Overboring vs sleeving (on HD U/UL/UH/ULH)

[sveh2117]

In the process of resurrecting my 46U2774 I want your advice on sleeving vs overboring.

I have 5 cylinders with the following bores (in mm, sorry for this!): Rear (88.5, 85.5 [sleeved], 88.8) and front (87.0, 89.4). As you can see these jugs are overbored well above the standard for UH/ULH (3-27/64" = 3.421875" = 86.92 mm STD).

Bringing any of them back to STD for model U requires sleeving, but people on the forum suggest to avoid sleeves if at all possible. And obviously previous owners of my cylinders have managed to ride bikes with these very overbored cylinders.

Which are the main drawbacks of the two options? What options to minimize problems? Careful honing will obviously be needed whatever solution is used.

Sven from Sweden

[T. Cotten]

In the process of resurrecting my 46U2774 I want your advice on sleeving vs overboring.

I have 5 cylinders with the following bores (in mm, sorry for this!): Rear (88.5, 85.5 [sleeved], 88.8) and front (87.0, 89.4). As you can see these jugs are overbored well above the standard for UH/ULH (3-27/64" = 3.421875" = 86.92 mm STD).

Bringing any of them back to STD for model U requires sleeving, but people on the forum suggest to avoid sleeves if at all possible. And obviously previous owners of my cylinders have managed to ride bikes with these very overbored cylinders.

Which are the main drawbacks of the two options? What options to minimize problems? Careful honing will obviously be needed whatever solution is used.

Sven from Sweden

The drawbacks for sleeving, Sven,

Include insulation for poor heat transfer, along with extra effort, and extra expense.

The drawback for large overbores is that most poke-and-hope machinists ignore proper techniques of applying torqued plates to stress the casting as if installed.

Honed and fitted while stressed allows a return to normal clearances, for pistons as large as are currently available.

....Cotten
PS:
For reference:
https://forum.antiquemotorcycle.org/...-cylinder-bore

And even though Chiefs are a lot friendlier:
https://forum.antiquemotorcycle.org/...chief-resleeve

[chuckthebeatertruck]

There is another way, Sven.

Rather than sleeving to fit standard pistons -- why not choose non-standard and easily obtained pistons to fit your bore? No, they won't be HD pistons; but it will solve your problem without worries.

In this case, I would not hesitate at all to buy Chevrolet Corvair pistons in the sizes you need from Clark's Corvair Parts. You can buy single pistons from Clark's in several different styles from cast to forged. For example, your 88.5mm rear cylinder is 10 below a 60 over Corvair piston (3.484 vs 3.497 inches). So, I would order a 40 over piston (3.4775) if the bore was clean and simply run with large clearance. Or, buy the 60 over and fit the bore to your desired clearance.

There is NO problem running two different sized pistons. You can run say a 20 in the front and a 60 in the rear -- the bike will not care. Two pistons at $40/ea is a lot more cost effective than sleeving and avoids all the problems with sleeving these very hot running motors. Remember, they will run at least 50c hotter than other bikes. I routinely saw 230c on my cylinder heads and 180-190c on the cylinders. The bike never missed a beat and was used on AMCA National Road Runs.

The advantage of a corvair piston is the cost effectiveness vs sleeving and buying new pistons. All you have to do to run a Corvair piston is open the small end bushing to .801 and then make up a set of pin buttons from 6061 alloy. You'll find that you can then deck the cylinders to bring the piston closer to the top of the bore, which has the added benefit of unsinking the valve seats. You can always sleeve them later if you like. The other benefit is that corvair pistons are lighter than std UL pistons with the steel strut. On spindly little U/UL/ULH connecting rods; I prefer the lightest possible piston.

I'm not speaking hypothetically -- I ran two differently sized Corvair pistons in my 90" stroked 1946 UL. I'd still be riding that bike if it were not destroyed in an accident. I ran those pistons with .009 clearance and never had an issue and used no oil.

[T. Cotten]

...There is NO problem running two different sized pistons....

Absolutely, Chuck!

I wouldn't even re-balance (unless somebody other than the Factory screwed with it.)

Who bought up all of DIXIE's huge oversized productions?

....Cotten