I'm currently rebuilding my bottom end on my 2wd saph, I bought the bottoms end already bored .5 over bored and honed, I have measured the Pistons in the bore and its is .38mm which I beleive is a good tolerance? But when I put the Piston rings in, push them down the bore slightly with the piston and then measure the gap it is between 0.80mm - 0.97mm, the workshop manual says it should be 0.4 - 0.6mm, the rings are the correct 0.5 oversize so what is goin on??

Regards, Kieren

 

Instead of parts of numbers, make proper measurements.

And if you're trying to say you have a piston to bore clearance of 0.38mm which is 15thou in old numbers....then you really have a serious problem as that is huge.

 

Just reread my post, then went and re measured, the bore measure 91.41mm and the pistons measure 91.25mm, so that gives me a gap of 0.16mm or 0.0062 thou, is this acceptable in a 350bhp road car with hard use on 4x4 Mahle pistons?

Regards, Kieren

 

You'd need to consult the piston manufacturers data to be sure, but 6 thou is a fairly large clearance if indeed the measurements are correct.

 

are you measuring these with a proper bore gauge and micrometer? also, are you measuring the piston in the correct place (around half an inch up from the bottom of the skirt, perpendicular to the gudgeon pin)?

 

Taking the block and Pistons down the machine shop tomorrow to have them properly measured. Fingers crossed!!

 

Ok so had it all measured up properly, worked out closer to 8 thou of clearance, so god knows what the block was machined for before?? Anyway, I have had the Pistons measured and they are 91.257 (they have had about 2k of use) and having spoken to Mahle they recommend a 91.32mm bore size which is what I will get my spare block bored out to, that would give me 0.0026 inches, is this good for a 350bhp road car?

Regards, Kieren

 

If you spoke to Mahle and explained your build to them, surely they will know if that clearance is fine or not ?

8 thou certainly wouldnt have been though !

2.6 thou could sound a little tight, but you dont state piston material etc

 

They are standard Mahle 4x4 Pistons, but the book clearance for these and Mahles spec shows it should be 1.5 thou, so 2.6 sounds loose still, yet I have read e few threads on here with people quoting that it's too tight.

 

There are many variables, but I wouldnt have any concerns of 2.6thou being too loose

 

The standard clearance is best and make sure you have had it bored with torque plate have you checked bores are straight and true.

Mark

 

I'm going down to the machine shop tomorrow, will check and ensure everything with them before machining, so I would be better off going for 91.319mm with Pistons measuring 91.257?

Regards, Kieren