons which seem to come pretty well recommended.. my uncertainty came from me not starting with a RXP powerplant.. it's a 2007 130HP 4-tec powerplant that i'm building up for the turbo.. and when i caught the piston size differences it may me start thinking to further investigate my clearances
04-04-2008, 05:42 PM #7
If you are not in a hurry and could afford a week or so send the pistons to these guys.
I know it is really not a clearance thing but worth the time to do it IMHO.
Last edited by 1 FAST VE; 04-04-2008 at 05:42 PM. Reason: spelling
04-04-2008, 05:44 PM #8
according to their company info and piston systems website, the mahle pistons already come with their own coatings and such
Mahle forged gasoline aluminum pistons...
they also have a link on the site for powersports stuff
but yes... i am in a hurry, a week is valuable time i dont have to spare, lol...
04-04-2008, 05:52 PM #9
That is really not the same deal, biggest thing is the thermal coating on the top of the piston that you can do. Swain claims any coating on a stock piston is no more than a break in thing.
Theirs spreads heat more evenly over the piston top and slows heat soak into the piston as well.
It is a little insurance when setting up clearances on any motor. It also lessens the likelihood of detonation because hot spots don't really form well when the coating is applied.
Last edited by 1 FAST VE; 04-04-2008 at 06:00 PM.
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