Topic: K1 750 Restoration - Engine questions

[mwvachon]

Hi All,

Beginning to tackle the engine on my resto project. The top-end was rebuilt about six or eight thousand miles ago (but about 20 years ago!) Even then, the mechanic asked why I had him do it (about 28K showing on the odo at that time) - he reported all parts appeared to be in great shape. The main reason I had him do it was a slight oil weeping in the head. He replaced the cam rollers, checked the valves and put in the new top-end gaskets. Bike has always run great, no issues relative to the inner workings.

I scoured the forum posts for information on head gaskets & leaks - this one does have some slight weeping between the two cylinders on the point-side of the engine now. My question is relative to the oil weeping. I have no reason to believe there is a need to tear down the top-end, although I'm taking the valve cover off to polish it up. Wondering if I'll be all set with just re-torquing the head?

Ideally, I'd like to just clean up the cases, polish the covers, touch up the silver-painted areas, and just put it back together. I do need to inspect the clutch, had some issues with jerky starts in 1st gear just prior to starting the restoration. But other than that, I believe the motor is solid. Can anyone comment on this line of thinking? (please be nice  ) Thanks to all in advance for your usual great input...

[volz1fsu]

You can always try to re-torque a head to seal a gasket.  You MUST heat the engine to operating temperature to accomplish anything this way.  Many engines with aluminum heads recommend re-torquing the head bolts after warming up the engine to seal a new gasket.  It might work for you but then again the head may be warped and leaking because of it. Hopefully not in your case.

[Clyde]

It would certainly be worth re-torqueing the head once the engine is out, but you need to make sure where the leak is coming from.
I re-torqued a head and it still leaked as the rubber button seals under the cam boxes were leaking not the head gasket. A pain as you have to pull these engines out to do anything with the top end.
Regds Clyde

[volz1fsu]

Does the Engine really have to come out on a 750 to do top end work?  I can do head work on my 550 with the engine in the frame.

[bwaller]

It all depends on much you hate an oil leak. You will have to pull the engine to even retorque, and quite frankly that probably won't eliminate your leak. It's the o-rings between the cylinder & head that generally cause the problem, so if you absolutely have to cure the leak, remove the top end, clean all gasket surfaces very well (be careful not to gouge the aluminum) and install new head and cylinder base gaskets. I have read that people have tried teflon o-rings to replace the original as they might withstand the heat better, but someone else will have to let us know about that as I haven't used anything but what's in the kit. When it's apart, do a close inspection but it doesn't sound like you'll need to call the parts man for many replacements!  Good luck.

[Gordon]

Does the Engine really have to come out on a 750 to do top end work?

Short answer, yes.  Long answer, yeeeeeeessssss.

[bryanj]

Alloy heads are re-toqued COLD cast heads HOT and Honda dont specify a re-torque anyway BUT do make sure that the nuts spin free on the studs as any binding  will alter the torque

[Lumbee]

...it being such a pain to pull these motors I'd say spend $20 and just put a new gasket in it.  I imagine you won't be happy if you only retorque, get everything back together and you still have the leak on your freshly polished motor...   

[Clyde]

If the leak is clearly from the cam box rubber o-rings/buttons, I would not touch the head gasket, except for re-torqueing.
It is a quick job to take the cam towers out and replace the o-rings/buttons. (Once that engine is out of the frame that is)
The parts are not too expensive either.
Regds Clyde

[mwvachon]

Additional question - Anyone got a list of torque specs for the CB750's? I have 2 shop manuals and only the older one lists a torque spec here & there (they at least saw fit to print the head bolt pattern and torque). My Valkyrie shop manual provides torques for almost every single assembly!!!

I've decided to replace all the top-end gaskets. I tore off the valve cover to polish it and was surprised how brittle that gasket was. Then I realized that the last time the engine was opened up was 25 years ago! It's out of the frame and on the bench, so what the heck! As I examined the top end more closely, I could see the head gasket had been weeping in more than one spot. I just hated to do it because the bike ran flawlessly before the tear-down for this restoration. Placed my order with Parts-n-More a few moments ago.

Thanks to all who have or do respond! I'm so itching to start putting this back together!

[Ted Nomura]

Never reuse old head gaskets. Leaky head gaskets on early CB750 engines were common. All my CB750 suffered from this, even after careful 16,000 miles and it's such a pain to change it because you need to remove he engine first. It's hard to believe that early CB750s were more reliable than the BSA Rocket III/Triumph Trident 750s of those days but you have to give Honda a credit for maintaining strong customer service back then. They always fixed my Honda quickly without charge.

[Ted Nomura]

Does the Engine really have to come out on a 750 to do top end work?  I can do head work on my 550 with the engine in the frame.

Yes, that was a common early complaint. Honda redesigned for the CB500 Four (pre-CB550) so it doesn't have to, not to mention eliminating the dry sump feature. The best way, actually the only way, is to remove the CB750 engine from the right. Some guys even glind the front right engine frame mount down a bit to make the removal easier. I usually put a wooden block over the right side of the engine and just tip the whole bike that side to remove the engine. This prevents paint scratches on the frame. Of course the best way is to completely strip the frame and reassemble like the factory guys.

[lassenc]

A "stop-leak" additive worked out for me, no leaks anymore

Stopped use it now, and still no leaks

[mwvachon]

Well,

Questions have been answered! I pulled the head off the engine anyway. Since #1 - This is just about a full-on restoration project - and B: The only way to service the 750 SOHC is to pull it from the frame, I decided to go the safe route and check out everything that could be an issue here.

After pulling the head and closely inspecting I was able to determine that the head gasket did indeed have a few spots that were allowing a tiny bit of oil to escape. I also determined that the rubber disc seals that reside under the two cam holders were also weeping oil.

The bores have been inspected and measured (all in the 2.403" and under range and still round!) and the cylinder walls still have their cross-hatch markings (not sure if it has ever been honed since new).

Head appears to be straight and other than deposits on the valve faces, the valves appear to be fine. Took the head to the local machine shop anyway - just to verify all this. Plus they will install new valve seals and clean the entire assembly so I can refinish it.

Also replacing the clutch friction plates while I'm at it. They were within spec, but they are original (34 yrs old!), so thought it would be worth doing it now.

While waiting for parts, occupying time by polishing the valve cover and case covers... Now it will end up being both pretty & mechanically sound! Thanks to all for contributing to this thread...