Do I need a rebore?

[ade]

Hi everyone, I have a barrel which I can only measure at either end as I only have a cheap Vernia gauge which only goes to 3 decimal places. I make it 2.764. Do I need a rebore and if so what piston do I order and what do I ask the engineering firm to bore it out to? I did not have a piston supplied in the box of bits. the barrel came from Ebay and was sold as a standard barrel.

[Rob Harknett]

The first thing you need to do is get literature for your bike. You will then know bore sizes etc. Which are measured in m.m. ( metric. ) Do not do anything until you first make sure the barrel is correct for your bike.

[Mick D]

Hi

You need to measure the wear on the bore, unfortunately there will be no representative wear at the very top or bottom of the cylinder as the piston rings do not sweep these areas.

The easiest, (but not most accurate), method is to use a telescoping gauge and subsequently measure the gauge with your vernier calliper. You will need to take several measurements at varying depths down the bore and various angles around it, bores tend to wear towards oval shapes, and then have the cylinder bored to the lowest increment that will return the bore to a perfect cylinder.

Telescopic Gauge.png

Failing that any engineering works should be able to measure the bore wear for you.

Edited to add: Yes you will need a re-bore, you have a worn cylinder and no matching piston assembly.

Regards Mick

[alanengineer]

MickD is correct in what he says. In reality, you need to find a piston as close to that size first. You may have to go up two sizes or three if pistons are hard to find.

[Stuoyb]

All pistons in sizes from standard to +.060 are available from Club Parts Service.

[ade]

Thanks everyone for your input. I have a relative who works in a engineering firm who will measure it properly and can machine it for me, but they have no idea what to machine it to. I have searched through several manuals and cannot find any reference to tolerances as in max dia before a rebore is needed. if I find out its exact existing dia will anyone be able to tell me what to machine it out to and what piston to buy?

[Janet]

Thanks everyone for your input. I have a relative who works in a engineering firm who will measure it properly and can machine it for me, but they have no idea what to machine it to. I have searched through several manuals and cannot find any reference to tolerances as in max dia before a rebore is needed. if I find out its exact existing dia will anyone be able to tell me what to machine it out to and what piston to buy?

As I understand it, when you find out what it measures, you buy the next piston size up and then have the barrel bored to fit.

[Mick D]

Hi

An engine begins to wear from day one, the piston, / rings, and the bore wear at an equal rate and maintain an efficient seal as they do so.

In your case I believe you no longer have an associated piston and bore, in which case your only option is to return your bore to a cylinder by re-boring and fitting an appropriate piston.

You need to determine the current dimensions of your bore. Once done you will know which oversize will return it to a cylinder, (I think pistons come in +20, +40 and +60 thou).

Re-bore to that diameter and source a suitable piston.

Regards Mick

[ade]

thanks mick. I was having a dim moment out of frustration. I have determined that the original dia is 69.85mm and pistons come in 0.5mm increments (0.020) therfore if mine is 70.10 then I need to machine to 70.35mm (69.85 + .5 = 70.35) and use a 0.020 piston.
agreed?

[Mick D]

Hi

Can't fault the maths there.

I would probably hold off purchasing the piston until the re-bore has been completed - just in case you have to go larger for whatever reason.

Regards Mick