A10 Big Valves - Rare?

[trevinoz]

My two-bob's worth.

              There is a lot of confusion as to what was originally fitted to the "big valve" heads.
I have both, 67-1549 & 67-1571.
The inlets are the same size, 1.455".
I asume that the difference in the heads is the inlet tract as some export machines were fitted with an 1-3/16" carby.
I have attached a page from the factory tuning notes which states that the inlet valves are increased in diameter [1.455"] by fitting valve 67-961 which is listed in some publications as a Super Rocket, '58-'59, 1/16" os, making it 1.5".
I am again assuming that the author means to increase the inlet valve from 1.455" to 1.5".
I think that the Atlantic Green site is wrong in stating that the RGS & Big Valve were fitted with the over size valves as I have never seen a parts book which would back it. Nor the heads in my possession.
Logic tells us that BSA would give a head, with different size valves to standard, a new part number.
My parts book only lists the 67-1551 valve and the book includes all markets machines.       

   Trev.       

[Rocket Racer]

If the valve seats have enough meat on them to cope with the std 1.455 inlet big valve I'd be using that.
I fitted large valves to my earlier '55 head, supplied by draganfly and am now having to fit smaller valve seats on the exhaust so I can go back down on the zorst valve diameter. I didnt have valve to head issues with 8:1 pistons but was getting valve clipping.
My racing oracles opine that the big A10 zorst valves are too big in relation to the larger inlets.
I'm running with a 357 cam and looking forward to better reliability with a smaller exhaust valve.
That triple port head is just sad.

[chaterlea25]

Hi Trevor,
You maybe correct surmising that different size inlets were used for different "export" markets ???? ????
I compared two 67-1549 heads and one has a 1 3/16 inlet port 30mm) and the other 1 1/8 (28mm) or so
Both did not look as if they had been worked on by "home tuners
The 67-1571 on my SR has a 1 3/16 port and had 1.5in valves fitted when I first got it??

All this tells us is that more or less "anything goes" when waffling on about these heads

Cheers
John O R

[jfligg]

Hi Guys
  My 1959 Brocher advertises a big valve head.  I'll let you guys know when I get it apart.  Jeff