594 ccs 500R & 126 Engines

[Rusty's Uncle]

On the 652 ccs cylinder heads there is a raised cross hatch pattern in the recesses of the exposed surfaces of the head. The 499 ccs heads are just plain. When I showed some cylinder heads to a specialist he pointed out that that pattern was there to increase surface area exposed to air to assist with cooling. I bought a 500R engine a while back and the cylinder head was plain like the 499. A recently aquired 499 engine had a cylinder head fitted that had the raised pattern but no 650 on the casting. The inlet port has a 26mm hole like the earlier 499s.
So the question is did the 500R share the same head as the 594 126s ?
I should add that both engines were not original.

[grubscrew]

 They were pretty much back to back in production, so I recon Yes.

[Bar Vitelli]

I agree, there is significant parts overlap with the 594cc 126 and the 500R and when I replaced my cylinder head and pistons the NOS parts I bought were described on the packaging as applicable to both.

[Rusty's Uncle]

I think that the hard bit here is finding someone with an original 500R to check it out. I remember reading somewhere that the 500R was restricted power wise by Fiat as they did not want it to be faster than the newly launched 126. So the 500R was rated at 18 bhp @ 4,400 rpm & the 126 23bhp @ 4,800 rpm. The 500R was fitted with a 24IMB carb, smaller than the earlier 500 models and the 126 with the 28IMB. I have also compared 500 F & L camshafts with the 594ccs 126 and they are the same.