Aluminum or steel Pistons for a 28 JD?

[SteveAdventure]

I bought my 1928 JD in April 2012 and the engine was claimed to have had 1500 miles on the rebuild. I adjusted my exhaust valves after about 600 miles of riding and found that the valves had no clearance. The manual says they need to be .08 to .010 so I adjusted them to .010. I discovered that it had the shorter 61ci exhaust pins and then immedeatly got a thudding knock in the engine. We dismantled the engine and found the front cylinder was melted and the top ring was seized to the piston. I think these events are related; or are aluminum cylinders prone to this? Should I switch to forged steel? What do you guys think?

[sirhrmechanic]

I would not go near a forged steel piston.

If you can get a cast-iron piston, that is likely to be your best bet for longevity. The aluminum ones are good and the lightness doesn't hurt. But if you fail to lubricate even once... or overheat and squeak one, it's toast. Cast iron are bulletproof...

Cheers,

Sirhr

[sirhrmechanic]

I would also add that you should be careful to set enough piston clearance in the bore. The rule of thumb is that you want .001" clearance for every inch of piston diameter. That's for water cooled engines. But especially with aluminum pistons, it is a good idea to go a bit bigger. On an air-cooled engine, that is probably even more critical. You have to remember that the coefficient of expansion between aluminum and cast iron is dramatically different. When cold, the clearance is one thing.... once that aluminum piston heats up, it expands at a much greater rate than the cast iron. Thus you get lockup or squeaked/scuffed piston.

If you can get a set of original cast iron pistons... that would be my recommendation for a bike you want to ride.

Cheers,

Sirhr

[Steve Slocombe]

The JD has the same bore and stroke as the VL, where piston clearance needs to be matched to piston material. Clearance of about 0.004" was recommended for the early iron pistons, then 0.018" for the solid magnesium/aluminium pistons, then 0.001" for the 1934 T-slot pistons, then 0.004" for the 1938 style strutted aluminium pistons. This style is being reproduced and is the type you are likely to buy. The front cylinders run hotter and I now allow 0.005" clearance on the front and 0.004" on the rear, but would not think another thousandth is too loose. I'd use the modern repro pistons, allow extra clearance, and check your oiling system is working correctly.

[rwm]

have someone who can do a blind bore take the taper out of you cylinders. then you can use a german made evo piston (mahle)
make sure the person who did your motor used equal length rods.
if you want to ride this in the cannonball it's going to be hard to find someone to have your motor done in 2 months.