Tapered Bore

danfr · Apr 24, 2023

Hi there,

Can someone provide the specs for getting a cylinder rebored with a taper? And can someone also explain why it is necessary?

Also if someone can suggest a machine shop that is familiar with these old air cooled bikes. I'm in Victoria BC Canada, but willing to ship out.

Thanks
Dan

worntorn · Apr 24, 2023

Unless things have changed on me, you dont want any taper. Worn cylinders have taper. We bore them out to remove the taper and get everything all nice and round again.
My Vincent has just over 1 thou of taper after 60,000 miles. That's quite good.
Hastings ring Co reckons that by the time there is 12 thou taper you need a rebore.
I suspect that for optimum performance you would need to rebore long before it gets to 12 thou of taper.

Glen

concours · Apr 24, 2023

OP, could you please clarify?

kommando · Apr 24, 2023

Its not necessary, Commando bores are parallel bored with no taper. Some borers will emulate the barrel being fully assembled and bolted to the crankcase and the head by clamping between 2 plates before boring but that is not a taper bore.

danfr · Apr 24, 2023

Gotcha. Ok thanks all. I thought it was due to a discrepancy in temperature between the top and bottom of the bore.

Cheers
Dan

Fast Eddie · Apr 24, 2023

There is a school of thought like that, Hobot used to talk about it, but I’m not sure if he did it or not. Either way, I think it is a technological idea that’s pretty much wasted on these old lumps.

JimNH · Mar 7, 2024

Aircraft cylinders are bored that way. It's called a choke bore and the reason is as you state. The top of the cylinder near the head expands more due to the heat so the choke bore is straight when running. Here's an explanation. Slow talker, run at 1.25:

Stephen Hill · Mar 7, 2024

Danfr asked why you need to remove taper:
Two reasons you don't want taper in a bore:
-The rings will expand and contract as they go up and down leading to poor sealing resulting in oil migrating into the combustion chamber and blow by.
-Over time the movement of the rings will wear the grooves in the piston oversize, increasing the above issues.

concours · Mar 8, 2024

Stephen Hill said:
Danfr asked why you need to remove taper:
Two reasons you don't want taper in a bore:
-The rings will expand and contract as they go up and down leading to poor sealing resulting in oil migrating into the combustion chamber and blow by.
-Over time the movement of the rings will wear the grooves in the piston oversize, increasing the above issues.

No, he didn't.

marshg246 · Mar 8, 2024

concours said:
No, he didn't.

Actually, you could read his posts two or three ways but since it wasn't clear, your post #2 asked for clarification. That was not given and I, therefore, did not post. Had I, I would have said what @Steven Hill said! Others posted thinking of the more common practice of using a torque plate to ensure that the cylinder is round and not tapered when the head is installed and torqued - post #4.

Tapered Bore

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