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piston clearance

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hvacr

hvacr

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I'm re-building my 1500 engine after a valve scattered itself throughout the piston chambers (previous posts ). Today I recievd my pistons from Moss and I now have more questions. I have used my precision gauge test blocks to be sure of my bore before I ordered pistons. The spec. for my 1500cc bore is 2.899-2.9". Mine is 2.900. The piston says original spec. but the piston is smaller, ( for clearance, I'm sure) BUT how much, and why don't they say ???
 
hvacr, If my memory serves me right, it should be between .003 and .0045 all the way around or max of .009 if shoved to one side. I hope you have a manual and the STD piston on some makes does not have a stamping, just the +.010 and such. Wayne
 
Thank Wayne, Yes I'm reading the manual. So does the piston maker decide how much tolerance is best?
 
hvacr, if you get 4 pistons, most times 2 will be out a thousands or so unless you really serious about it you won't go to the local big three and get the perfect pistons. Getting a +.010 means you got a set that is .010 over size and you will have to have had a rebore, but on crank and rod bearings a +.010 means the bearings are over size because you had the crank ground down .010 and so on. I'm sure you know this, but sometimes things don't get said on some post, because we all take things for granted that everyone knows this, but then again they my not. Sorry to rattle on. Wayne
 
One more for you Wayne, I've always wondered about the dynamics of "twist" in a piston as its headed downward. What force really keeps it from tilting from a turbulent downward pressure?
 
[ QUOTE ]
hvacr, If my memory serves me right, it should be between .003 and .0045 all the way around or max of .009 if shoved to one side. Wayne

[/ QUOTE ]
Hi Wayne,
Better check your memory. I would guess that .002" to .003" total clearance when shoved to one side would be much closer to correct. Clearance is usually measured at the bottom of the skirt at right angles to the piston pin. I think .009" would rattle something awful. /ubbthreads/images/graemlins/smile.gif

Ed,
It's best to get the recommended clearance from the piston manufacturer & then have the cylinders individually bored/honed to this clearance. The required clearance varies quite a bit from manufacturer to manufacturer & type of piston. If you ordered standard pistons & plan to use the old bores, the fit is almost certain to be less than optimum, as is the bore condition. Far better to get + .010 pistons & have the bores fitted to them. Unless the bores are nearly new, there is likely to be some taper & out of round in the bores.
I'll be quiet now.
D
 
Dave, you are right on the money regarding piston clearances on the 1500. .002"-.003", bottom of skirt, 90 degrees to the wrist pin. Even on the race engines, that I built loose, I only ran about .0045" total.
Cosworth pistons actually give you the bore size to use to fit their pistons.
Jeff
 
So much for the memory, thanks guys for the correction and like you I have always had the bore to match the pistons done at the machine shop, I think (?) I need more fish oil or what ever they are saying is good for you today, tomorrow not good.
Wayne
 
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