Topic: New Pistons - K1 cases, F1 head, 392 pistons - what the?

[nancy]

Guys - I have the CB750 motor out - 1st the camchain was getting bad. While out - I found that the K1 engine (s/n: 111xxxx) actually has a head stamped 392 and pistons stamped 392. I gather these are F1 generation - is that correct? Going back 2or 3 owners - back to 1999 this was a K2 bike and someone reconditioned the motor..and that's all the histroy I can gleen from the PO I bought it from 3 years ago. My guess is, at that time they pulled the K2 motor and refitted a different motor - K1 bottom - hence the s/n staring 111... but put an F1 head and pistons. Don't know how to determine what the cylinder block generation is - if that matters.
Being it is F1, with 392 head...am I limited to buying only 392 piston/ring sets..or can I refit original K1-k6 type kit....without issues with the head clearance?
I've looked at kits on Z1 and they seem to offer one set - to cover both the 300 and 392 sets.
Appreciate comments.
regards
Mark

[Don R]

The F1 parts would be considered an upgrade. I'm thinking the parts are basicly interchangeable. K7 is very much like F1, The big F changes started with F2.

[Jerry Rxman Griffin aka MuthaF'er]

F or F1. measure the ring lands. You will probably use those pistons with -300 rings. 1.5, 1.5, 2.5mm. Keep the 392 pistons, a little higher CR. The head is better too due to the combustion chamber and psitons.

[nancy]

Thanks guys.
Well, Jerry, I hope I can keep and use these 392 pistons...but I was assuming that they may be US - out of spec. Or if I need to rebore and go oversize - will need to buy new pistons.
Am taking it all into machinist on Tuesday for internal boreoctomy - measurements of piston to bore etc.
Are u suggesting retaining somewhat (maybe 50-60% at a pure guess) worn 392 pistons and fitting just new 300 rings? Pistons are genuine and maybe old original HONDA parts, but I cannot see any markings on any of the rings..so assume perhaps rings were replaced back in 1999.
The pistons lands measurements - are u are suggesting getting the lands/grooves resized for 300 rings? Or are they same as 300?  I tried measuring with digital caliper - they seem to be Top=1.40mm to 1.45mm, mid= 1.28 and bottom = 2.5mm to 2.8mm...variances across the 4 pistons.
Edit: Rechecked with feeler gauges - top and middle lands - seem to be 1.2mm maybe 1.25 at most.
Dunno why the results differ from top to middle lands - thought they were the same - did it 3-4 times over and get differnent readings with the caliper - so I'll wait till I see the machinist.
Cheers
Mark