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by Mal Marsden - 06/16/22 7:00 pm
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Original Post (Thread Starter)
#851484 06/13/2021 3:23 AM
by chris b33
chris b33
Hey all!
My bikes have been running so good thanks to all the help along the way that I’ve received upon this here forum. Knock on wood.
So haven’t been on the forum for a few years so forgive me for asking a question that might get asked pretty frequently.
My question;
A65 heads, different years, is there much difference between the years? Obviously dual and single carb but is there any improvements through the years?
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#851492 Jun 13th a 09:31 AM
by Mark Parker
Mark Parker
Hi Chris, yes there are differences. The earlier heads had no balance tube between the carbs. They also had smaller inlet valves, smaller cast in valve seats only able to take up to 42mm valve. And smaller diameter rear studs. These were all changed by around 1967.

Then in 1968 they made a special head for the Spitfire with part number 68-915 this head was modified in the bowl area of the intake port and made to suit a 32mm concentric, flow increased to probably around 120-122cfm from the std Lightning's 109cfm or about 105cfm through the carb. The special head allowed the Spitfire to make 56.5hp at the crank on std 9-1 compression. Parts books did not pick up the change and listed the std part number 68-701. Twin carb heads stayed with the std 701 ports till 1971 when a new casting for the OIF was introduced. The ones I've seen have the improved port with std 30mm carb, one of them I measured.

Umberslades tested a new OIF Firebird in 1970 with a cylinder head machined to the drawing, giving 8hp more than production line engines, with much better response. It looks like the drawing was for the 68-915 head. And they may have done all late twin carb heads like that. Why more restrictive mufflers did not seem to adversely effect performance.

The BSA casting is very good with plenty of room for improving the ports. Some contemporaries are either thin or crowded with head studs. An A65 30mm round port can be modified to flow around 135cfm, 32mm around 150 with a 42mm valve. 34mm, if oval with a manifold, depending on valve size much more.

34mm oval port head and the choice of valves, std 40.5mm, 42mm or 44.5mm. The big valve being more suitable for big bore motors. SRM do a 43.5mm valve but I do not have one.
[Linked Image from]

The problem with these on an early bike is the length and where the carb sits close to side covers. OIF would be the same if the original air box and covers were fitted. Amal 32mm on a round entry port could keep it looking stock.

[Linked Image from]

This shows the oval port entry where the manifold joins the head.

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Vizard has a graph for port velocity it only reads to 400 fps which equals 38" water on a manometer. Testing on the 34mm with 44.5mm valve its around 380fps just in the port. In further toward the guide it's way faster pulling much more. That's with the bench test pressure on 28"w. Good thing I made the manometer go so high.

I guess I should check other heads. Interesting that speed in the port is so fast. I need to check and compare to low bench pressures because a running engine will vary that at different rpm. It may show why the 42mm valve head goes so well seeming to boost midrange. The critical speed is around 600fps parts of this look to be approaching that without actually extending the conversion graph.

Just looked stuff up: According to numerous engine builders, optimum port velocity is at least 320ft/s and maximum volumetric efficiency is achieved at as high as 670ft/s. When I get time I'll check the other heads. I can guess the 42mm will also be high speed.

Being a little more precise I wanted to see what happened at half the test pressure if the engine is not pulling so much vac 30" @ 14"w which is around 360fps inside the actual port. At 28"w it pulls 40"w which is over 400fps and toward the guide area 50"w which must approach 600fps or more. It varies according to where in the port I probe, the short turn is faster than the roof which seems odd.
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