Hi,
Im building a 3sgte motor and I have a set of JE pistons which are 0.020mm oversize. Since I plan to run the motor on high boost, what should be my piston to wall clearance? I need to bore my block so I have consulted various forums and the rule of thumb is that it should be 0.004 or 0.005 for boosted engines. Can anyone with some experience help with this?
Thanks
Apr '13
Apr '13
Just for the sake of info
Bro what gauge are u using for the measurement ? Is it oversized considering the piston rings or the wall to wall difference ? the above mentioned clearance seems to be legit but approx what increment would u adopt for boring ? all of a sudden to the approx dia or repeating it a couple of times until u come somewhere near ur required size ?
BTW the piston that u mentioned - are they meant for the 3s engine u mentioned ? are they oversize only in dia ?
If im reading correct, you bought pistons with only 0.02mm oversize? Im sure you mean 0.25mm oversize.
its best to consult with the manufacturer because of the alloy used in aftermarket pistons is not the same as OE. anyway - a target of 0.065mm can be used - but even with this you may hear slap, but with forged you will always hear slap. Please use a micrometer and a bore gauge to measure, not feeler gauge.
if u got pistons in .020 over as in internet speak than thats .020 inches which translates to 0.50mm over or what ur local khiraad wala will call zero pachaass over bore.and yes the ptw clearence will be good enough from .040 to .050 inches.anything less pistons can bite and more will make it slap and noisy and maybe use a bit of oil.how are u going to bore and hone?torque plate?
Let me clarify a bit further. My 3sgte comes with 86.5mm stock bore. The pistons are 86.5+0.020 oversize (thats factory spec I havent measured them myself) and no its not 0.25mm. So I need to bore my cylinders to accommodate the 0.020 pistons plus a clearance of lets say 0.0005. While I am certain of the how much to bore for the pistons I am not certain on how much clearance to keep as forged pistons tend to expand more than cast one as Xulfiqar says. I have already sent an email to JE for some proper clearances but I thought someone might be able to help me here as well. Thanks
I am sorry its a bit confusing but here are the specs for the pistons:
Applications: Toyota- 3SGTE 2.0L 90-95 MR2 Turbo
Bore MM: 86.50 +.020
Compression: 9.0:1
Why should I read it as 0.020 inches when it is in mm?
86.5 is in mm while .020 is in inches.oversized pistons only come in .25 .50 and .75mm sizes..020 in gets .508mm .to get ur required ptw u need to add 86.5mm plus .020 in plus .040 in. which is 88.10. although this sounds very loose.anyone else ?
piston to wall clearance depends on many variable factors...
1- amount of boost to run
2- type of alloy of the piston manufacturer
Run it too loose and you have piston slap,run it too tight and you have possible engine seizure in the future.
i have seen biased comments on forums on this issue,but i have also seen that you learn with experience.
talking about boost here just for your reference here,n/a engines have different specs for the ptw clearance
Found this on the JE site:
Clearances listed below are minimum. Some applications such as supercharged, turbo, nitrous and endurance applications may require additional clearance (.001-. 003"). Coldwater pickup marine applications may require an additional .002-.004".
Table 1 bore range min clearance.
Sport compact 2.500 to 3.625" .0022 to .0028"
Sport compact 3.626 to 3.999" .0025 to .0035"
SB applications 4.000 to 4.200" .0025 to .0035"
BB applications 4.200 to 4.600" .0035 to .0045"
My pistons come in the first category Sport Compact 3.425512 (3.405512+0.020). Now I take the minimum value 0.0022" plus additional 0.002" it equals 0.004" which verifies the 0.004 to 0.005 range if my calculations are correct. 
yes thats the figure .but how are u going to do your bore/hone?u need a torque plate and a very very good machinist.
Yup. Will have to discuss with my mechanic on whats available. I am for torque plate honing but I dont knwo the costs associated with it. Any recommendations?
it would be far cheaper to go to a steel market and get a regular MS plate about 1 inch thick, and ask them to plasma cut it according to your headgasket and cylinder bolt holes - get 8.8 bolts for your plate not 10.9 - 10.9 are harder but brittle too. Get your machinist to polish the plate dead square on his mill.
Install and tighten as per spec, similarly install a junk crank and tighten the main caps - do all this while the block is resting on its own weight and not hanging off a stand.
For the clearance, you need a bit loose on full forged pistons and use a big mircometer to measure the skirt and a bore gauge to measure then calculate the clearance - most (read as all) machine shops use steel feeler gauges to determine clearance, it may be OK for a jing long generator but not any engine im building. The really oldschool machinists would use string type feeler gauges and push a piston in the bore, It was good but nothing beats precision tools.
The most important bit is the ring gap, rings expand a lot under heat and you need to account for it, You also need to chamfer the edges of the rings when you set the gap. Otherwise it may pinch it and you then lose compression as the ring is not pushing into the cylinder under combustion pressure.
You have got yourself 0.50mm oversize pistons aka 0.020" (imperial), you cannot buy 0.02mm oversize pistons for almost no engine - the difference would be really small and useless IMO.
