Advice needed - 1971 BSA A65 Head. Will this hold compression?

[adriancw]

This may be a question I don't want to hear bad news about, or get told there's no hope, but here goes...

I have a 1971 BSA A65 Oil in the Frame engine and was not able to buy the head for it, although I have the rest of the engine and a matching frame.  That's the good news. As you may know, the engine for the 1971/72 had different studs when compared to earlier A65's, hence the rarity.

Sight unseen, or based on some poor quality photo's I found and quickly bought a head that was correct for the year and model, no broken fins and with the correct stud size. The heads are few and far between as these are only for 1971/72.  Now that I can inspect it in detail I noticed that there's very little by way of  actual metal on the front of the underneath of the head, i.e. where the head joins the engine and where a gasket would be between the head and the cylinders.

I don't know if this is the "norm", or if a previous owner cleaned the head by sanding/smoothing the head down to point where the front now is virtually the same as the rest of the head...  I haven't started the build yet, and before I do attempt it I thought I'd get some advice/guidance from here and on other old British bike forums.

I hope the photo's show what my dilemma is.  Any constructive advice willingly accepted!!   Cheers,  Adrian

[Bess]

Hi,
     They are pretty much all like yours, here's a pic of mine which has been skimmed.

Best wishes...

[chaz]

may get away with it but get a good head gasket and would keep an eye on any rebore size.
failing that, is there any room to fit sleeves, have done that in the past.

[adriancw]

Hi,
     They are pretty much all like yours, here's a pic of mine which has been skimmed.

Best wishes...

Wow, That head looks great!!  I'm going to probably try and build it out and see how it fits... After I clean it and replace the valves !!   Cheers,  Adrian

[adriancw]

may get away with it but get a good head gasket and would keep an eye on any rebore size.
failing that, is there any room to fit sleeves, have done that in the past.

Thanks!!  I think I'm going to rebuild it after doing some repairs.  I hope to get away with what is seen here (albeit much cleaner!!

The sleeves option is something i would follow up on with a shop, if it comes to that.   Thx !!   Adrian

[Mark Parker]

The head should be fine. It will likely need the guides done and cleaning up.

I have a question though if it's a twin carb head. I have one twin carb head from an OIF model with factory porting. The casting pad is blanked with no number and no number stamped on as they usually did on special ones. Like the MkIV Spitfire head with 68-915 stamped on.

This late head looks to have the same port as that special Spitfire and I'm wondering if they were all done like this. It flows 139cfm compared to a std '70 Lightning head's 109cfm. Umberslade built a std motor using this porting which gave around 8hp more on their dynos than production line engines, and maybe BSA did all the late twin carb heads this way.

This one doesn't look much different but it's much wider near the guide 32mm compared to 17-18mm on a 68-701 std head. It's interesting because Norton Commando owners often claim the Commando has the best flowing twin carb head. And they are good, but not as good at these special BSA twin heads. The one I have is a factory machined head, but I don't know if all those heads were like it.

The heads can go 39mm wide there and be ported oval to suit a 34mm carb and flow up to 170-180cfm with a 42mm valve. It's primarily shape that adds flow which increases gas speed and so bottom-end/midrange/top-end.