Topic: cb750f bottom end 76 vs 78 which is better??

[Hockers Choppers]

I have a 1978 cb750f that I have all apart and was going to big bore,cam it this winter. I bought a 76 f engine off ebay and it was "supposed" to have a webcam 63a, wisco big bore, spring & retainers, new chain, etc. I just got it and tore it down and found stock pistons in it!?!?!?! It does have the cam,springs and retainers so I guess all is not lost. My question is which bottom end should I use? The 78 bottom end had larger rod bolts but is anything else better? Trannys the same? Anyone know of a good place to get a piston kit, valve guides, gasket kit, timing chain and head studs? Any other tricks now that its all apart? Thanks for any advise.

[Geeto67]

I have a 1978 cb750f that I have all apart and was going to big bore,cam it this winter. I bought a 76 f engine off ebay and it was "supposed" to have a webcam 63a, wisco big bore, spring & retainers, new chain, etc. I just got it and tore it down and found stock pistons in it!?!?!?! It does have the cam,springs and retainers so I guess all is not lost. My question is which bottom end should I use? The 78 bottom end had larger rod bolts but is anything else better? Trannys the same? Anyone know of a good place to get a piston kit, valve guides, gasket kit, timing chain and head studs? Any other tricks now that its all apart? Thanks for any advise.

The countershaft is sturdie on the 78r too and you can run a wider spaced sprocket. However the advantage with the 78 F is the pistons and head which are different than the 76F. If the 76 F motor is complete and a runner - just run it like it is, otherwise I would put the 78 back together and run that instead. If you go big bore you are going to need a K head anyway.

[jtb]

Hi Jesse,
If you're gonna put an 836 kit in it, you may want to use the 78 bottom end, and the jugs and head from the 76.  the flat pistons from the 386 kit will create a lower CR with the 78F head.   I think that's the case, Mike Rieck or Sean Condon can tell for sure.

John

[Hockers Choppers]

OK, the 76 engine has the heavy springs, retainers and the hot cam. I'm just asking if I should use the 76 or the 78 bottom end with the 76 head? I will definatly use the 76 head since it has all the goodies. I'm just wondering which bottom end would be better with the big bore kit, which I don't have yet, anyone know of a good place to purchase?

[scondon]

  Wiseco raised the price of their piston kit this year to something like $512. Dynoman in Texas lists it for $350. You can get the rest of the stuff you need from them as well.

http://www.dynoman.net

   As far as which bottom end to use I'd go with the one that has less wear and tear. What little the '78 might have over the '76 is meaningless if the PO never changed the oil or adjusted the cam chain. I like the '78 bottom 'cause a lot of these didn't see more than 30,000 miles before the valve guides burned  out so some of them are in pretty good shape despite their age.

[Jerry Rxman Griffin aka MuthaF'er]

The trannys have the same ratios according to my service manual. The 78F rods are stronger. All things being equal I'd probably use the 78 due to stronger rods. I think you'll need to use the 76 jugs, pistons, and head. The cam, etc sounds good.

[Terry in Australia]

Agreed. The 78 bottom end with the stronger rods and beefed up clutch etc, mated to the 76 top end is the way to go. Cheers, Terry.

[Hockers Choppers]

Thanks guys,
I was hoping you'd tell me to use the 78 since it is the original engine for the bike and only has 13,000 miles and the ebay one is unknown.

[MRieck]

 I've heard the 78F crank is supposed to be stronger though I'm not sure why or how. Possibly a better radius on the crank pins where the meet the counterweights and better material?? Anyway The 76CB crank is plenty strong....that's for sure.

[joeb]

What sleeves are  you going to use the 78 sleeves if so you may want to machine the oil return holes to open up the closed off holes on the bottom to get better oil flow if you use an older 750k head.  Hope I explained this right.

[Hockers Choppers]

I'll probably just use the 76 cylinder. I heard that something is different with the oiling but if you use both the head and cylinder it is ok?? I haven't tore down my 78 engine yet so I'm not shure.
Anyone know of a cheap set of pistons for sale?

[joeb]

If you are looking for a set of 75 over I think , at least thats what they said at rice paddy when I purchased them I have a set the only thing is they have  one piece oiling rings.   

[Terry in Australia]

Yeah mate, use the F cylinders with the 76 F head, don't use the F2 cylinders with the 76 F head or vice-versa, I did, now I have to pull it apart and start again. You can use any K pistons, or F/F1 pistons if you're gonna use the 76 F head. Cheers, Terry.