Piston to bore clearance... seems tight...

[handyman]

Hmm, just been trying to accurately measure the cylinder bore after machining and the size of the piston skirt. It's not as easy as it looks on YouTube! 

Since the piston is slightly tapered and the book says measure 5mm up from the bottom of the skirt I was getting slightly different readings every time. Same in the bore, the T bore gauges are tricky to get exactly right, but I managed in the end (with a little tilt and side to side wiggle to find the exact centre before levelling up) and I now get consistent readings (within 3 or 4 thousands of a mm). 

So, now I've got some good-ish readings I've taken one from the other and I'm left with 0.013 (or about half a thou' in old money). 
Specs in the book say "standard is 0.010mm to 0.040mm with a service limit of 0.10mm" (0.4 to about 4 thou').

 

So, my question for any engineers out there is, I'm very close to the lower/tighter limit, would you hone a tad more out to bring the measurements closer to the mid point in the specs, or leave it as it is? 

 

Ta muchly, and incidentally, the ring gaps are pretty much in the middle of the specs as it is... 
Cheers

Ian 

p.s. the lubed piston slides to the bottom of the cylinder a treat if that means anything lol! 

 

 

Edited May 13, 2022 by handyman

[Old-codger]

There is a very good article in this  Mays Classic Bike Guide about this very subject.

[Stu]

If its within spec then it should be good to go

[Bianco2564]

The machine shop that did the boring and honing should have done it to achieve the correct clearance?

[handyman]

4 hours ago, Bianco2564 said:

The machine shop that did the boring and honing should have done it to achieve the correct clearance?

Kind of. There are not many guys doing this kind of work around here and he wasn't the more clued up guy I don't think... 

I was just hoping to find out what an engineer would do when given a range to fall into. Do you aim for the lower, middle etc.

I hate it when this happens, I get this in construction a lot too. Why not just give an exact figure with maybe a plus or minus tolerance either side?

[TimR]

And to add more oh fuckery ....

 

Remember your body temperature can affect the readings ...

 

Holding the bore gauge or micrometer in your hand  for too long can cause fractional expansion ...

 

Which dealing with mm's is fine but thousands of an inch or sub millimetre it can really do your head in ... so consistent temperature and remeasure .

[handyman]

22 minutes ago, TimR said:

And to add more oh fuckery ....

 

Remember your body temperature can affect the readings ...

Lol! That's definitely me then, I must have measured a hundred times  

I could not get the same reading each time, always different by a few thousandths of a mm. Especially on the tapered piston  

I wish I could have afforded a dial bore gauge, way better than measuring the T gauges with the micrometer...

Hey ho, happy weekend all

[billy sugger]

Throughout my machining career I always aimed for the middle tolerance on anything I made. Not only was that the size things were meant to be, it gave you leeway if there was any play in the machining equipment and allowed for wear on the cutting /forming tools. 

Anyway, on topic, have you got a digital vernier to hand? A 12" one should have Jaws big enough to measure the skirt, though it is very difficult to get an accurate reading on a taper. My suggestion, get a metal plate and cut a hole in it to the central tolerance and put the piston in it and measure how far it has dropped in the hole. If the hole is cut to the mid limit and the piston drops in 5mm,or whatever the measurement point should be then the piston will be fine

[handyman]

1 hour ago, billy sugger said:

Throughout my machining career I always aimed for the middle tolerance...

That's my thought also. Hence being a little nervous that the lower limit is 0.010mm and I have, to the best of my measuring capability, 0.013mm. The piston is tiny, only 52mm and some change across...

I have the measuring tools I posted in the pic. so I'm quite confident in the numbers. It is just deciding whether I should use a cylinder hone to eke out a little more and head away from that bottom limit into the mid ground somewhere around 0.025...  (0.010mm to 0.040mm is the 'standard range').

Edited May 14, 2022 by handyman

[billy sugger]

Have you tried checking the ring gap with them in the barrel as it is now? If that is correct I would leave it be

[handyman]

37 minutes ago, billy sugger said:

Have you tried checking the ring gap with them in the barrel as it is now? If that is correct I would leave it be

Yes, that is mostly the reason for leaving it. They are about mid way, but I'll measure them all again later and write them down. 

One guy said that although it seems a little clearance (0.01mm), to not forget that it is not just the piston which expands in use, but the cylinder expands too, so the clearances can be tight as they will expand and contract somewhat together...

Thanks for chipping in  

[Bianco2564]

5 hours ago, handyman said:

One guy said that although it seems a little clearance (0.01mm), to not forget that it is not just the piston which expands in use, but the cylinder expands too, so the clearances can be tight as they will expand and contract somewhat together...

True but at different rates.

A piston will expand roughly twice as much as a cast iron liner and the piston will run hotter.

 

If you are confident with your measuring, then it's in tolerance and should be ok.

Can you find someone with a bore gauge like these?

https://www.amazon.co.uk/Bore-Gauges/b?ie=UTF8&node=6286320031

How accurate is your micrometer?

You should check it against a setting gauge or some slip gauges.

[handyman]

1 hour ago, Bianco2564 said:

How accurate is your micrometer?

Thanks for that. Locally I could only find the telescoping type bore gauges. They and the micrometer are new (bought specially for this job), it came with a 50mm setting bar and it looks spot on. 

 

I'm going to triple check it all again tomorrow, including measuring all the ring gaps. 

Hopefully then I can put this to bed, and just put the engine back together  

[handyman]

Thanks for the input guys. Did some more practicing with the Telescopic bore gauges and micrometer today. Turns out you need quite a light touch with the bore gauges to get a true reading, it's easy to measure short. I found it's best to hold it at a little angle, lightly nip the lock, find the widest point with a little side to side 'shimmy' and rotate until perfectly level. Nip the lock a tad more and lift out at an angle again.

This gave me slight higher readings than I had before, and now I'm very happy with the clearance as it's gone up from 0.013mm to a better 0.023mm, bang in the middle of the specs. 

 

Ring gaps all good to go too. It's my birthday tomorrow and since I rarely work on by birthday, I can't think of a better way to get the birthday started than to have a go at getting the engine back together

Cheers all. 

 

p.s here are the specs from the manual if anyone else is reading this to build a CB125F engine...

 

Edited May 15, 2022 by handyman