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#788390 10/27/19 10:25 pm
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I have discovered that my Bsa cylinder head intake valve seat has been cut to many times. The stock replacement valve will not cover the entire seat area . My machinist has informed me that i could use a stock valve but it might not hold up very long. I have seen Kibble/ White Black Diamond oversize valves sold by our vendors marked for Bsa B25 and A50 models. Would this work? The machinist also could cut down a A65 intake valve to size or install new seats. Any suggestions ? Thanks. Jeff

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might be best to get a new hardened insert (seat) , do exhaust ones at the same time i would say


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I have fitted oversize valves to stock seats, yes it should work, the original seats are worth saving if you can, its a fair old job to replace them . if your machinist is willing , go for it. Seats were originally cast in , machining them out, then making a new one fit so it doesnt fall out is a tricky job. My pals trident motor was wrecked when a replacement valve seat dropped out. Do watch out for installed spring length.


71 Devimead, John Hill, John Holmes A65 750
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I had stellite valve seat inserts put in the T160 Trident I raced back in the 80's
I was running some W&S valve springs and after maybe 2 laps of Amaroo park I'd lose 1500 to 2000 rpm as the valves climbed into the head.
Once I had the seats done (Bernie at Rams Head Service) I'd go from meeting to meeting without having to adjust the clearances.
I think a cheaper option would be to try and source a good condition replacement head though.

Last edited by John Goodwin; 10/28/19 5:21 am.

Current: 2 x 1966 A65S, 1 x 1967 A65SA, 1 x 69/70? A65LA space Y, 1 X D14/4 & 1 x B175
Past: 4 x 1976 T160V, 1 74/5 T150V, 83 model GSX 750 ESD, Z650, Katana 1100(Bathurst Model), 79 T140V, 70's TR6, 2 x 1971 BSA 250 Gold Stars, 50's 350 Goldie, A65 Spitfire semi basket case, 1965/6? A65 LC, Tiger 21 350 & a D14/4 Bantam, 175 Bridgestone Twin with Zimmerman discs!
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If you can find suitable oversize valves for the bike the beezer seats can be cut a fair bit bigger than they are as standard.
I've had seats put in an a65 that fell out and several other blokes i know have had the same problem. The inserts are put into
the original austenitic iron seats as machining them right out is very difficult. If you go that way i suggest you use albronze inserts.
That way the rate of expansion keeps them in.
Getting the job done by someone who has a Serdi or similar is a good idea, finding someone who'll gaurentee it, is harder.
If you use A65 valves they will need to be shortened from memory, that will mean using lash-caps and re-cutting collett grooves.
You may be able to find some that will fit if you can persuade a stockist to let you rummage through his stock, it may mean fitting
different colletts springs etc etc. For instance I have fitted Triumph 500 daytone valves in a B40 with no issue, but you need all the bits.
If you do fit bigger valves, check your piston clearances, when done, you may have to cut a little more out of the pockets.

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Originally Posted by NickL

If you use A65 valves they will need to be shortened from memory,


Interesting point Nick, never realised that A65 valves were longer.


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Many nitrided auto valves available for less than $10 that can be modified to fit.

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Are you using the correct A50 valves? Early valves are smaller than 66 onwards.


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1965 A50 Cyclone Clubman
1966 Victor GP
1967 Victor Special
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The new valves are in correct spec for a 1970 Bsa A50. Ordered 2 oversize intake valves, my machinist say's this will solve the problem. Thank you for the input. Jeff


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