Topic: 77 CB750 Engine Rebuild - What to replace?

[greddm]

Hi All,

I'm getting ready to tear into the engine of my 77 CB750k. This is my first rebuild, and while I am pretty handy, I have never done an engine before. My questions are as follows:

1. Cylinders: How do I know if I need to bore them, or if I can get away with a hone and new rings? (bike as 33k miles)

2. Oil Leaks: Bike has tons of old oil all around the exhaust openings, plugs and fins - basically  the whole front of the engine is covered in oil. Will the new gaskets solve this problem?

3. Lower Case: How do I know if I need to crack the lower case?  I would rather not mess with it if possible.

4. Valves: Do these need to be replaced? Lapped? Any other associated parts I should replace, or signs of damage I should look for?

Thanks

[nancy]

Hi Greddm,
1. Take your cylinders/pistons into local motor machinist and have them mic measured - they will tell you if clearances are OK or not. Take your service manual with you so he knows the Honda tolerances.
2.New gasket set comes with O-rings, gaskets and PUCKS - the pucks tend to solve most of the leak problems.
3. You do. Sorry - get over it...I know it sounds hard..but it's not...and you have to do this to replace primary chain tensioners - these were rubber wheels back in 1977, but will be black concrete now. MUST be replaced. Same for camchain tensioner (plus chains - primary and cam).
4. Depends. Dismantle, fit valve and wiggle if you get slop - may need new valves/guides (see your machinist). If they don't..great - check for valve face wear (2mm max from memory) - if greater - chuck em and get new valves. If OK...(mine were), head with valves popped back in (loose - to protect seats from blast damage) needs to be cleaned and media blasted - lapping needed and leak test afterwards. New guide seals are needed - these come in the gasket set.
Happy wrenching.
Mark

[ZanVooden]

Why are you tearing the engine apart? Because of the leaks? Were there other compression problems?

I opened up my 78K when I bought because of compression issues. In my opinion disassemble the engine down to the jugs. Take the Jugs and head and the pistons into a shop. Have them measure everything and machine the surfaces.

My guess is your cylinders are probably going to be fine. Valve guides may need to be replaced. Cutting new valve seats is never a bad thing (at the very least lap them). New rings only if you need them (if there were compression issues then you probably do).

Replace the cam chain tensioners and guides. All new gaskets. This should take care of the oil leaks.

Unless there is a reason, why open the lower cases? Was there knocking or transmission issues? I wouldnt open it up if you dont have too. Never had an issue with the primary chain tensioner.

my 2 cents.

[mrrch]

Primary tensioner can be inspected thru the oil pan. Alot of things are decided by your budget.

[nancy]

Unless there is a reason, why open the lower cases? Was there knocking or transmission issues? I wouldnt open it up if you dont have too. Never had an issue with the primary chain tensioner.my 2 cents.
[/quote]
I think a person who says they are doing their first REBUILD - 1st should be looking at opening cases and doing a thorough job and 2nd definately involving a machinist inspection of jugs/pistons. Don't be lead astray by people who have already learnt how to do it themselves...get help for the all important job of measuring.
He hasn't said he is planning on his 1st "quick bodge job". A rebuild if tackled properly would include a dismantle to inspect the crank and bearings, replace the tensioner (without question) and possibly the primary chains (not needed if inside tolerance). 35 years of various oils (changed how often??), crud and mileage have gone on here. What you DON't know about your engine is what you might live to regret once you zip it back together. Why would anyone spend hours unbundling their engine (1st time), only to tackle the top half only? It took me I'd estimate - 30 mins to unbolt my case bolts and 30 mins to split open my cases (1st time). For the sake of one hour of your time - do it right. If you are planning on a "quickie" or a "cheapie" - agreed,...leave the bottom alone. But then - it wouldn't be a rebuild...and you'll be doing it again - later.
As stated - primary chains and tensioner can be AND SHOULD BE checked with the cases intact - by removing the oil pan.
Mark

[greddm]

Thanks for all the replies and advice. I have torn down the top end of the engine, but thats I'll I got to last night. I'm going to pick up some feeler gauges tomorrow and check the clearance on the pistons. I'm not hopeful after my initial inspection - there is a significant ridge at the top of the cylinders. The pistons themselves are in great shape, but that won't matter if the cylinders need to be bored.

As to why i'm tearing it down in the first place:  I was told that the reason there was so much oil leaking out the front was because the rings were worn and oil was blowing by them and out the exhaust. Seemed like the compression was crap as well.

Money is the key factor here, so this is my next question: Is it cheaper to get new sleeves and stick with the stock pistons with new rings? or to get a new set of pistons and have the cylinders bored to the next size over?

