typical piston to cyl wall clearance- TRW .030 psitons

[Quick6'n'-K.C.]

we took the heads off a friends 6500 mile motor last night and i noticed that there is a 1/8th inch gap between the piston and cyl wall --

they did a cyl leak down test on the motor and said there was nothing to be alarmed about-

but from everything else i have seen wrong with this engine, i am starting to get alarmed with how it was put together-

so what is the typical cyl wall clearance?

 

[KendallF]

TRW specs .0025-.0035", if I recall correctly. I went .004" with the the ones in my GN currently. If you have more than .006" or so, I would be concerned. Did the motor have piston slap on start-up?

 

[Quick6'n'-K.C.]

kendall-

there is a gap about 1/8th of a inch between all the piston to cyl wall clearance-

there is no piston slap

the moron who built the motor put B series bearings in it and all the bearings showed more wear on them than my 120k motors-

i can move the piston top over a fuzz enough to show a gap at the top of the piston to cyl wall-

motor lasted 6500 miles

has studded heads and mains-

 

[FJM568]

Bryan,
The crown of the piston is smaller than the skirt of the piston...The crown is the top of the piston above the top ring...

The .030 over TRW forged pistons I have are about 3.78 at the crown...Then down on the skirt below the oil ring, above the pin, it's about 3.79 and the 90 deg from there it measures like 3.825...

So there will be what looks like a HUGE gap and that the pistons look too small at the crown...The clearance that Kendall is talking about needs to be measured at the skirt diameter of the piston where it's the largest...The pistons are machined oval so that when they heat up, they will be round...That is why you check side clearance at the largest diameter on the skirt...

Hope this clears it up a little bit...

edit...The above measurements were just using a pair of dial calipers...If you want more precise measurements, I'd have to dig out my micrometers...

 

[Quick6'n'-K.C.]

JIM-
thanks for pointing that out for me- i didnt realize that the piston was made like that-

i havent had to redo rings in any of these motors so that is something that i just havent dealt with before-


thanks for the tip jim

 

[Reggie West]

If the motor sounds like a diesel truck when it is running, you have piston slap......