A7 alloy head on A10 barrels?

[rocket man]

yes she looks great i would definitely put an alloy head on will just finish her off



dave

[tombeau]

Good looking bike.
Welcome.
Cheers,
Iain

[A10Boy]

Jim

I'm not about to mess up an original Gold Flash

Oh thank god for that, now I can sleep soundly at night.  

If its purely for cosmetic reasons and your Iron head is otherwise good, I would go LJ's route of a good quality black heat resistant paint on the barrells and Silver on the head.

It will be a great looking bike, and I cant help thinking I've seen it before somewhere.

Good Luck

Andy

Catalogue Nazi [whatever that is]

[MG]

The idea of fitting an A7 alloy head to A10 barrels is an interesting idea I have stumbled upon when rebuilding my GF. But I couldn't bring myself to do it with the otherwise very original bike. 
But afaik it was common practice to fit the 350 heads to the 500cc singles. The smaller diameter combustion chamber in the head creates a so-called pinch-off edge which creates turbulence and therefore better combustion, esp. at low revs. Back then this was called "poor man's Goldstar" 

The same effect would occur when using the 66mm diameter A7 head on a 70mm bore barrel (or even more when bored out). The piston should get as close as possible to the pinch-off edge in TDC in order to make this set-up effective. Furthermore you have to do some calculations concerning compression ratio (the compressed volume of the A7 head is smaller due to the smaller combustion chamber!). Probably flat-top pistons could just be fine.

The question that remains is the valve diameter. Maybe changing to larger A10 valves would be a good idea, considering the lack of resonance charging caused by the ignition spacing of our twins.
Otherwise one could compensate with more valve lift (Spitfire cam). Hard to tell without trying.

Nevertheless it would definitely be an interesting project, so if you decide to do so, please keep us updated!

Best wishes, MG

[900triple]

The question that remains is the valve diameter. Maybe changing to larger A10 valves would be a good idea, considering the lack of resonance charging caused by the ignition spacing of our twins.

MG  - interesting idea with the A10 valves in the A7 head...whats involved? New valve seats, guides? Would it be possible in an iron Star Twin head? Does the inlet track need to be opened up and if so to what size?

Cheers

[MG]

900triple - Well, it's a bit delicate. Definitely the valve seats have to be recut, maybe new seat rings have to be installed, depending on the size of the original ones. I have no idea whether they were the same for the A7 and A10 heads. But that's no problem if you have an iron head, you don't have the cast-in seat rings anyway.
You have to look what size of valves can be acommodated in the head. You also have to mind that the valves don't touch each other during operation (on TDC, during valve overlap). I just can't remember whether the valve angle of A7 and A10 is the same. The valve size mainly is limited by the head diameter and the valve angle. I guess the rocker boxes and rockers are interchangeable, so there must either be a difference in valve angle or valve distance.
Naturally you have to do some port work, otherwise flow would be restricted by the small diameter ports.
I run my A10 on a 30mm diameter carb and inlet, which works pretty well with the SR-camshaft (67-356) and the original valves. Could also be 32 or 34mm, or maybe twin carbs, depends on what you want. A smaller diameter and longer intake tract leads to more bottom-end torque and cuts power at high revs, which is not quite what you would want to have for top-level racing, but it's great for everday use.

Maybe an A7 head with original size could work pretty well, too, maybe with the hotter cam, like I said before.
I wonder if someone ever tried this?

I can't actually remember if the valve stems were different in diameter, but the A10 valves can actually only have a thicker stem, so you could just ream the guides to fit, if necessary (BEFORE having the valve seats cut!)

Nevertheless, could be an interesting experiment. Unfortunately I have restored my A10 and the A7 after each other, so I don't have a direct comparison of the heads.