What usually needs doing in an engine rebuild...apart from the obvious

[Gabby]

Hi all. I've stripped down the engine in my 1977 model. It's actually a P7 police model but basically no different in the engine department. It was a runner but not very well, probably only due to carbs I think. The only other thing I noted was a very clunky gearbox when putting it into gear.
Since I haven't had any real experience of it and it was sitting for many years, I am keen to hear what might need doing apart from the usual. It might have 70,000 km on it (44,000 miles or so). Are there common failures or things that wear in these engines that really ought to be addressed even if they look ok. I only have time to do this once so would like to know of anything learned by others' experience. The primary chain and cam chain guides etc seem obvious, even if ok. Oddly enough, there seems very little wear in the pistons or bores. Are main bearings and rods something that ought to be done anyway?
Thanks, G

 

[dirtdigger]

If you haven't done so I suggest getting a factory cb750 shop manual....they are sometimes hard to find and a little expensive but they have the most accurate info and better pictures then the cheaper brands. Absolutely do the cam and primary chains. The cam chain tensioner wheels get very hard and brittle so need replacing as well as the primary chain tensioner gets hard and brittle. If the bearings look good with no grooving then I would just plastigage them to check clearance and see where you stand. You will need a manual for the chart to select new bearings if needed. Bottom ends in the 750 very rarely have any issues. At a minimum in the cylinders have them honed and fit new piston rings. The gearbox will clunk when putting it into gear...this is normal. Inspect the tranny gears and the engagement dogs for chipping or rounding. The cylinder head is a place I suggest having a competent motorcycle machine shop rebuild because of the tools needed for rebuild. With its age and hard use being a police bike it probably has worn guides and is probably warped. Inspect rocker arms and shafts for wear. The shafts are more than likely worn and good used ones will need to be found. Couple things...I suggest using heavy duty cam chain and heavy duty cylinders studs on any 750 taken apart. The stock studs stretch and cause loss of head gasket tension which results in oil leaks. Above all use NO RTV on this motor, use only Hondabond or similar on the case halves. RTV softens and plugs small oil passages in the head and destroy the top end. Any issues post or message me and I will help when I can.

 

[Gabby]

Thanks dirtdigger. Some good points there; much appreciated. I have the Honda shop manual. Regarding your comment on the rocker shafts likely being worn, is that normal for a bike with that mileage? And are you saying that there are no NOS or aftermarket shafts available, hence I may need used?

Just another thing I've been worried about: the cases, head, cylinder block etc need cleaning back to bare metal as they are a mess. I'm aware of not getting them done with an abrasive product as it gets in everywhere, but what is the general thought on having these treated on the 750? I've seen others talking about having them re-painted in some sort of silver finish but I'm not keen to do that. Regards

 

[dirtdigger]

All of the models that the rocker shafts have the two extra bolts that hold the shaft into the tower wear the shafts on one side. The early models let the shafts float in the towers hence they rotate and dont have a concentrated wear. No new ones are available unless you find NOS on some shelf...only other option right now is finding good used shafts. I am working with someone to correct the lack of new shafts but I can not say at the moment. Is this a aluminum color or a black motor? You could use a paint type stripper or put some time and hand sand the surfaces and repaint.

 

[Gabby]

Thanks again. Understand what you are saying about the shafts. The engine paint is an aluminium colour and is flaking off everywhere. I thought getting it back to bare metal with a vapour blast etc might be the way to go.