Piston weights and balancing

Afternoon Gents, and ladies if there are any about.

I have a 71 750 roadster pulled down at the moment. The measured piston to cylinder gap is out at 6 thou so i am going to do a re-bore and fit 40 thou o/s pistons. Ultimately this will affect the balance factor but by how much and is it worth doing the balancing act again. The old Hepolite pistons are the ones with the "comma" oil groove and the new ones are Hepolite but with the holes instead. Understand there will be a weight difference due to the comma area and the O/S but is it worth worrying?

Common sense tells me to just get it done anyway.
Thoughts?

Regs

Is 6 thou all that big a clearance?

If the bores are good, and if the 6 thou clearance has been arrived at by proper measuring by a proper shop, personally, I would not rebore it. It will run just fine. And you’ll have a nice safety margin against seizure.

Remember that the sealing is really done by the rings, not the pistons, so ensuring good rings and correct assembly and correct ring gaps is more important than your 6 thou.

Regarding weight, +.020 pistons are unlikely to be ‘much’ heavier than std sizes. Re balancing the crank, especially on an iso Cdo, is not necessary (you’ll most likely still be well within factory balance tolerance limits / margin of error anyway, so you’ve as much chance of it being better as worse)!

If you want to do anything, just ensure both new pistons weigh the same by removing weight from the heaviest until it matches the ligntest. Using a small dremel type tool, you can safely remove metal from the inside of the piston skirt area.

The pistons should be closely matched anyway, if you’ve bought new ones that aren’t, send ‘em back with a stern explanation of why and request for better replacements!

Thanks Fast Eddie,

The other reason i wanted to change the pistons out was as i thought the perception around the "long oil slot" and later type 'comma' oil cutout pistons was that they are prone to breaking at the top. And seeing this isn't my engine i don't want to take any risks.....
Also, i thought acceptable limits for piston to barrel were between .0035 to .005.
.006 seems a tad loose and could piston slap a little on startup.
Correct me if I'm wrong of course

Cheers for the replies

Yes, I also understand those pistons are weak, so fitting new ones is a good idea IMHO.

Yes, .006 is a tad loose, but my point is that I really don’t think that extra .001 inch will even be noticeable, and it’ll give you many thousand of miles good service.

If you’re building a top end racer, or Bonneville bike, then I might say different. But if you’re building a bike for occasional use and fun, I say you’ll be just fine with your slightly sloppy .006.

Cool, so fitting new pistons which are 20 thou bigger than the ones in there currently so 40 thou. Im going to measure the new pistons and make sure they are the same weight and not worry about the balance factor.

Thanks for the advice Fast Eddie

Connie73 said:

Cool, so fitting new pistons which are 20 thou bigger than the ones in there currently so 40 thou. I'm going to measure the new pistons and make sure they are the same weight and not worry about the balance factor.

Thanks for the advice Fast Eddie

Click to expand...

Listen to Eddie about not re-boring. I would suggest you obtain another [updated design] +0.020" piston set & check to see what the bore clearance is with new pistons. If you go to a "high volume" [hey, I can dream, can't I?] seller, have them check their stock and give you the set with the largest O.D. Once you bore those cylinders out to +0.040", your friend/customer has only 1 re-bore left. Have you priced new cylinders lately?

Connie73 said:

Afternoon Gents, and ladies if there are any about.

I have a 71 750 roadster pulled down at the moment. The measured piston to cylinder gap is out at 6 thou so i am going to do a re-bore and fit 40 thou o/s pistons. Ultimately this will affect the balance factor but by how much and is it worth doing the balancing act again. The old Hepolite pistons are the ones with the "comma" oil groove and the new ones are Hepolite but with the holes instead. Understand there will be a weight difference due to the comma area and the O/S but is it worth worrying?

Common sense tells me to just get it done anyway.
Thoughts?

Regs

Click to expand...

I suggest you read this thread before having those cylinders bored. See

https://www.accessnorton.com/NortonCommando/jcc-std-piston-size-for-750.24607/

Almost forgot, make sure you weigh each piston assembly [wrist pins, circlips & rings] & lighten the heavy one.