Author Topic: 1977 Honda cb750F rebuild (Read 3301 times)

lessthanzero · « **on:** May 10, 2014, 08:37:06 AM »

This happened:

I am trying to salvage the top end of the motor, but I can't do much because I can't get the motor to top dead center because of the damage. Any suggestions?

I will be putting the top end on a different motor that has a good bottom.

madmtnmotors · « **Reply #1 on:** May 10, 2014, 08:50:14 AM »

Remove the bolts from the cam sprocket. If that is not an option then cut the cam chain. Remove each rocker shaft as each pair of valves permit. Rotate the cam by hand until you are able to remove the rest. Don't force it since your cam/crank will be out of sync, but if you remove the rocker shafts as they are freed you should be good.

Keep them sorted so they go back in the same location...

lessthanzero · « **Reply #2 on:** May 10, 2014, 10:40:29 AM »

Thanks for the reply. I'll try this today.

madmtnmotors · « **Reply #3 on:** May 10, 2014, 03:35:23 PM »

If you get resistance rotating the cam one direction, then rotate the opposite direction to see if the tension on the rockers can be relieved to allow removal. If you reach a point that you can't rotate the cam to relieve the tension on one or more of the rockers (if a valve(s) touches a piston) then you could still pull the bottom half of the lower case, then remove the crank and pistons from below. This would eliminate any interference with the valves.

lessthanzero · « **Reply #4 on:** May 10, 2014, 06:21:24 PM »

Your suggestions worked and I was able to free everything up. Tomorrow will be spent continuing to tear down and maybe start pairing the two motors.

madmtnmotors · « **Reply #5 on:** May 11, 2014, 07:21:11 AM »

Good news indeed. Once the valve train is removed then the head/cylinders/pistons are now able to be removed without splitting the lower cases.

ekpent · « **Reply #6 on:** May 11, 2014, 08:03:10 AM »

That's gnarly. Make sure the valves didn't get bent, looks like a couple pistons may have been floating around a bit. We need more pics of the carnage

Were you driving it at the time ?

lessthanzero · « **Reply #7 on:** May 11, 2014, 06:12:48 PM »

I was driving down the highway at the time. I think I'm pretty lucky.

lessthanzero · « **Reply #8 on:** May 12, 2014, 04:11:19 PM »

Any tips for getting the top case off? I don't want to hit much harder with a rubber mallet and break any fins off.

edwardmorris · « **Reply #9 on:** May 12, 2014, 04:25:56 PM »

Quote from: lessthanzero on May 12, 2014, 04:11:19 PM

Any tips for getting the top case off? I don't want to hit much harder with a rubber mallet and break any fins off.

How bad is the gasket stuck? Some suggest running a very fine chisel to break it loose but it makes me too nervous so keep that as your last resort. On mine, I was able free it up by gently lifting it vertically/straight up with a lot of grunting and cursing. You could also ever so slightly try to "turn" the head till you hear the seal give out, then pull up vertically. Mallets near fins are risky too, so be careful however you go about this.

lessthanzero · « **Reply #10 on:** May 12, 2014, 07:07:44 PM »

Here are some photos of the damage:

A piston broke off and sort of melded to the jug

Will I be able to salvage any of these?

This was my last ride one morning a couple of summers ago. Engine blew up and I rolled to a stop.

SOHC4 Cafe Racer Fan · « **Reply #11 on:** May 12, 2014, 09:09:58 PM »

Holy crap!

Hon3ybadger · « **Reply #12 on:** May 13, 2014, 01:15:00 AM »

Your engine picked a beautiful day to die. I would consult Cycle X about that top end.

martin99 · « **Reply #13 on:** May 13, 2014, 02:59:35 PM »

Re. the head, can't see any glaring problems in the pics, could well be salvageable IMO. You'll need to remove the valves and clean everything up before you can assess it properly.

lessthanzero · « **Reply #14 on:** May 13, 2014, 08:19:16 PM »

no standard piston rings available for this. exhaust valves seem to have too much play...pretty discouraging day. wondering what course of action to take.

madmtnmotors · « **Reply #15 on:** May 14, 2014, 03:43:06 AM »

The 77/78 F and K models were hard on the valve guides. Sounds like:

>Valve job with new guides
>836 overbore. There's a good thread on the forum here about the Ebay 836 kit. Seems overall positive reviews.

lessthanzero · « **Reply #16 on:** May 19, 2014, 04:16:02 PM »

Started reassembly.

Can someone tell me where these belong?

lessthanzero · « **Reply #17 on:** May 20, 2014, 02:53:16 PM »

also, after tearing down a 1977 and 1978 cb750f, i noticed that both have knock pins that do not sit flush with the cylinder surface. i'm pretty sure neither have been decked before either. after reading user brandEn's leak free top end tutorial, this has me worried. any advice/insight would be appreciated. also, where can i buy new knock pins if i do need them?

CB_Jeff · « **Reply #18 on:** May 24, 2014, 07:50:05 AM »

+1 on the holy crap. I would think you would need to either overbore the cylinders or look for a used set of jugs. How many miles are on your bike? As for the head, based on my research ~40,000 miles is the bright line when valve guides fail and the head needs an overhaul. ... bigger valves in the F3 and F4 motors that was not kind on the guides.

Don R · « **Reply #19 on:** May 26, 2014, 06:52:58 PM »

Unless I misunderstand what you are referring to, the pins need to engage the block and head, they won't sit flush. They just can't be so tall they bottom on both ends.

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Author Topic: 1977 Honda cb750F rebuild (Read 3301 times)