Help with measuring a bore

I am in the process of restoring a older motorcycle and need to machine new pistons to fit into the fork sliders, unfortunately I am having trouble measuring the internal bore of these as I only have telescopic bore gauges and I'm not getting consistent measurements, as the pistons need to be 3 thou under the size of the bore I need help in getting the correct measurement so is there anyone within a hours drive of Corby in Northamptonshire that could accurately measure these for me. The internal diameter of the sliders is approximately 31 - 32mm. any help with this would be very much appreciated

Joe

Just a thought Joe, could you not turn up a plug out of Ali or similar that is a snug fit in the slider and then mic that up.

If you're getting inconsistent measurements, do you think the bores are actually round and parallel, or are they worn unevenly?

Is it practical to ream to a known diameter and make a piston to suit that?

As Mick B1 said the bores may no longer be round or straight due to wear. Try to borrow a bore gauge with a dial indicator.. Use it to explore the bore to see what condition it is. It may no longer be round or cylindrical. It may be necessary to hone them to make them cylindrical.

What he said.

Machine DRO on this page sell bore guages at very reasonable prices.

You don't share the make or age of motorcycle with us, but generally speaking, many old bike tele forks were made from seamless tubing. The inside diameter was not machined, merely left as rolled. So they may well not be perfectly round nor perfectly parallel. Mostly the pistons were made a loose fit and relied on the piston rings, or O rings in later years, to seal.

+1 on make an ali plug that you can fit the end of the fork leg over while plug is still in the lathe and size it that way. Then bung it on the end of a bit of rod and run it up and down both fork legs to check it will slide all the way up and down without binding. If it binds, knock a couple thou off and try again etc. Then turn your pistons to that final size.

Where are you i use digital and analog bore gauges all the time and have several maybe i can help i am between Grantham and Newark.

Bob

Bob,

Joe has no profile in his settings, but did say ‘within an hour of Corby, Northants’ in his opening post.

I do a lot of motorcycle fork work, they are never very good for being round. Go with the Ali plug idea, you don't want them that tight that they won't slide. I now use glass filled PTFE rod to make the bushes and they slide well.

Hi Bob I have sent you a message.

Joe

I think Joe is making his life hard for himself. If it were manufactured in imperial measurements, it makes sense to measure in imperial measurements. Then at least the start measurement becomes obvious with simple measuring tools.

Italian motorcycle therefore manufactured in metric, maybe I should have said 0.07619 mm instead of 3 thou tolerance.

Work in metric and imperial sometimes even at the same time with no problems just have to make use of a calculator now and again.

Wouldnt be Marzochis would they.

He is referring to the bore in the aluminium precision machined legs (unless worn oval) not bore of the precision fixed harder steel tube that slides within, no play.

Analogue bore guages just as good as digi. Much easier for me to put you on to a good guide regarding usage and eliminating errors on youtube. One of many https://www.youtube.com/watch?v=R6YLK0F3ytc
Have reservations non twist/rock to find centre before locking up. If off centre bore will be smaller!

Joe why not consider a bore gauge see this link, not too expensive and useful afterwards ,

**LINK**

Another alternative measure best you can with the tele gauge then make a multi stepped plug gauge in 0.002" steps, the steps don't need to be very long, then when you are confident of being very close to the size make or alter the gauge to longer steps and test again, last make/alter the gauge to the final size and test. About 2" of aluminium bar should suffice.

Edited By JohnF on 15/04/2018 15:54:40