Topic: ‘76 CB750F, 17K miles, new rings?

[BuffaloBill]

Looking for advice:

I’m the second owner of this bike.  I bought it with 10K miles around 2010.  The engine had never been apart before this event.

Recently I thought I noticed an increase in top end noise, and a new oil leak appeared at the front of the engine by #2.  So I tore it apart a couple days ago.  The left side cam bearings, cam lobes, and rocker arms are toast.  I found a small metal shaving in the oil jet but not enough to totally block it.  The cam bearing near the cam chain is smeared, to the point that there’s no visible oiling hole.  Or was that hole missing from the factory?  I wouldn’t think that would be possible (?).  The rubber seal disk under the tower at that end, was heat damaged I think, accounting for the recent oil leak.

I just found and ordered from an eBay seller, cam towers, rocker arms & shafts, and camshaft from a 1976 F that apparently was parted out.  So am hoping those parts look good when they arrive, to fix the top end.

About the rings & cylinder condition:
I don’t have recent compression readings.  It was running quite well, but a bit down on power, for how much the cam lobes are worn.  I have not yet pulled the cylinder but looking at the cylinder bores (pistons still in place), I don’t spot anything bad looking.

Should I leave the bores & rings alone, just fix the top end?  Or should I hone (Lisle rigid hone) and use new rings on original pistons? 

I’ve read on here about the bores wearing egg shaped during the first XXXXX miles, and that the best fix is to bore for the first over size pistons, etc.  But I don’t have a good feel for at what mileage one would make that jump.  Would the egg-shape likely be a few tenths or more like a thou or two?  I understand clearance should ideally be less than a thou.

Thank you,
Bill

[bryanj]

Metal shaving must have come from somewhere after the oil filter so probably a good idea to pull at least the block and thouroughly clean all the oilways, then inspect and decide on rings which unless physicaly damaged will probably be ok

[scottly]

17K miles is barely broken in.

[newday777]

Without measuring the bores with a bore mic you cannot get the answer if you should or not.
You said it had 10,000 on it in 2010 you bought it, so how many miles on it now? Edit... now I see in the title 17,000...

Personally I would suggest to go ahead and pull the cylinders and check the bores to make that decision and inspect the rubber wheels and guide slide of the cam chain,  check the oil pump screen for debris and the primary chain and rubber wheels/guide for wear and possible damage. It is close to 50 years old and hard rubber and the damage of the top end could drop shavings into the bottom end. Easier to check now than have a failure later.

[BuffaloBill]

Hello everybody, I have an update on the repairs:

Thanks everyone for the feedback and suggestions.  Since the mileage is low, oil consumption was minimal, and because the ideal piston-cylinder clearance is quite small, I elected not to hone and re-ring. 

I bead blasted the combustion chambers, ports, and valves.  I lapped the valves and installed new valve stem seals.  The old original ones were understandably hard and cracked.  New gaskets, etc.  I pulled the oil pan and found almost nothing on the pick-up screen.  I finished the reassembly today and test rode 72 miles.  It runs great.  It hasn't run this smoothly for quite a while I think. 

The partial blockage I found in the oil jet must have been allowing some oil flow, as not to fry things quickly.  The cam lobes in the affected side were worn nearly 50 thousandths, relative to the other lobes.  The used eBay cam, rocker arms, and cam towers were in very nice shape, thankfully.

I'll need tires before too long.  I have Michelin Active Pilots on there now.  What other other tires might I consider this time around?

[newday777]

That is some deep wear on those lobes. Good to hear you are back up and running good again.
Tires to use..... I don't have an answer for you,  I only changed the rear on my K5 to a 16" Metzler on the Invader wheels.  Been holding off on the front until I get the stock wire wheels true'd up before I get new sneakers for those rims.