750 cylinder bore

Just getting ready to visit a harely machine shop to see if my 1970 commando 750 cylinders are still usable. He says he can do it but needs specs like any weard proceedures like taper ect. Right now it is .020 over and one cylinder is pitted so not shure if it will machine out. I dont have new pistons yet. I want to at least provide him with specs. Not shure if I should just buy the .040 over pistons and take a chance. I just want a good running stock performance. Also what piston brand is good. old pistons measure 2.879 to 2.890 depending where I put the caliper and the bores are 2.894. Thanks in advance

Peter Restaino said:

Just getting ready to visit a harely machine shop to see if my 1970 commando 750 cylinders are still usable. He says he can do it but needs specs like any weard proceedures like taper ect. Right now it is .020 over and one cylinder is pitted so not shure if it will machine out. I dont have new pistons yet. I want to at least provide him with specs. Not shure if I should just buy the .040 over pistons and take a chance. I just want a good running stock performance. Also what piston brand is good. old pistons measure 2.879 to 2.890 depending where I put the caliper and the bores are 2.894. Thanks in advance

Click to expand...

Any qualified machinist can look at the pitting and recommend over size needed to clean up.
And, yes, then buy pistons & rings size he recommends. He'll need them to proceed with work.

ok thanks so just dont buy .040 pistons until he checks it.

Peter Restaino said:

ok thanks so just dont buy .040 pistons until he checks it.

Click to expand...

Correct.

ok

Once he's sure +040 will work, order the pistons. Do not let him bore without them. I specify .004" clearance. There is nothing special, just a straight bore and hone job. You may get others saying tighter or looser. I use Hepolite piston and ring sets and .004" works well. (JCC Pistons, Federal Mogul Rings).

BTW, the manuals give "inch" measurements and close but not always accurate mm references (when they give them) and suppliers are often worse - make sure he measures in inches.



So, there was a large range of bores in factory cylinders: 2.8746-2.8754 (72.01484mm - 72.03516mm) Which is often listed as 73mm pistons.

Ok thank you Greg will do and if say it needs .050 or .060 should I try it.

Peter Restaino said:

Ok thank you Greg will do and if say it needs .050 or .060 should I try it.

Click to expand...

AFAIK, .050 is not available unless JS can have them made. In the old days many more options existed but not today. It's best if you can do +040. +060 works, but start to makes thin cylinder skirts.

ok lets hope .040