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Check/grind valve seats

Submitted by graham_cooper1 on Sat, 25/03/2023
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Commando

While I’ve got the head off my 1972 750 I’d like to check the condition of the valve seats and lap them if needed. I’ve got a general purpose valve spring compressor. Is it necessary to remove the rockers and are any special tools needed? Thanks

Submitted by robert_tuck on Sat, 25/03/2023

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I had the valve seats of the atlas  worked on by a very good engineer . They were  a bit sunken. He recut the seats and removed the masking that too many "decokes " had produced .  The action of the rockers was even improved as apparently Norton got it a bit wrong. The compression ratio may be slightly reduced which is all to the good with an Atlas on Commando pistons . I tested his work and no lapping was needed. I could not have had a better result. A good engineer can work marvels and is worth cultivating.

Thanks for the reply. The engine was rebuilt many years ago and has done very few miles, so all i’m really concerned about is possible light rusting of the valve face or seats: I know that any blow past an exhaust seat can rapidly erode it.

Submitted by david_cooper on Sat, 25/03/2023

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You don't have to remove the rockers. Certainly not on a Dommie.  If I remove valves again I'll order a new compressor, designed to fit, from RGM.  My several general purpose ones are all either difficult to use or actually useless.  (Unless that is because I don't remove the rockers...they need a spacer to get down into the rocker box).

The alloy heads have hardened seats already.  With a bit of luck you'll find them to be perfectly OK.

Submitted by graham_cooper1 on Tue, 16/05/2023

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In the end I fitted the spark plugs and inverted the head, filled the combustion chambers with petrol and left it like that for 10 minutes. Then I looked into the inlet and exhaust ports with a light (no naked flames!) and there was no sign of any wetting, so didn’t attempt valve removal. Pity I didn’t assess the exhaust port thread more carefully before reassembling the engine…..

Submitted by philip_hannam1 on Wed, 17/05/2023

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The very early Dominator cylinder head (1949 on) had valve springs which when released from the valve collets wedged up on the rocker ends. It was near impossible to remove them without pulling out the rocker spindles and pushing up/back the rocker arms to allow space.  Possibly this was due to the rocker boxes being taller on the early cylinder heads. Four difference lengths of valve spring were also available over the years.

 



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