Topic: Rebuilding bottom end ('71 CB750)

[mrosso]

I'm rebuilding my '71 CB750.

The bottom crankcase is in pretty bad shape. I have access to a '76 lower case. Can I use a '71 upper and a '76 lower case combination?

Should I replace the crank bearings (journal, plain, shell bearings)?

How the hell do you pry that bearing out of the countershaft holder (the thing that has 4 or 5 countersunk screws near the shift mechanism on the lower case)?

Thanks for all the help, guy, couldn't do this without you...

Mario

[rhinoracer]

I don't think you should mix n match case halves unless you plan to do a line bore job even though the halves have locating dowels for mating.

If you're talking about the bearing inside the casing with the four countersunk bolts it should slide out after removing the four bolts - I'm asuming you already removed the shifting mechanism.

[azuredesign]

I have also been told that mismatched case halves will not line up correctly in critical areas allowing seals to leak etc. Better to look for a pair of cases and work from there.
All best,
Ben

[750deepsouth]

I have a cracked upper case on my cb750 and found a replacement in perfect condition. I've also been told that not a good idea to replace just one half and I'd love to hear the espereiences of other owners.

Andy

[Terry in Australia]

Well I reckon the only way you'll find out is to try it mate, I've stripped and rebuilt one engine in the last month (K6) and am now cleaning up the two crankcase halves on #2, (K3) and I've studied just about every nook and cranny in each casting, and can't see how it'd be a real problem, apart from the obvious concern about main bearings.

Having said that, if you still have the bottom main bearings from the K6 lower crankcase and they're in good condition, then by using the good 71 upper bearings and the 71 crank, it should work fine? everything else should line up ok, the crank seal and countershaft seal are the same, so as long as you use the correct size dowels, and a smear of "Threebond 1104" on both the case halves, it shouldn't leak.

Make sure you install new primary chains from Honda and a new cam chain, (Z1 Enterprises are selling the best you can get, Tsubaki race cam chains for $28.60 right now, woohoo!) and one of Cycle Exchange's new $59.00 full gasket sets (excellent!) and your bike will live another day! Cheers, Terry.

[MRieck]

There is a nice set on ebay now for 24.99. Good seller too. I have seen mixed halves on a few newer GSXR's and a 600Fi that were roadraced. They worked OK (as far as I know) but I never saw the mains after a season. Anyway...I'd go with matched halves and not worry but that's me.

[mrosso]

Hey guys,

Thanks for the help and great advice. As luck would have it, I found the top half of the '76 bottom I bought off of ebay and bought it too through a buy it now. Still a bit confused, worried about the main bearing thing. I'm a real newbie at this. I'm tempted to buy new bearings, but I keep reading about how they can differ from on journal to the next.

[754]

UMMMmmm Has any one here looked at a manual, or changed bearings??

Cranks and Case SETS are graded... remember the word sets.

That is to say, the crank has a code marked on it that is the graded sizes of the journal, and the case has the same marking on regards to bearing bore sizes. Thes are marked under the ledge at the front of the case, above the oil filter area.

These codes used in combination are used to select proper bearing size from the chart in the manual. If the crank still falls within tolerance.

So you cannot mix and match... in some cases it could work, but in other cases it would never work.. so if you had a pile of bearings to use you may get by.. till next time..

You can do it right the first time.. or wish you had..

[Sweep]

One other small thing was there was a chain oiler in the earlier models that was not installed on my '76 but it probably wouldn't make a difference anyway.

[Terry in Australia]

Hey guys,

Thanks for the help and great advice. As luck would have it, I found the top half of the '76 bottom I bought off of ebay and bought it too through a buy it now. Still a bit confused, worried about the main bearing thing. I'm a real newbie at this. I'm tempted to buy new bearings, but I keep reading about how they can differ from on journal to the next.

Well done mate, as much as the mismatched halves might have worked in a pinch, I'd feel more confidant with a set.

Re: your main bearings, flip the top case over and near the front engine mount there is a 4 (I think) letter code stamped there, and another code stamped on one of your cranks counter weights. Take these codes to your Honda dealer, and he'll match you up with a new set of bearings. Cheers, Terry.