Author Topic: Valve Question - A7 vs ST  (Read 364 times)

Offline Happyhenry

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Valve Question - A7 vs ST
« on: 02.01. 2021 17:47 »
Hi,

I'm building one (hopefully even two) A7 long stroke engines from piles of iron in various states of decay.

I have an assortment of cylinder heads, two of which are definitely boat anchor fodder. All of the heads have the same part number: 67-24. I have selected the best two, taken the valves out and cleaned them up.

One set of valves are A7 marked: Inlet = 67-30 (67-0030) Exhaust 67-29 (67-0029)
One set of valves are ST marked: Inlet = 67-189 (67-0189) Exhaust 67-190 (67-0190)

The A7 and ST valves look the same with the exception of the portion of the stem at the top above the collet groove. On the A7 valves this portion is 3/16" and on the ST valves it is 1/4".

As the camshafts (and I assume the pushrods) are the same for both engines, did BSA obtain an extra 1/16" valve lift for the 'sports' model by simply increasing the length of the valve stem?

Looking at the tops of the stems of the ST valves they seem have been used without the caps, which would probably bring them back down 1/16" to the equivalent of the A7 valve length.

Would ST valves work in a bike that is otherwise standard A7 but with a 7:1 compression ratio if the ignition timing was set as for the ST at 3/8" BTC as opposed to the A7 at 5/16" BTDC?

Would the increased valve lift give any performance improvement?

Thanks
H



 
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Online JulianS

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Re: Valve Question - A7 vs ST
« Reply #1 on: 02.01. 2021 18:09 »
The standard A7 valves had valve stem caps the Star Twin had welded Stellite valve stem ends.

Online RDfella

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Re: Valve Question - A7 vs ST
« Reply #2 on: 02.01. 2021 18:36 »
Hi HH. One cannot gain extra valve lift by increasing valve stem length. All that does is mis-align the rocker / valve contact. To gain extra lift one needs a higher lift cam or alternative rocker geometry.
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Offline Happyhenry

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Re: Valve Question - A7 vs ST
« Reply #3 on: 02.01. 2021 21:03 »
Thanks for the replies.
So the only difference, as far as I can tell, between the cooking A7 and the ST valve train is that the former had valve caps and the latter had hardened valve stem tips?
I guess then that the only difference between the the two engines was compression ratio, ignition timing and price?
Seems a bit of an anticlimax.
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Online bikerjohndavies

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Re: Valve Question - A7 vs ST
« Reply #4 on: 03.01. 2021 10:04 »
The Star Twin long stroke also had twin carbs with a different manifold. The head part number was also different 67-182 as the inlet ports had recesses cut in them to allow the stubs of the manifold to fit. It also looked a lot better in my opinion with its silver and chrome tank.
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Offline Happyhenry

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Re: Valve Question - A7 vs ST
« Reply #5 on: 03.01. 2021 15:09 »
Thanks John,

I've definitely got A7 heads with the 2:1 manifolds. Odd that one of them was fitted with ST valves and the others have standard A7 valves with caps, but I guess it doesn't matter as far as the valves are concerned - if you can source them. They appear to identical but for the 1/16" at the top of the stem.

Draganfly has both ST valves in stock, but only the inlets for the A7.

H
"Every time he put his key in the door he wondered what he was letting himself in for." - Spike Milligan - Puckoon.