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

[joe fxs fxr]

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

....Cotten

BMI Karts https://www.bmikarts.com/

[chuckthebeatertruck]

BMI does not have big twin flathead pistons.

[T. Cotten]

.... then make up a set of pin buttons from 6061 alloy....

I'd bet, Chuck,

PEEK would be lighter, and gentler on the cylinder walls.

If anybody wants some, let me know, I have plenty of scraps!

PEEKscrap.jpg

Although I suggest that stress plates are critical for bores .060"-over and larger (~.153 mm),
A boregauge will show that sleeved cylinders will benefit even at stock bore.
The extended spigots of U models distort dramatically from fastener stress, allowing sleeves to behave like a springy cylinder inside of another springy cylinder.

The bore can only be made round while stressed to assembled torque. It will go squirrely when the plate is removed, but return to round when installed to the same torque.

.....Cotten

[chuckthebeatertruck]

The trick on pin buttons for this application is radius. For a flatty running corvair pistons, you wind up needing to create extended pin buttons which resemble lycoming buttons. The pin fits the slipper design and leaves a solid 3/8th gap to fill. Buttons can easily do that and .060 total float is enough with 6061 to leave a good gap when the piston is pushing 500 degrees F.

By radiusing them and center drilling, you wind up with a very tiny donut contact patch about 1/4” od and with an edge margin below .020 depending on the radius. I have several custom ground hss tools that create radii so it’s a bit easier for me. Anyways, it leaves a very light track on cylinder walls. The weight winds up very similar to triple locks instead of singles. They look spindly, but work very well as there is no actual load on them. They simply take up space ;-)

PEEK would be lighter and capable of the extended length. Hideously expensive, but lighter for sure.

I’d be curious how it picks up material from the oil.

Teflon buttons always wind up with stuff in them, but the ptfe is generally soft enough that it erodes vs the bore. I usually find Teflon buttons wipe a line but rarely streak unless there’s significant metal contamination.

I do have a 500 guzzi single top end to do soon. Maybe we might just have to play with those scraps . . .the goose is low revving and heavily oiled, but with no oil or air filtration, so almost a perfect test bed for seeing how the material performs in such an application.

[sveh2117]

Dear Cotten (and others)!

Thank you for your comments on this and similar topics on boring, sleeving and maximum overboring. My situation is as follows:
My 74" cylinders are out of round and not even 090” oversize will be enough to get them clean. Overboring to STD 80" OHV (3-7/16" = 87,31 mm) could work.

You state in another topic that sleeving always should be avoided, but my choices are overboring to very thin cylinder walls (and spigots) or sleeving. I understand that people have been running 74" flatties overbored with 80" STD pistons, but not sure how that has worked out on extended trips. What option would you choose?

Sven

[T. Cotten]

I know this seems counter-intuitive, Sven,

But if you feel that your cylinders have worn out-of-round, I would measure them again torqued to a base plate.

At larger overbores, the spigots will flex both inward and outward when disassembled from the cases.

Please remember the thinnest part of the cylinder casting is always directly above the base stud holes.

....Cotten

[sveh2117]

I know this seems counter-intuitive, Sven,

But if you feel that your cylinders have worn out-of-round, I would measure them again torqued to a base plate.

At larger overbores, the spigots will flex both inward and outward when disassembled from the cases.

Please remember the thinnest part of the cylinder casting is always directly above the base stud holes.

....Cotten

Cotten,

The bore ovality was measured in a cylinder-boring machine so I assume that the cylinders were torqued (will know that after the weekend). So my question remains: should I go for sleeving back to 74" STD or overboring to 80" STD? I get mixed responses from different parts of the community.

Sven