Micrometers

[luccass_j]

Im rebuilding an engine i took it apart some time ago and i bought micrometers to measure the wear of the engine components i use telescopic gauges to help me measure cylinders bores. The question ive got is why every time i measure cylinder bores ive got slightly different reading? What do i do wrong if anything? Any help is welcome

 

[500e]

How wrong ?

 

[Swift&Bold]

Cylinder bores wear to an oval which is more pronounced at the bottom.

 

[cocker]

The bores could be not round , as in slightly oval with wear .
Telescopic bore gauges are a bit tricky to use correctly , are you useing the ratchet on your micrometers ?

 

[brightspark]

once u have set and locked the bore gauge move it up and down then turn it quarter and do it again . as said oval bore or worn from top to bottom u will feel the difference on the gauge as u move it

 

[Robotstar5]

I push a ring up the bore using a piston skirt, makes the measuring point nice and square.
I once took a bike cylinder into work to see what the metrology dept had that could measure the bore, they decided the easiest option was to use a CMM, it was funny to see a £000's machine used to produce readings to 6 decimal places

 

[luccass_j]

Ive got rachet on a thimble and i used it to the first maybe second click they are metric micrometers and the difference is sometimes quite small .02-.03 but sometimes up to .05 thats a big difference

 

[cocker]

That's 2 thou . How accurate do you need to be ? Rebores are normally in 10 thou increments .

 

[luccass_j]

I just want to find out if ive got to rebore the cylinders which i already belive ill have to so the question is to what size i also want to learn how to do it proprely i believe if the taper exceed .10mm or .004in the engine sshould be rebuild my first reading gives me taper of .21 thats the taper between the top and the bottom but as i said the readings are not consistent

 

[ronan]

If you have a ridge at the top of the cylinder then a rebore is likely, as Robotstar said putting a piston ring down the bore and measuring the end gap is another method. What engine is it ?

 

[luccass_j]

1.8 ford pinto it does have a ridge on the top and i did a piston ring methods that already indicated reboring...i want to try to push the piston ring to make the measuring points square the way Robotstar suggested because thats where the errors probably occoure

 

[cocker]

Are you trying to convince yourself that a rebore is not required ?

 

[wristpin]

Measure the bore at three places, just below the top of the area swept by the rings, mid way down and at the bottom . Take two readings at each point, one parallel with the piston pin and one at right angles to it. Write them down and compare with the manufacturer's spec.
The trick is to rock the gauge at each position and take the maximum reading.
My purpose made bore mike includes includes the instruction COCK, ROCK and LOCK!
The alternative is to use a bore gauge that does not include a dial test indicator and once locked measure it with a vernier caliper

 

[luccass_j]

Thanks for all the help so far...and no im not trying to convince myself that rebore is not required i want to do it the proper way and learn as much as possible while i do it

 

[BrokenBiker]

I push a ring up the bore using a piston skirt, makes the measuring point nice and square.
I once took a bike cylinder into work to see what the metrology dept had that could measure the bore, they decided the easiest option was to use a CMM, it was funny to see a £000's machine used to produce readings to 6 decimal places

they use CMM's where i used to work to measure the parts we were churning out on the CNC...once an hour a random pair had to be put on test...the programs were supposed to be locked but when you work with chimps that use 'password01' as the password its quite easy to bring it into manual mode and measure whatever you want with insane accuracy...like your lunch time apple...

 

[ronan]

Was the engine burning oil before you torn it down ? My old BSA 650 twin when i bought it, had bores well worn out, but the bike ran really well and didn't burn oil, no blue smoke.

 

[gordon stephenson]

I have measured lots oj bores to close tolerances and never did like telescopic gauges. you get a much better touch and therefore accuracy using an inside micrometer. Don't believe what the inside mic says but transfer the size to a calibrated outside mic. As Brightspark says if you have to set the telescopic then move it around slightly to get a good touch making sure that the telescopic level in the bore and just touching diametrically opposite each other, Then just repeat this in several places to check for ovality.
Bowers bore gauges are extremely accurate and can be extended to measure long cylinders etc but not many people will have them.

 

[luccass_j]

I dont know but i bought it really cheap and the top piston rings were stuck so it is a run down engine the valves were covered with carbon as well its all clean now

 

[luccass_j]

I dont know if it was burning oil thats what i mean...while using telescopic gauges i do try to see if they touch opposite sides but ill try to rock them a little bit

 

[ronan]

Have you priced pistons and reboring ? It might just be cheaper to buy a second hand engine. Chances are if the valves were covered in carbon and the bores are knackered, then the engine bearings have seen better days too.