Engine rebuild questions...

[ProfoundlyItchy]

I've been lurking around the forum a while but haven't posted until now. I'm in the process of tearing down/rebuilding the engine on my '81 CB750K. I took apart the pistons/rods last night and noticed dark rings around the wrist pins. The wrist pins also have a ridge near where the connecting rod sits. Needless to say, I have to replace them...

I was able to find wrist pins for $14.95 on Partzilla. My question is, do I need to replace the pistons at the same time? If so, where can I buy a set of new STD size pistons at a reasonable price? I've only been able to find big bore kits, or used pistons on ebay....

This is my first engine rebuild, so I look forward to learning from you guys. Thanks for the help!

 

[amc49]

You may well be out of luck on standard pistons as most were sold out long ago.

You have to positively and absolutely locate the pin ridge location as to being at piston or rod, it is more likely to be at rod meaning the rod tops are loose, that usually goes first, the rods are softer than the pins.

Get your new pins and do a fit check in pistons, anything looser than an easy slipfit (same at rod, ANY wobble is death there) and you have loose pistons too.

Give the engine mileage.

 

[fg1972]

New over size pistons are much more common but you can get new aftermarket standard size pistons like these. If it were me, (I’ve just done a rebuild and had similar questions) I would first have the cylinder bores measured. If they are in spec and just need a hone, then standard sized pistons would be fine but if the cylinder needs to be machined, then you’d have no other option but to go over size.

https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com.au/ulk/itm/172497632374

 

[dirtdigger]

If you dont have a set of micrometers..inside and outside...I would take the parts to a machine shop and have it all measure before I spend any money on parts. I agree if the pins are worn the rods and piston pin bores are probably shot. Why sink money into parts only to find out the pistons and the rods need replacing as well. If the wrist pins are worn the rest of the motor is probably in pretty bad shape.

 

[amc49]

X2, the pins are generally in good shape when other parts are junk. You almost always can use them over. Them being worn says uh-oh.

 

[ProfoundlyItchy]

X2, the pins are generally in good shape when other parts are junk. You almost always can use them over. Them being worn says uh-oh.

I probably over exaggerated when I called it "ridges" on the pin. When running my finger over the pin, I can feel slight waves. But the waves aren't enough to catch on my fingernail. Do you think that is still cause for concern?

If it turns out the piston pin bore, and connecting rod small end are all out of spec, what do you think about having the machine shop bore the piston pin bore and rod for an oversized pin? Could this be a way to savage the motor?

To add insult to injury, I also found another problem when tearing into the head last night. One of the exhaust valves on the #2 cylinder has a chunk broken off the face. It looks like something litterial took a bite out of it! The missing piece is nowhere to be found (i guess blew out the exhaust?). Surprisingly the valve itself is not bent and there is no play that I can feel in the valve guide. I'm was hoping a new valve and valve job would be sufficient. What do you think?

 

[amc49]

Pic the valve, give bike mileage. Valve may be severely burned and if so somebody ran bike into the ground to get there. Don't even think of oversize pins, the piston bosses will be too small and the pin clip retainers.....................just don't even go there, you are showing you know nothing of engines. No insult intended there at all, just the way it is. You want to know when you are going off a cliff I assume........you won't find anyone that agrees to do the work and if you do I guarantee utter incompetence doing it. The rods are well known for failing at the top where the metal is too thin, boring for a bushing or bigger bore only makes it worse. In normal well-used but perfectly rebuildable engines the pins commonly color up rings where the oil cooks on them and simple 600 sandpaper on them will clean them up to like new with zero ridging at all. The pins are like the hardest parts in a motor and them being worn tells you scads about general engine condition. As in, it's bad.

 

[amc49]

Just as an aside, you picked one of the most complicated engines to decide to rebuild on, that one will give you fits before it is over. They are relatively expensive to rebuild as compared to others too. You will need likely all valves and the entire chain tensioner assemblies in 3 places if engine has high mileage. Anything over 40K is high on these if run in a hot climate, Texas here cooks the tensioners in 25K miles to crack them up. The carbs WILL give major trouble, they always do on these. The engines burn many valves as people refuse to set them because they needed shims to do so and it takes longer than 5 minutes, about all the average owner can stand. The burned valves then push all owners into the male testosterone thing, or, 'it must be the carbs' and then the damage gets even worse. You have to choke people to even get a compression reading to then show them why the engine NEVER will run right (because of the burned valves), and then they insist you have to be wrong and spend $$$$ on theirs until they give up in frustration.

Don't get me wrong. The engines run amazing well if one knows how to approach them, they are just VERY picky about HOW you go about that. If you think you will beat it into running right like your lawn mower you are wrong, these respond well ONLY with well thought out attention to detail and unless you luck out to get a 'good' one you will have fits ever getting it started, they rag out easily to never run right again. Easier than most.

 

[ProfoundlyItchy]

Pic the valve, give bike mileage. Valve may be severely burned and if so somebody ran bike into the ground to get there. Don't even think of oversize pins, the piston bosses will be too small and the pin clip retainers.....................just don't even go there, you are showing you know nothing of engines. No insult intended there at all, just the way it is. You want to know when you are going off a cliff I assume........you won't find anyone that agrees to do the work and if you do I guarantee utter incompetence doing it. The rods are well known for failing at the top where the metal is too thin, boring for a bushing or bigger bore only makes it worse. In normal well-used but perfectly rebuildable engines the pins commonly color up rings where the oil cooks on them and simple 600 sandpaper on them will clean them up to like new with zero ridging at all. The pins are like the hardest parts in a motor and them being worn tells you scads about general engine condition. As in, it's bad.

Hey, no insult taken! I'm a software developer by trade; about the furthest thing from a mechanic...I'll probably ask more than few stupid questions before it's all said and done! But hey, that's how I learn...

I'll get a pic tonight of the valves, pins, pistons, etc.

I have no idea about the mileage unfortunately. The bike itself is in rough shape - The previous owner started tearing it down as a project but quit midway through. I got it in pieces for next to nothing.

Thanks for the advice about the chain tensioner. I live in Arkansas so I'm all too familiar with that summer heat. The service manual shows a process for checking the chain stretch - I'll add that to my list of things to check.

 

[amc49]

The chain checking method the manual describes is pretty much undoable in the real world unless one devotes a custom made fixture to set it up. With tensioners it's more the rubber part that flexes, you flex it and look for cracking. Or step wear in the running notch.

You really REALLY need to itemize each and every part there at least in the head, each tappet, shim, valve, locks, retainer and spring needs to be matched to location or you will have utter fits going back together. Same with pistons and rods. Each part reused must go back in the original location.