Topic: Top end job on my '75 CB750

[Ira]

Hey guys, I'm starting this thread to document the process of sorting out my engine.  I got it as a replacement two years ago with unknown miles. At that time I tested compression (it was acceptable, though I don't recall the numbers now), dropped it in the frame, and rode it for a season.  I believe I put 2k miles on it last summer, but it began leaking oil badly from the front (cam tower pucks?) and there was a little bit of white smoke in the exhaust on acceleration.  It's sat all of this summer because I recently completed an 81 Suzuki GS550 to ride around town, but I'm really missing the big Honda!

Last night I pulled the engine, with the help of two strong buddies.  It's resting in a homemade work stand, which I believe is the work of Big Bob (of this forum and ChiVinMoto fame).

   

I'm going to post about what I'm doing and hopefully get some pointers in the right direction. 

[SOHC4 Cafe Racer Fan]

I'm always interested in seeing detailed engine builds, probably because I am not adept enough to undertake one alone.  Are you planning on simply refreshing it, or modifying it for improved performance (big bore kit, port and polish head)?

[Ira]

At this point, the minimum goal is to keep the oil in   However, I won't know how much needs to happen until I crack it open and measure everything.  I don't have a large budget, but I'm willing to invest some time and do things right.  At a minimum I'll be replacing all of the top end seals and gaskets.  I'm hoping to not do this again for a few years, so I want to hone the cylinders and replace the rings while I'm in there.  Who knows what else I'll find?

[BCBarker]

watching this with great interest as my 77 is leaking from the top end too and I've finally decided to just fix it this winter. post away!

[evanphi]

Also watching with interest. Same motor, same year. I have a sneaky suspicion that maybe my oil pucks are bad, or a bad head gasket seal. They can look the same (oil on the top fins), but nice pics of a teardown will be helpful!

[Ira]

Thanks guys.  Now that the motor is out, it should take another evening to pull the top end apart.  I only have one bike night per week, so bear with me...

[Ira]

I started on the tear-down last night.  This is what the valve cover and hardware looked like.



I freed nearly everything with an impact driver.  The breather cover screws were gnarly beyond belief (covered in rust and filth) and i had to cut slots in two with a dremel.  All of the rest came out fine.

[Ira]

The gasket peeled off in one piece.

[Ira]

I've removed a valve train before, so this was quick.  I was alarmed to find two of the cam holder bolts were finger tight, and one came out with either stripped threads or part of a helicoil on it.  It appears I'm not the first to have my grubby hands inside this case



I stored all of the hardware in a sheet of cardboard (a trick I read about here) with grid drawn on it.  I reassembled the valve train on my bench to make sure all of the bearing surfaces stay correctly matched.



I wrapped a piece of wire around the cam chain to keep it in place.



 

[Ira]

The cam tower pucks, as expected, were brittle plastic.  There's one hanging out on the far left.



You can see my wired cam chain at the bottom.

All of the nuts came off the studs (pinggg!) and I stored them in a parts case.  Removed the 6 remaining bolts, all came out easily.  There was a lot of crud and gravel in all the recessed spots, but fortunately none in the case.

[Ira]

It took some persuasion to separate the head from the cylinders.  I put a block of wood against the head and tapped with a big hammer. Again, the gasket came off in one piece.





This valve was completely black.  The others were white.



There's a #$%*load of carbon on everything.

[Ira]

The next step is to disassemble the head, then clean and measure everything.



That's it for one night!

[evanphi]

bravo! Great pics. Really shows nice detail.

[SOHC4 Cafe Racer Fan]

Thanks for the detail. Great progress for one night!

[Ira]

I took a close look at the pistons in my engine.  They're not OEM, but I'm not sure what they are.  They are domed and say "AP" on the side.  Any ideas?



I haven't measured the bore yet.

[Ira]

I also noticed someone attempted to port the head.

[70CB750]

I believe - and I was wrong before - that those are late pistons, such as 77/78K or maybe even 78 F pistons.  My F3 pistons are domed and have a cut out for valves.

Hopefully somebody will pitch in with more info.

[Ira]

They are not '77K pistons, I know that much.  They also don't have "Honda" cast into them.

[70CB750]

Here are 78F pistons.

[Ira]

Yeah, they're something else. The valve cutouts are different, and they don't say Honda.

[MRieck]

Something was going on in that #4 cylinder judging by the piston crown and outer area of the chamber.

[SF]

interested

[BPellerine]

is it stock bore .bill

[Greggo]

Subscribed!

[Ira]

is it stock bore .bill

i haven't measured the bore yet.  doing that this weekend.