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Discussion Starter · #1 ·
I’m rebuilding the top end of my 1978 cb400t. New valve seals, lapping the valves, cleaning out carbon, new pistons, rings, etc.

So far everything looks good on the head except for pitting around the valve seats so I’m lapping the valves.

While doing this I noticed that two of the valves don’t slide smoothly in their valve guides. None of the valves wobble. They all fit very snuggly but 2 of them kind of hang up when sliding in and out. I’m worried they might stick when the engine is running.

Is there any way to clean out a valve guide and restore it to original condition?
 

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Before you start reaming out the valve guides, are the valves straight (chuck them in a drill and give them a spin)? Any raised burrs where keepers fit,in the grooves?
 

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try the sticky valves in a non sticky valve guide in the head. If you are putting pistons and rings make sure your cylinder clearance is still within specs otherwise it will smoke. Hondas like them tight.
 

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Discussion Starter · #6 ·
try the sticky valves in a non sticky valve guide in the head. If you are putting pistons and rings make sure your cylinder clearance is still within specs otherwise it will smoke. Hondas like them tight.
Thanks for the suggestion and heads up. Even though this bike has 9000 miles on it, the cylinders are measuring at 2.774-2.776 in. Which is within spec but seems really tight to me since the new pistons at the skirt are at 2.776 (70.5mm). Someone tell me how 2.774 is within spec for a piston that is 2.776?? Anyway, the pistons that came out measured 2.776 also so they do fit!
 

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Are you replacing the pistons because the rings are broken or stuck or are the pistons looking worn or scratched.
Are you replacing with what type of pistons. Some aftermarket pistons can be interesting to say the least when fitting to original honda clearances.
The types of measuring clearance vary and can be deceptive. I used to bore cylinders and it can be hard to measure tight tolerances in cylinders and pistons that have various tapers on them. I used techniques that the techies would complain about.
This post is mainly to give people a bit of info and not give advice for your particular situation.
 

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Discussion Starter · #8 ·
I’m replacing the pistons and rings because the PO said compression was very low. I chalked up the low compression to the valves, they need badly to be lapped, but to be sure I’m replacing the pistons and rings too. The rings were quite worn.
 
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