Topic: CR750 resurrection

[davomoto]

Hello all! Been a member here for a while. I’ve been restoring and modifying vintage Hondas for 25 + years. My main focus was the 60s CB and CL77s. Last year came into some CB750 stuff including the bike pictured. It was torn down to the bare frame and I’ve been slowly reassembling it from the pictures. It has a title and it matches the engine I’m tearing down. There is a lot of carbon build up in the exhaust ports, and lots of oil down the front of the cylinder. It was stored without spark plugs and no covers on the ports. I could see a small amount of dirt and debris in the ports and figure a proper cleaning and seeing what’s in the engine is in order. So far only the top end has been removed. Bore is 70mm. The copper head gasket is stamped RC ENG. bores look good will re ring it for sure just have to figure out where to get rings to fit those pistons. Speaking of which, anyone recognize what brand they are? Rods appear to be Carillo but still learning the CB750 hot rod parts. Any advice, suggestions etc more than welcome! This bike was raced out of a shop called Vanguard in San Rafael Ca that closed its door many years ago.

[davomoto]

Pistons and pic of the bike years ago

[PeWe]

If ring end gaps are good in bores I should not change the rings.
Pistons look good. Not much visible wear.

How does the head look like?
Valve size, chambers and ports.

I guess the chambers are made wider for the piston domes.

Which cam?

[davomoto]

Still learning what is stock or oversized on these engines. Obviously big overbore. Pics of valves

[davomoto]

Head pics

[davomoto]

Pics of cam   Also has K134 stamped in and J 8

[davomoto]

And where it’s at currently.

[PeWe]

No stamps on cam ends or sprocket ear area?

It looks like titanium spring retainers

[davomoto]

YEP, on cam end 25X1

[newday777]

Nice looking project! Kimtab Snow flake wheels, rear disc(F model?).
Valves appear to be stock size, 28mm exhaust, 32mm intake.

[Jerry Rxman Griffin aka MuthaF'er]

Not F0 - F3 rear disc setup

[davomoto]

Not F0 - F3 rear disc setup

What is F3 setup? It appears to be all aftermarket parts to me?

[davomoto]

Nice looking project! Kimtab Snow flake wheels, rear disc(F model?).
Valves appear to be stock size, 28mm exhaust, 32mm intake.

Thanks for the info Stu!! Any opinion on weather or not those Kimtabs are safe to ride?

[newday777]

Nice looking project! Kimtab Snow flake wheels, rear disc(F model?).
Valves appear to be stock size, 28mm exhaust, 32mm intake.

Thanks for the info Stu!! Any opinion on weather or not those Kimtabs are safe to ride?

As far as I know I haven't heard of any problems with them, they are highly sought after.

[newday777]

Not F0 - F3 rear disc setup

What is F3 setup? It appears to be all aftermarket parts to me?

F3 is the 1978 version of the Supersport F model.
I bought this 78 F3 a couple years ago

[davomoto]

Got it, Thanks. I believe Kimtab sold rear disc brake kits but usually with Hurst Airheart calipers.

Any ideas on piston and rods brand?

[newday777]

Sre there any markings in the piston pin relief area? If not underneath the piston might have markings.
I'm not sure on the rods, can't see enough of them.

[MRieck]

Rods are Carrillo....early straight beam. Decent porting in regard to port volume, Manley valves, bronze guides, Ti retainers. Valves don't have much wear on the tips. More than likely Arias pistons. Aluminum wrist pin buttons. Measure the ring thickness...you may be able to get rings from Total Seal etc....maybe even Wiseco or MTC. Lots of duration in that cam that's why the valves are stock size.
Those cam tower center hold down bolt holes don't look very good thread wise. Smooth out the chipped area on the upper case and do the same to the other 3 areas. It gets thin with the boring for oversize sleeves and it will crack and you can lose chunks of metal as sen in the first pic.

[scottly]

Might also be RC pistons with the grooves on the skirt?

[davomoto]

It does have a copper head gasket stamped RC ENG. Thanks everyone for the info and insights!

[scottly]

What does it say on the rear caliper?

[MRieck]

It does have a copper head gasket stamped RC ENG. Thanks everyone for the info and insights!

The pistons were probably sold by RC which used Venolia and Arias. They used more Arias especially in the home stretch.

[davomoto]

Rear caliper is a Fontana

[davomoto]

Base gasket measures 1.2mm thick. Perhaps an attempt to lower compression? Any idea where to get a replacement gasket that thick for the 70mm bore? This bike will see little use, definitely won't be raced by current guardian anyway.

[PeWe]

You need to verify how it looks like with piston at TDC. How much it will stick up. A quick look with one piston, no base gasket.

I got a set of 70mm JE 12.5:1 cheap.
A pre-owner did not get it right. Too much dome, high cr.

The domes had enough thickness to mill some off, 2-3mm. Your pistons might not have that thick domes.

Dynoman has RCS base gaskets. But only 0.01" and 0.02" thick.
MLS head gasket 0.03" (good price) and 0.04" plus a thicker fiber gasket.

The oil leak might be either from oil feed at base gasket if no o-ring sealing it, or oil returns.

Or the bored stock cylinder where bore will go thru the aluminium between 1-2 and/or 3-4. It will weep oil if not sealed.

If head gasket is not sealing the oil feed/return holes. Oil will enter the chambers  when piston goes down plus it will weep oil when going up.

The sleeves need to be sealed.
Here my 70mm cylinder. O-rings pressed in before the greay seal.


Thin RCS gasket I have reused several times.
Used with a billet block with 78mm "sleeves" OD.
Very thin layer of RTV on both sides, let it cure for 30 minutes before  assembly.