[nancy]

Forget the feeler gauges. They are too hard to get any accuracy with. Tolerances are not much with these engines. Machinist will likely use an internal mic to measure bore walls inside wall to wall. He can compare that to tolerances stated in your Honda service manual.
In particular with cylinders that have become out-of-round. Assume a rebore - get a quote at your machine shop for the rebore and ask them to do a quick measure up for you - so u know whether to order 1st, 2nd oversize piston sets. Ask as though "Free measure" is your expectation. That's what i was given..in return I went back for the rebore and handed them $250 for the rebore.
Regards
mark

[PeWe]

Open the crank cases too.
There is no additional cost except damaged parts inside and sealer for the crank cases.  All O-rings inside tightening the oil channels might be tired and need new to ensure the oil pressure from pump all the way to the cam.
The years and not the mileage is the repair factor here.

I found Hondabond sealer on eBay for about $20US, shipment included.
Take out most av the visible axes and clean the inside of crankcases. I let half of the gearbox to remain in the case while cleaning it with detergents + high pressure. After that sprayed the bearings and axes with thin oil spray like 5-56 to avid corrosion.
The crank shaft/ case bearings might be damaged due to oil problems, parts in the oil.

[Thousandson]

My engine is also in parts. Take your time and follow the manual, you'll be fine. Your saving a lot of money, and gaining great experience with this motor. Ask these old guys, they know what they are talking about, even if they give you a little gripe.

[greddm]

Ok, I have removed the pistons - there was a thick layer of buildup on the top, but after 3 days in a soak, here is what they look like:

When I fit them in the cylinder they are really snug - I cant even get the smallest feeler gauge in at the skirt (rings also pass according to the manual).  Does that mean i'm ok to keep using them?

I've also attached a shot of one of the valves - what is this cement crap on them? This guy also soaked for 3 days with absolutely no result. I'm going to try a wire wheel but is this salvageable?

[Thousandson]

I have 38K on my motor and I'm going to replace the piston rings, and the piston pin rings to. Its cheap parts fora great motor for many more miles.

The pistons look great, the valves need a little love. Take some sand paper and sand the crud off. Don't sand the valve seat, it could cause a compression loss if sanded. Don't forget to replace the valve guide seals.

I need to do some cleaning on my 78' Cb750f before it all goes back together. Keep turning those wrenches.

[Jerry Rxman Griffin aka MuthaF'er]

Your valves don't look wet with oil. Much lighter than Thousandson. His look dark and one is wet. Things like oil getting past rings and/or valve seals or bad/weak spark in a cylinder or rich condition can attribute to the darker color. Your rings may be/have been fine. Seals also. However I might speculate that your bike was not ridden hard enough to clean out the deposits you see.

What to replace? Nothing to everything. You didn't mention that you did a compression check??!??!! That would  have told us a bunch. What are the ring end gaps in the current condition? Are they beyond servicable limits? It's a crap shoot now. depends on your philosophy and how deep your pockets are at this point. If you wanted to roll the dice you could clean it up, hone the cylinders, lap the valves and put it back together as is with new gaskets and seals then see how it does THEN/AND do a compression check then hope for the best. You mention a mess of oil but you didn't mention how tight the bolts on the cylinder studs were. You didn't mention the condition of the 6 rubber pucks under the cam towers. You don't mention how many miles you wish to put on this bike either. That might be the best indication of what you spend on it. If your pockets are a little deeper than that you might want to install new rings and spend $80 on a set of heavy duty cylinder studs to keep the top end firmly attached to the bottom end and keep oil leaks to a minimum. The stock studs tend to stretch and the nuts will be looser upon disassembly. The later narrower rings could be an issue to find and I'm not sure if your engine has them. If they are the -300 part number you're good. If they are the -392 or -405 (-410??) good luck finding them. Or you could actually spend $7K - $8K and do a serious full performance build. Seriously it's possible.

[Thousandson]

After pulling off the heads and the jugs. I found water in the cylinder studs, and a heavy amount of corrosion that held on to the jugs for a bunch of pulls. My number 2 cylinder isn't firing, the spark plug looks new, like way new never used. So yea that cylinder is very wet.

One question I have is, I just measured my pistons, and I am right on that cusp of an over bore, But its only in one direction, on the 90 degree measurement its a few thousands over minimum requirements. I'll get exact measurements tomorrow, I'm also planning on doing an over bore, next tare down which will be in a year or two. Is it going to be ok, or should I overbore, or find new pistons and jugs?

[Jerry Rxman Griffin aka MuthaF'er]

Think I'd try to determine why #2 isn't firing. Since it's not 2 AND 3 it's wire, cap, plug, carb, valve clearance, etc. Minor problem that needs to be fixed. Since you're 'close' to an over bore and you want to go bigger in 'a year or two' check your ring end gap within the cylinders. Maybe just fresh rings and a hone for now then worry about details in that year or two. Or same old rings and add oil when needed for now.

[greddm]

ok, since my last post I have finished cleaning up the pistons and I may have more problems than I thought. They have some pitting on the the top and on has some pretty bad scratches around the head. Are they shot? They are still within tolerance as far as the fit in the cylinder goes. Money is tight so I would rather not replace them if I dont have to.

thanks

[mrrch]

They look better than the ones I reused, some cameras will make light marks worse than they are.