Board index Flathead Power-Technical Questions, Answers, and Suggestions Pans How Many Thousandths Can Taiwan Cylinders Be Re-Bored ?

How Many Thousandths Can Taiwan Cylinders Be Re-Bored ?

Moderators: Curt!, Pa

Posts: 1536
Location: S.Calif.

I've already bored my (wear seasoned) 74 cu. in. Taiwan cylinders 0.010" and run them 26K miles. Has anyone bored them to 0.020 over with great success? Don't want them to frag in service. Don't want to have to buy new cylinders either.

Posts: 184
Location: Naples, Florida
Plumber,I highly recommend you bore them to .020 over. You can be the test bed for the repop industry and report back to us as to how it worked out.

Posts: 3010
Location: Central Illinois, USA
Fragging isn't really a concern; Daylight is.

I have an older Oriental cylinder that ran fine for the owner at .030".....even though the wall was paper thin over a basenut flange. I put my fingernail through it upon inspection.
It was seriously gouged elsewhere by a thrown wristpin, yet the casting never failed.

The core float might be different for every cylinder, so just measure the wall over each flange before you make any decisions.
A real HD cylinder can have as much as .070" left after a .100" overbore!

(And remember, the thinner the casting, the more important torqueplating becomes.)

Daylight at .030"
PERFDCYL.jpg (68.13 KiB) Viewed 2962 times
Last edited by Cotten on Fri Oct 05, 2007 6:59 am, edited 1 time in total.

Posts: 1536
Location: S.Calif.

Thanks Cot. I'm forwarding what you said over to Stett. I like to go in there armed with as much info as possible and hear what he has to say. Mechanics, like myself, like to hear themselves talk about procedures. It's a mental audit for "wrenches" that are called upon to work on a lot of different motors. A "sorting hat" for each particular job, if you will.
Torque plates will come up as a topic. Makes good sense to bore under the stress of the actual fastener pressure, especially on something that's going to want to move around, with 0.020" of the original material gone. Rarely do you find an early Big Twin mechanic, in our region, that's has an analytic quality, and the shop equipment to match his knowledge and talents.
On torque plating the cylinders, the torque plates for early OHV only support the bottom half of the cylinder (unlike an Evo where the torque plate supports the bottom of the cylinder as well as the top). You would have a bottom plate, where the head bolts go through the underside of the cylinder, and a then a top torque plate on top of the cylinder. So, you're only torque plating half of the cylinder. How about the other un-supported bottom half of the cylinder where there is no torque plate compression?
We went with Wiseco 0.020" over pistons and Hastings rings.

Posts: 3010
Location: Central Illinois, USA
Whoa now...

Evo's use compression plates.
Their design does not suffer from the kind of fastener torque-stress that earlier models do.

Earlier OHV models also benefit from applying top plates: Fastener stress pulls the wall outward in five spots, robbing horsepower/ring seal. Bottom plates are just more critical for piston clearance, as the wall will go both inward and outward.
(Top plates on a flatty seems futile.)

Whether or not Oriental repoop cylinders will distort at less than .060" overbore remains to be measured by your boremeister. That's about where it becomes measureable on real cylinders.

The bottom line is: measure everything before you cut or order pistons.


Posts: 1536
Location: S.Calif.

measure everything before you cut or order pistons.

Indeed. Attention is being focused on the on the walls in the areas of the flange.
When I mentioned Evo torque plates, I was speaking out of my element. I know nothing past 1959. But torque plating an early cylinder will leave the bottom of the cylinder un-torqued plated, just because of where the head bolts sit on the bottom of the top half of the cylinder. Where's my camera when I need it?
My biggest concern and the total absolute bummer of my life are those CCI gas tanks. Not for just me, but for the entire repop world of future builders :!: If I can find a way to clip-up a set of Tedds Knuckle tanks and make them look right for a Pan, I'm doing it.

Posts: 646
Location: Detroit
Everyone I know and trust who does boring always told me not to bore Taiwan jugs to any OS because the walls are so thin to start with.
New Knuckleheads? Thank, you, Jesus!!

Posts: 1536
Location: S.Calif.

It's a dice roll. Stett's done a lot of them with none of them popping. I'll let you know if I'm unlucky.
Seems like I've read in V-Twin's catalog before about not boring them out either, but I can't find it now.

Posts: 3010
Location: Central Illinois, USA
Plumber wrote:
"but torque plating an early cylinder will leave the bottom of the cylinder un-torqued plated, just because of where the head bolts sit on the bottom of the top half of the cylinder. "

Use two plates, one for the top, one for the bottom.
The bottom plate is the same for all OHVs (and BTSVs); the top must be appropriate for each particular bolt pattern, spigot, etc.

Please remember: stress fixtures are meant to simulate the distortions caused by fasteners and assembly only. The same torque specs should be applied as will be used when installed.

PS: I'll trust a measuring instrument over hearsay any day.
Measuring cylinder wall thickness
cylwall2.jpg (40.22 KiB) Viewed 2980 times

Posts: 1536
Location: S.Calif.

Nice pics. Always a plus to go along with an explanation. Makes sense to torque pate cylinders.

Return to Pans