Aluminum verses cast iron piston

Dennis P

Member
I am setting up to bore my blocks in a UTS (4 3/8). I was told to bore/hone it & end up with 2.5 clearance for aluminum or 4.5 clearance for Cast Iron. What are others doing?
 
I believe the rule of thump is .001 for every inch of diameter. You probably want
.005 on skirt clearance. I would say .01 would be too much.
 
I agree, More clearance is needed on the aluminum pistons. I didn't go check my notes, but if i remember correctly when we were boring blocks for U tractors for M&W pistons, we had to bore them for .009 clearance to keep them from galling.
MMDEL
 
Delbert is rite about the clerance with alum. pistons.One of my " U " s did not have enuff clerance and it stuck with in an hour from start up.
If IRRC , the shop had about .5 clreance. I was concerened about that . Wound up with .9 that worked Was able to salvage some of the pieces .
 
Dennis P,
Piston clearance is not a constant. As the bore changes, the clearance changes. An iron piston has a coefficient of expansion that is almost half of a forged aluminum piston. The old rule of thumb of a thousandths of an inch per bore diameter generally works for softer aluminum pistons. Most iron pistons need half that clearance.
If your piston is cam ground; it needs less clearance. If your aluminum piston is made from a hypereutectic alloy (hard); it needs less clearance. This is because pure aluminum will pick up heat more rapidly than iron. Hypereutectic aluminum has silicon added to insulate the piston from the combustion heat and make it extremely hard and scuff resistant.
Good luck.
Charlie
 
Years ago I installed a set of Jahns aluminum pistons in my UB. They were about 4.375" in diameter. The recommended diametral clearance was 0.008". In doing the rebore of the jugs, the machine shop erred and bored one with a 0.006" diametral clearance. I didn't realize it at the time and didn't check. After assembling the engine and running it, I found that the engine got tight when it got hot and actually temporarily seized up. When it cooled down it ran fine. I ended up disassembling the engine. I found that one piston was seriously scored. I replaced it and had most of the the scoring on the one cylinder honed out. It was reassembled with a 0.012" diametral clearance (larger than recommended to help remove the cylinder scoring). Never had a problem with it after that and used it quite successfully in many, many tractor pulls. Of course, I wasn't building a 2000 hour engine either so having a slightly larger than recommended diametral clearance was not an issue.
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top