Topic: Cb750 Block/Barrel Question

[BenelliSEI]

A local shop asked me if I have pistons/barrel (cylinder block) set for a cb750 they are working on. All I have are the stripped down bits from a cb750A. Are they the same as all the other cb750’s?

[Don R]

 Yes, they're 392 like an early F

[newday777]

What year/model 750 John?

I just looked at the fiches.....

The A, K3-K8 and F0/F1 cylinders are the same.

Piston part numbers are 393 on the A, which is different than the K and F0, F1, both list 300 pistons. I don't know what is different about them.

The A rings are 392, same as the K7/K8 which use 392 rings. F0, F1 use 300 rings(same as up to K6). I'm not sure what the difference is in the rings either. I haven't compared any of them fiscally to each other.

[bryanj]

My aged memory tells me the later compression rings were thinner

[BenelliSEI]

Gentlemen…… Much appreciated. Turns out the shop has a new new set of oversize pistons and rings and needs a “block” to bore. Seems this will do and at least a few bits from the cb750A engine will live again!

[newday777]

Gentlemen…… Much appreciated. Turns out the shop has a new new set of oversize pistons and rings and needs a “block” to bore. Seems this will do and at least a few bits from the cb750A engine will live again!

👍

[newday777]

My aged memory tells me the later compression rings were thinner

Can you narrow that down a bit? On the A or???

[Jerry Rxman Griffin aka MuthaF'er]

One of the rings on the earlier set was 1.5mm vs 1.2mm on the later set

The pistons have different compression. Even the -392 come with 2 different crowns while both are labeled 392 on the skirts. My K8 pistons, 392, do not have a "full" crown whereas my 392 from my F0 have a full crown. I don't know the proper descriptive but my K8 pistons have a small depressed outer ring around the circumference whereas the F0 crown is full across the entire diameter.

I have never broken an A apart so I can't really speak of that from firsthand knowledge like I have above. I do know it is loosely based on the 392 engine just as the K7/K8 are. 

[newday777]

One of the rings on the earlier set was 1.5mm vs 1.2mm on the later set

The pistons have different compression. Even the -392 come with 2 different crowns while both are labeled 392 on the skirts. My K8 pistons, 392, do not have a "full" crown whereas my 392 from my F0 have a full crown. I don't know the proper descriptive but my K8 pistons have a small depressed outer ring around the circumference whereas the F0 crown is full across the entire diameter.

I have never broken an A apart so I can't really speak of that from firsthand knowledge like I have above. I do know it is loosely based on the 392 engine just as the K7/K8 are.

Can you narrow down earlier vs later?

[Jerry Rxman Griffin aka MuthaF'er]

I "believe" the few late 76 F and then the 77/78 F and K started using the 1.2mm thickness compression rings while all the previous rings were 1.5, 1.5, 2.5mm ? Sorry, I am >3000 miles from my reference materials. I am confident on the dimensions though. I think that is where you would find the different part numbers.   

Measure the ring lands/ring thickness and order based on that if you do not know exactly which pistons you have.

[newday777]

Thanks Jerry 😊

[scottly]

My K8 pistons, 392, do not have a "full" crown whereas my 392 from my F0 have a full crown. I don't know the proper descriptive but my K8 pistons have a small depressed outer ring around the circumference whereas the F0 crown is full across the entire diameter.

K7 392 pistons:

[Jerry Rxman Griffin aka MuthaF'er]

That is what I meant Scottly. Same as my K8 engine. Do you have a picture of the "other" 392 full dome piston also?

[scottly]

Sorry Jerry, I don't have a pic of the "other" 392 piston. Care to make a guess about the compression ratio of my K1 head 836 with TRW pistons?

[bryanj]

Er high?