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Valve Job on 1968 CL77 Honda 305 Scrambler

shadtews

Member
Joined
Apr 14, 2021
Total Posts
20
Total likes
0
Location
COLORADO SPRINGS, CO
Hello, everyone,

I am in the middle of rebuilding the motor on my 1968 CL77. I am currently working on the valves and was hoping to validate a few things before I proceed.

First, I did the liquid test and found that all of my valves were leaking. Not too surprised as the bike sat for a long time and I had to do a lot of carbon and rust cleanup. After cleanup, I lightly lapped the valves and that seems to have stopped the exhaust ports from leaking but the intake valves are still allowing the liquid to bypass. At this point I want to get the seats redone and replace the valves if they need it. Although I know a machine shop that can perform this service I'd like to give it a try myself in hopes I can do the same on future rebuilds.

So first question. The FSM says the "Cylinder head valve sheet" is an angle 45 degrees. If I am looking to buy a valve seat cutting tool is there anything else I need to know besides the angle when shopping for one? Has anyone here ever done this before and if so, what cutting tool did you buy?

Second question. The exhaust valves have a flat contact surface but the intake ones appear concave. Is this right or are the intake valves worn out? Hopefully the pictures will help make sense. (fyi, it is hard as heck to hold everything and take those pictures but hopefully they display my point)

Intake (notice the gap)
20210425_092822.jpg

Exhaust (no gap)
20210425_092851.jpg

Last question. I know it is important to measure the thickness of the mating surface where the valve contacts the head. The FSM has an entry for "Ex. In. valve" with the width listed as 1.0 ~ 1.5. Is that the spec I am looking for? If so, my valves are way off as it measures about 2.5.

20210425_100915.jpg

20210425_100927.jpg

And here is the head I am working with if anyone has anything else to point out that I should be looking out for.

20210425_094456.jpg 20210425_094503.jpg
 
Ever wish you could delete a thread? I found what I needed in the manual after all, not sure how I missed it.

20210425_102438.jpg

I would still love any recommendation on cutting tools or any advice from someone who has done this before.
 
No worries on deleting any thread, ones like this are still advantageous to others in the future who are looking for similar info. I moved it to the 250/305 Engine and Transmission section. I prefer to avoid cutting seats unless absolutely necessary as it can have the negative side effect of reduced valve adjustment clearance in the 350 and DOHC 450 engines due to the eccentric valve adjusters, but you would not encounter as much of an issue with the tappet adjusters on the 305 rocker arms. That said, it is necessary sometimes and the tools to use can be pricey. I'll defer to Jim (LDR) for further advice as he has infinitely more experience in that area than I do.
 
Longdistancerider mentioned something in another thread that I found interesting
was something along the lines of, if you took your head to a shop for a valve job and the person mentioned that this would be their very first head then how would you feel?

being said, since you plan on doing others then it I understand the interest in investing in a setup to tinker with

I have a Sioux grinding stone setup that I've used about 3 times. My first seats "serviced" were ones where the spark plug melted and the valve seat on that side was all beat up from debris. It was put back in service but I can't offer any follow up results as after a few hundred/thousand miles the bike/engine taken off the road and is now lost in a pile of projects

I would replace or resurface valves that are concave
 
I think I'll buy new valves, install them and see how well they do on the leak test. Plus I took my cylinder in Saturday to get the new pistons bored to the new sleeves and I'll get the opinion of the guys at the machine shop, see what they recommend.
 
New valves are definitely in order, especially those intakes.
Do Not install new valves with out cutting/grinding the valve seats, lapping if done is the very last step of a valve job.
Look carefully at the FSM picture (visualize it inverted like it would be in the engine) and notice that the lower edge of the seating area is 1/2 way across the valve face, this is very important. The portion below the seat is called the "margin" and is critical to cooling the valve. Less or no margin means the valve will burn from the edge in.
How do you set the margin and seat width? First the valve seat is cut at a 45 degree angle. Now spray Dykem Blue on the valve seat. With your Vernier caliper adjust it so your points are at the 50% point corresponding to where the lower seat edge should be. Use the 60 degree cutter to grind the valve seat down to match the Vernier setting. Once this is done the 30 degree cutter is used to narrow the seat to the width specified in the FSM. Typically it's .060"/1.5mm Exhaust and .040"/1mm Intake, these numbers are pretty much universal to all engines now.
If the valve guides are worn they have to be replaced prior to doing a valve job.
Valve seats cut very quickly so when grinding them is a slowly done process, cut a few seconds and check, cut and check, repeat until it's just right.
Finding cutters/stones small enough can be a problem. For actual grinding most shops use Sioux and for manual cutters they use Neway. If used properly both can do an acceptable job and improperly they can ruin the valve seats very quickly. The best valve seat cutting is using a Serdi machine which is out of everyone's budget.
 
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