Inlet size on A10 iron head

[KiwiGF]

Hi, it looks like the cylinder head on my bike might have been modded as it's about 1.25 inside diameter so there will be a step between it and the carb. The bike came with a 389/67 monobloc carb, which is off an A65 (I think) and has a 1 1/8" nominal bore.

I've read the late GF's had the 389 1 1/8" carb and as I also have 7.25:1 pistons and a 356 cam I'm sort of hoping I'll end up with a late spec top end and be able to use std 389 carb settings/jets with the 389 I have.

Std for my bike, a 56 GF, would be a 1 1/16" 376 monobloc carb (I think).

I'm concerned about the effect the inlet might have on the std jetting if it has been increased above the standard size, so my question is please:

What diameter inlets did the iron heads come with? Were they all the same? Did they get increased when the 389 carb was introduced?

Is there any way to tell apart "early" and "late" A10 iron cylinder heads? or are they all the same?

[trevinoz]

Kiwi,
               There is a way to tell the late iron heads from the early ones but I'll have to give the skull a good scratch to think about it.
I'll get back to you on it.

Trev.

[Brian]

In 1960 they started using the 389 which was 1 1/8", up from the 1 1/16" 376 used up until then. Is the difference in the size of the inlet tract ?

[trevinoz]

From 1960 the iron head was part 42-0180 and previously 67-1065.
I think the difference visually is the 4 horizontal fins. The early heads fins are mostly parallel on the sides but the late ones have the 2 centre fins slightly recessed.
I haven't got a late iron head in the shed now, I gave it to a mate some years ago for a project of his.
It stands to reason that the inlet on the late head would be larger than the early one, doesn't it?
But then again, we are discussing a pommy bike! ???

Trev.

[KiwiGF]

Thanks for the info I'll look later for a casting number to see if it relates to the part number at all. I did not realise this was  bit of a left field question. If the inlet had been 1 1/16" I would have assumed I'd got a 56 cylinder head.

Are the threads a different spec on the 2 types of heads. The head I have has BSF stud holes for the rocker box.

But probably not as I've got a thick flange barrel with BSF stud holes, and those came out before 1960.

[Brian]

I just had a look at the head on my bike, its a 61' and as Trev says the two middle fins are recessed slightly, about 3/16".

[KiwiGF]

Great I'll a have a look at the fins when I get home. it will probably be a 56 with the inlet opened out to 1 1/4".

I need to look out for a better cyl. head for it for a number of reasons, not least of which is that that big chunks of the top horizantal fins are missing, they've been cut/ground back to look the same both sides, and almost like it's meant to be like that, but I know it's "wrong".....

Would I be right in assuming there is no "obvious" step between inlet and carb eg. the inlet is round, and 1 1/8" diameter on the later iron cylinder head?

[Brian]

Mine has a 1 1/8" hole in the inlet of the head.

One thing I cant find on mine is the casting number. Mine is still on the bike so it could be somewhere that just isnt visible with the head on the bike.

[KiwiGF]

Fins 2 and 3 are about 3mm smaller than 1 and 4 in area of plug. No visible casting number but there is what looks like a brand mark with the letters "IMC" about 20mm across, on the top right fin, near the plug hole.

[Topdad]

Mines got an iron head in which SRM fitted bigger inlet valves (SR I think) ,before installation I decided to take a bremel to the inlet track(a la Dutch ) and when rebuiilt fitted a 389 which a mate had found for me .She'd start well and using an old colourtune I have mixture seemed okay whilst being rev'd in the shed but it was awful on the road ,just didn't seem happy but just now and again used to come on song and you'd think ,whow, got it !! only for it next time to splutter and fart about and generally be a pain in the idiot .I thought I'd done something daft to the tract etc but in desperation fitted a concentric and it cleared the problem right away and is still running great 9 yrs later.I just think that these engines are more adaptable than we give them credit for at times . Regards BobH.

[KiwiGF]

It's looks like it's lucky I got a 930 concentric with the bike well then! (it came with loads of "spares"
.....including a box of amal jets.....)