cylinder bore measurement

[Drew Back]

I am new to using calipers so bear with me..I just measured the bore of my cylinders and I came up with 62.7mm is this .20 .40 over or are they trashed I think stock is supposed to be 62mm .I have allways sent this stuff out to get done so forgive the ignorance of not knowing what size pistons to get..

[RichardL]

My BSA Twins and Triples Book is telling me it must be a '46-'50 long-stroke A7 or Star Twin. At 62mm standard, your 62.7mm would be 0.028" oversized. If you have 20-over pistons, I believe this is too loose. Then, I'm assuming you are using a dial or electronic caliper across the top, which will give you a bad reading because of a ridge and the fact that the bore gets bigger  where the rings run. If you are using a classic inside caliper to reach in, I don't think you can get an accurate enough measurement. I am, right now, waiting to receive a dial bore gauge (Chinese knockoff) so I can get a decent measurement without struggling with telescoping gauges.

All this said, if the last bore was 0.020" over, as it seems it might have been, you should be in good shape for at least two more bore-outs, if you could find the pistons.


Finally, there are several people here who actually own the same model and can speak on the subject with much more authority and infinitely more experience with the bike.

Richard L.

[RichardL]

Couldn't help myself. I had to see if pistons were available. British Only Austria appears to have them at 20, 40 and 60 over ( with the 40 and 60 at 9:1).

[Triton Thrasher]

Measuring bores is an expert's job.

[muskrat]

My T gauges and micrometer get within 0.0005", but it takes patience, experience and a good feel. Verniers are for quick close enough measurements.
Most engine re conditioners have a book with piston size/pin height/pin size. Might find a car piston the right size to go +30. If you go with the +40 British Only Austria,I'd knock some off the top to drop the compression or use a plate under the barrels.
Cheers.

[Drew Back]

You are right about leaving this for the experts I just thought I would give it a shot I am using the t gauge also but its tricky measuring those little ball ends..Thank you all again for the help my next measure project will be the cam which does not look to bad..

[RichardL]

My T gauges and micrometer get within 0.0005"

It's going to be interesting to me to see the difference between my t-gauge technique and what the bore gauge tells me (which also comes down to technique, I suppose).

Richard L.

[RichardL]

Well, this came to my door about an hour ago. I didn't really have high hopes, considering $50, including shipping, but I must say, I'm pretty impressed with the general quality for something so inexpensive. Readily found on eBay. Don't have time to put it to the test right now. I'll let you know how it goes. (For those of you with bore micrometers this acquisition is trivial, but it's a big change from t-gauges for me.)

Richard L.

[Triton Thrasher]

You are right about leaving this for the experts I just thought I would give it a shot I am using the t gauge also but its tricky measuring those little ball ends..Thank you all again for the help my next measure project will be the cam which does not look to bad..

You're quite right to have a go, but I'd hesitate to condemn parts as worn, if there's doubt about tje measurements.  Is there a wear ridge?

[chaterlea25]

HI All,
Richard,
I have a similar bore gauge set,
In so far as their design goes they work well enough BUT do not actually measure the  bore without  having a set of "standard" setting rings
If you zero the dial gauge after fitting the assembly at the bottom (unworn part) of a cylinder bore  then they work well in showing where and how much the bore is worn,

Experience and "feel" are needed to get really accurate results with telescopic gauges or internal 2 point micrometers
For tool room applications 3 point micrometers are the norm, But these normally run to megsbucks
and have a limited range per micrometer 

http://www.ebay.co.uk/itm/50mm-100mm-THREE-POINT-BORE-MICROMETER-RINGS-INTERNAL-BOWERS-A-VAT-IN-/201029082345?pt=UK_Measuring_Tools_Levels&hash=item2ece445ce9

HTH
John

[RichardL]

Hey John,

I thought about gauging rings, but the instructions and some Youtube videos show calibrating by using an outside  micrometer locked to the starting measurement. Indeed, it was owners of 3-point bore micrometers that I thought would be snickering at me. Did I mention that nowhere visible is there indication of where my gauge was made. Maybe it's the main export from Uganda.

Richard L.

[Drew Back]

There is a little wear ridge at the top I did get a cylinder hone and ran it through them tonight they cleaned up real nice and shiny no scrapes or heavy scratches I will try and measure again soon with someone more experienced ..What should the exact measurement be to run .20 over pistons??

[chaterlea25]

Hi All,
Richard, Its difficult enough to set the gauges between the jaws of a micrometer, the area's involved are small
when fitted to a setting ring the gauge can be drawn through an arc to see where the gauge and setting ring are square to each other, I have saved some roller bearing inner rings as "rough" guides
Recently I invested in some 3 jaw internal mics on ebay, the best score was an electronic display set that measures from from 6 to 16mmm, or imperial at the touch of a button for just over £100 + post
New the equivalent kit is £1500 app
Its harder to measure smaller internal diamaters accurately than larger sizes where proportionally 0.001in makes less of a difference, (hope you follow that reasoning)

Drew,
Measure the clearance at the bottom of the piston at right angles to the gudgeon pin with feeler gauges,
place the piston firstly near the bottom of the bore, where the rings wont have been running
then move the piston to about a half inch from the top and  measure again,
compare the clearances measured with those from the service sheets and see if its within tolerances

This clearance is more important than actual bore size which may have been dictated by the size of the o/s pistons fitted
0.7m mentioned would have been nearer + 0.030 than +0.020 ??

HTH
John

[Drew Back]

Thank you again for all the imfo i dont have pistons which is part of the whol problem

[muskrat]

Looks like its +40's from Austria then Drew. I wouldn't be using them at 9:1 in an old Long Stroke without dropping the ratio to at the most 7.5:1.
As John said, bore is done to suit piston + clearance (4-5 thou).
Cheers