Topic: Bottom end rebuild questions

[zabzob]

I just took the engine out of my 75 750k with about 35,000 miles on it. It was awful and decided that I don't want to do this again for a long, long time. I had it out in 2015 to have the head rebuilt after a cam chain skip, but I stopped at the head because I didn't want to spend the money. It's still burning tonnes of oil. It's out now so I can do the rings and have the cylinders measured and honed. I am going to do everything I can so I won't have to do it again for hopefully many thousands of miles. So far I have thought to do:

-Primary chain
-Primary chain tensioner
-Cam chain tensioner/wheel (chain replaced in 2015)
-Check crankshaft and rod bearings, replace if necessary
-Measure cylinders, hone or bore if out of spec

Measuring and honing/boring will be done at a local machine shop, as I don't have the tools and would rather have it done properly.

Do I need to replace the piston pins? What about the pistons themselves, should they be measured? Anything else I'm missing?

[calj737]

Bores, rings and pistons all need to be measured with respect to each other. Ring gap and piston-bore clearance is a critical measurement. Once the machine shop has the bores measured, they will have the best guidance for other component replacements.

[millla03]

Going through the transmission wouldn't be a bad idea while you're in there. I followed Hondaman's book for guidance during inspection. Replaced all of the bearings in the transmission/final drive for good measure. Yamiya 750 out of Japan had all of the bearings for a good price, at least as of spring 2017 when I did the rebuild. All were in Honda OEM packaging or made in Japan NTN-brand, which I trust. They also had primary chains for best price I could find. The bushing on the gear that drives the oil pump is another good thing to check. Good time to go through the oil pump, too. A member here offers a nice rebuild kit for those.

It can be a slippery slope!

[pjlogue]

The ring/piston/bore clearance is different than automotive engines.  Be sure the shop you take it to understands this and has the information for proper fit and can do the work.

-P.

[zabzob]

@calj737 Okay, I'll bring the pistons to the shop as well and ask them to measure that too. For the piston rings, if the gap is out of spec, what is the solution to that?

[HondaMan]

Here's what the machine shop MUST know, because they "think" in terms of automotive engines, which are VERY different from these:

1. The piston clearance at the bottom of their skirts is NO MORE than 0.0008" new (that's not a typo), and considered significantly worn at 0.0022". Cars START at 0.0020" typically, so if they try to measure at car's specs, they will do it wrong. I see this A LOT, so it's up to you to force them to do it correctly, or they usually won't.
2. The rings MUST be replaced once you remove them from the cylinders. If you attempt to reinstall them, they will burn oil and will not seal.
3. The oil burning and wetness is likely due to worn valve guides. The K2 came 2 ways: some did not have seals on the exhaust guides, while some did. Replacement guides today should be from APE (#VG750H) or Dynoman (Kibblewhite), the latter being more race-bred parts and more expensive to install. The valve stem clearance is TIGHT at 0.0008"-0.0012" intake and 0.0012"-0.0014" exhaust, which is half of what cars use.
4. The valves likely need to be refaced (about $4-$5 per valve), and have the shop cut a 3-angle seat at 60/45/30 degrees, with a 1.0-1.5mm valve face seal in the end. The "valve margin" minimum is 1.0mm, and they need to know this: if they don't know what it means, find someone else.

Your best best is to buy the next size larger piston & rings kit and have the shop bore your cylinders for them. Tell them you want 0.0008" clearance, and if they balk, find someone else. Otherwise, it will burn oil again after it is all back together. You can get adequate piston kits from CruisinImage at eBay for about $125: I'd suggest using his 0.5mm oversize kit like this one:
https://www.ebay.com/itm/69-76-HONDA-CB750K-0-5mm-OVERSIZE-PISTONS-SET-4-PISTONS-INCLUDE-CI-CB750KPS-1/131588210647?hash=item1ea344bfd7:g:7ggAAOSwyQtV3EE6

as I just finished (another) engine using them, and they are fine.

[zabzob]

Hm... those valve clearances. I took the head to shop that does auto stuff, and maybe that's what is wrong. I will have to check them (or have them checked) before I button it all up again. If that was problem all along then I'm going to scream.

The machinist I'm looking at now is motorcycle specific, and I will come prepared with all the data about clearances.

[calj737]

Hm... those valve clearances. I took the head to shop that does auto stuff, and maybe that's what is wrong. I will have to check them (or have them checked) before I button it all up again. If that was problem all along then I'm going to scream.

The machinist I'm looking at now is motorcycle specific, and I will come prepared with all the data about clearances.

The shop I use, is extremely competent and does auto, moto, marine, and even some aero. These engines are not so darn particular that machining technology is unique to them. They will know how to measure bores and pistons, and based upon data in the Honda Service Manual, the decision to replace parts is easily made.

If you choose to replace pistons, have the cylinders bored to the piston's spec, with the pistons and rings in the hands of the machinist for accuracy. They should label each piston 1-4 for location installation. It's not Black Magic; its an internal combustion engine.

The head and deck should be milled flat though, irrespective of boring.

[zabzob]

Hm... I would hope that they would look up all the tolerances, but I didn't ask. I do think it is the rings as I get a lot of blue smoke on acceleration. My riding buddy couldn't stand to be behind me for more than a few minutes on my last trip.

[HondaMan]

Hm... I would hope that they would look up all the tolerances, but I didn't ask. I do think it is the rings as I get a lot of blue smoke on acceleration. My riding buddy couldn't stand to be behind me for more than a few minutes on my last trip.

If there isn't a broken ring, that comes from the intake valve guides.