Honda-SOHC

SOHC.co.uk Forums => CB350/400 => Topic started by: andeks on May 07, 2015, 02:03:48 PM

Title: Valve lapping
Post by: andeks on May 07, 2015, 02:03:48 PM

Hi all,
Have just registered but have been viewing the forum for a while and seen some great replies to calls for help and advice about restoration of  400/4's.
I am attempting a rebuild of an early model ( with rear footrests on the swinging arm)
I have taken head and barrels off and I am at the moment lapping the valves in.
Inlet valves are lapping in nicely but exhaust seats are pitted.
Does anyone know if there is a limit to lapping in, sure I read somewhere that the valves can only be taken back a certain amount.
Any advice would be gratefully received.
Dek

Title: Re: Valve lapping
Post by: martin_uk on May 07, 2015, 02:29:59 PM

Hi and welcome.

It is usual for exhaust valves/seats to deteriorate first as get much hotter.

General rule is that if you cant get rid of pits by lapping with coarse paste, will need to re cut seats.

You can try using an electric dril on stem to rotate, but keep lifting valve up and only do for very short period, say 10 secs.

Finish off with fine pasted by hand and make sure you remove all paste.

Wide seats will effect performance, but IMHO unlikely to notice on a 400

Title: Re: Valve lapping
Post by: Trigger on May 07, 2015, 08:36:34 PM

Quote from: andeks on May 07, 2015, 02:03:48 PM

Hi all,
Have just registered but have been viewing the forum for a while and seen some great replies to calls for help and advice about restoration of  400/4's.
I am attempting a rebuild of an early model ( with rear footrests on the swinging arm)
I have taken head and barrels off and I am at the moment lapping the valves in.
Inlet valves are lapping in nicely but exhaust seats are pitted.
Does anyone know if there is a limit to lapping in, sure I read somewhere that the valves can only be taken back a certain amount.
Any advice would be gratefully received.
Dek

Welcome, It depends how bad the pitting is on the seats ?  ( on close inspection on Mr T's head, he needed the seats cut ) Seats can be re-cut and valves dressed. These valves and seats can be cut due to they are on a adjustable tappet. You only have to be careful if the engine has shims, as the more you cut the more the valve sits higher.

Title: Re: Valve lapping
Post by: andeks on May 08, 2015, 07:17:42 AM

Thanks for great advice, just another question, where and how can I get the seats re-cut ?
Once again thanks for the replies.

Title: Re: Valve lapping
Post by: Bryanj on May 08, 2015, 07:27:38 AM

An engine machinist who specialises in motorcycles is the best bet, car one tend not to understand the tolerances on bikes

Title: Re: Valve lapping
Post by: andeks on May 08, 2015, 11:59:26 AM

Thanks Bryan.