• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

Valve and seat issues

motobarnvc

CB750 New member
Messages
3
Reaction score
0
Points
1
Location
Clearwater, FL
Hello all,

I am new here - I have come across a problem in the rebuilding process of a DOHC CB750 and I don't know where else to go!

A little background about the bike: 1979 CB750K DOHC engine fit into a SOHC frame, mileage is unknown, started to make cam noises (spinning noises) so removed cam cover and cylinder head to inspect..

A few of the intake valves were "mushroomed" at their tips and would not slide out of their guides. I didn't want to damage the guides by just hammering the valves out, so I made a grinding tool out of a small socket and some high grit sandpaper rolled inside of the socket, stuck it to the end of a drill, and very carefully removed just enough material from the tip of the valves to where they could slide out of their guides. It worked very well, despite being a very slow and tedious process..

I was able to remove all of the valves with this process and moved on to cleaning the cylinder head. After removing all of the carbon, I noticed that 2 intake valve seats have serious pitting and only have 2 angles! Ouch.

If I took this cylinder head to a machine shop to get those 2 seats cut to remove pitting and recreate 3rd angle, the 2 intake valves that correspond to those seats would then sit further "into" the head, thus making their tips even closer to the cam... they already have "mushroomed" tips, so I could only assume this would get worse if they were making even more contact with the cam.

This is a very confusing concept, and even harder to explain, so please let me know if I am not making much sense....

My question is: Should I start looking into replacing both the head and valves, or is there a way to salvage this head?

Thank you all!

seat1.jpgseat2.jpgseat12.jpg
 
You can have the head rebuilt. It will probably need some new seats and more then likely you will need to replace all the valves. Its not going to be cheap but you may have a hard time finding a used one that is in good shape. APE is the best in business http://www.aperaceparts.com/cylinderheads.html keep in mind cylinder head work is not cheap. I dont recommend going to a traditional automotive machine shop, alot of times they can not hold the tolerances on the smaller engine stuff and they are rigged more for getting parts in and out as fast as they can which is fine for general automotive but doesnt last in smaller motorcycle engines.
 
Thank you so much!

I will search for a cylinder head in good condition, even though it's hard to know about the condition of heads for sale online...
If that doesn't work, I'll send it out to where you have mentioned.
 
You MUST change any bad valves as they are barely surface hardened and cannot be ground on whatsoever. Like said a bike shop to do the work, car shops will not have the smaller valve seat cutting tools bikes normally need. Bike shops now can do the work but many have a head up the you-know-what and do not want to work on 'older stuff' and just being jerks. Having the proper 5.5 mm. pilot tool may be an issue.

Virtually ALL early DOHC valves will mushroom enough to just not slide out, I simply tap on them lightly and they do come out, any more than minimum force and not doing so and I file the end OD off with a point file. The ones I tap out work fine later but you have to know when to stop.

One of your biggest issues will be getting quality valve seals that don't leak oil in less than 10K miles, most out there do now and even the APE supplied ones for DOHC. They used to have good Viton ones but it appears their good ones are no longer available.
 
I see what used to be 3 angles there but utter crap now.

You will have to sink valves enough there to make shimming the clearances a tough sell, DOHC cannot get far off the valve set heights before you cannot shim them up for lack of shims in the correct range. I'd be looking for a head with MATCHING VALVES THAT RAN PREVIOUSLY IN THAT PARTICULAR HEAD. Hard to find, most want to sell mismatched stuff and back you are with the same problem again. The valves absolutely need to be kept and marked as to original location or fits getting them to seat as the valve takes a 'set' from the use of it under heat and then any seat mark becomes more like a groove that fits its' original seat location fine but leaks like a sieve in any other locations. People just don't get that and mix the valves up all day long and then the crap engine running results. I myself put every single valve/spring/retainer/washers/lock/tappet into marked freezer bags for each valve location, it simplifies things so very much and adds huge amounts to the quality running of a rebuilt engine.
 
APE mentioned above is best of the best work in the world but like said the valve seals from there likely an issue. They don't make those in house.
 
I always make sure to keep EVERYTHING organized and labeled so that I don't swap parts..., taking pictures to document everything helps too. Thanks for taking the time to reply to me. I'm going to just have to keep my eyes open for a head in good condition. I will use new valves too, as you have suggested.
 
You can use previously run valves as long as they are matched to the head. The valves CAN be lightly lapped to seal better but heavy lapping even will remove the hard outer heat treat, then the valve wears like lightning. They are so soft underneath that even 5K miles is a good life for them once you get to that. Why that exact valve match to the seat is so important, when you use mismatched valves the edge differences make you remove too much trying to get them to seat properly. The seats on every single valve are normally different from each other and a refit mismatching them up and leaks like h-ll.

The engines were designed in an era of low lead fuel, but it turns out that in reality even low lead helped greatly to extend the life of the valves, now with 100% no lead fuel they have trouble surviving as long as they used to. Why they got away back then with the low quality thin valve heat treats.

Used head with used valves and OK but mix them up and you got problems.

The valve tips mushroom out with even easy use again due to the crap heat treat there too, it's only up to .005" thick and if you have seats ground then instruct the guy NOT to remove any material from the valve tips as it is the normal thing for them to do. Better to readjust using the shim but then again you have to have them.

The valves are so flimsy it is common for extended running time and the clearances CLOSE UP rather than get looser like most valves do, because the exhausts recede so bad. That and the wonky original cam journal clearances then allow the valves to be held open when you think you have clearance and then they burn. Why you use .005" instead of the service manual .003" as the go-to valve clearance number, the engines then last MUCH longer before issues show up.

Follow the rules and you will be happy, don't and, well, you'll find out.................
 
Last edited:
Back
Top