Topic: Top end job on my '75 CB750

[Ira]

I measured the pistons at the skirt and they are about 65mm, which I believe means the engine is bored to 850cc?  Is that correct?

[70CB750]

836 says Satanic Mechanic:

http://www.satanicmechanic.org/bigbore.shtml

Check out the piston gallery.

[Ira]

Thanks!  So i guess I should mic the cam and see if the lift/duration is different, and if not, start shopping.

[Ira]

Kinda slow going yesterday.  I removed all of the valves and put them in numbered containers. I don't have a valve compressor so I made due with hand tools and got them out smoothly.  I didn't feel any play in the guides. The valves are heavily caked in carbon (didn't get a great picture) so I need to figure out how to clean them, I guess? Should this be a priority, as long as the face is in good shape?



When I pulled the valves out, I noticed there were only four seals total.  These were on the intake side.  The exhaust side had no seals, which doesn't make sense to me or match the fiche. Is there a reason someone would intentionally assemble the head this way?

[Ira]

I just searched the forum and it sounds like the lack of exhaust valve seals is normal on the earlier heads. Does this head predate my '75 engine?

The cam lobes measured 35 x 28 but I still haven't done the math.  How in the hell do I figure out which cam I have?

[Elan]

aren't those 76'-78' carb insulators?

[Ira]

aren't those 76'-78' carb insulators?

Yes, I have a '77 bike with a '75 engine.  They are a tight fit.  I'm probably going to replace them when I re-install the engine.

[bjbuchanan]

You have an early style head, they aren't machined for exhaust seals. My k2ish engine doesn't have them, looks the same as yours. My confirmed k6 has both seals.

If guides/valves/seats are good, just a generally good head then it doesn't really matter as much

[Ira]

Valves look like this.  Is the exhaust toast?

[70CB750]

You have to measure it to know.

[Ira]

Measured the bores for taper and round.  They are within spec (less than .05mm) of 65mm.

[edwardmorris]

Nice detail on the pix, looking forward to cracking open mine very soon here. I have a running engine but the bike is unridable at this time. Too many things wrong with it so I'm just going to take it apart and rebuild step by step. Detailed pix on these forums are what I'm relying on , hopefully will be enough once the Hondaman book arrives....

[Ira]

It's been a while since I got anything done with this.  Last night, however, I scrubbed nearly all the carbon off of the head, valves, and pistons.  I may have mentioned this before, but it looks like something hard was bouncing around in one of the combustion chambers...



Looks ugly, but should I be concerned with the odd texture of the piston dome?

Valves soaking in oven cleaner.  This took most of the chunky stuff off in 45min or so.  I might give them one more bath.



I used oven cleaner on the piston domes and head as well.  I read up on this quite a bit before trying it, and the consensus is that it doesn't hurt the alloy parts as long as they don't soak for hours.  I soaked them for around 5-10 min at a time and scrubbed with a soft metal brush.



And ended up with this:

[Ira]

Before/after:



I'll take some pictures of the valves when they are nice and clean.  I'll need to measure them as well, but then I should be close to the point where I start putting stuff back together!

[SF]

might want to take off the carb boots off when using that stuff

[Ira]

the boots are junk (like, rigid plastic), but yes, good point.