Hi all, I have a question about this oversized piston from the club, since I've never installed one before.
https://marston-sunbeam.org/sunbeam_shop/index.php?id_product=836&rewrite=piston-for-m990&controller=product
The piston crown extends all the way to the edge of the piston. If the combustion chamber in the cylinder head still has the original radius of 80 mm, the new 81 mm piston should touch the edge at TDC. Or am I missing something?
I also found an M90/9 piston at British Only Austria.
https://www.vintage-motorcycle.com/en/parts/sunbeam/sunbeam-piston-m90-m9-040-81-mm
However, its dimensions differ from those of the piston from the club. Can somebody explain that?
Cheers, Thomas
HI Thomas,
The head is not 80mm. The cylinder head has a chamfer on to accomodate oversized arrangements see pic . As your argument would be the same for the oversized cone shaped piston as well as the domes. Hope this helps.
(https://i.postimg.cc/NMhq5rcC/head-1.jpg) (https://postimg.cc/7CNQRhbS)
Ps the JP pistion is some 50gms heavier than an original.
Thanks a lot wessex_man for this very important information! It seems that my head doesn't have this chamfer and I didn't measure the inner diameter when it was dismantled. :( The cone shaped piston for 6.5 compression ratio doesn't even reach the head with its crown.
Cheers, Thomas
I see your's is a Series 2 engine. I've posted a picture of a 1932-35 bolt down type head. Think the piston would still fit. Was your piston at top dead centre? If not You could raise the barrel with a spacer on the bottom of the barrel spigot and adjust the pushrods to suit so the piston doesn't hit the head and the valves clear. infact I think the 1929 spare parts list a spacer.
(https://i.postimg.cc/gJ8jgTPn/head-2.jpg) (https://postimages.org/)
Yes, I have a Series 2 maschine. I believe the piston was at TDC but frankly I am not 100% sure. I asked Paul Hutton about the domed piston and he wrote me that the 7:1 compression rate can be reduced by a 1/8" compression plate between the cylinder barrel and the crankcase. Aha, as you mention! This would deliver a compression of around 6.7, pretty close to the present 6.5. Paul spends his holidays and I still await a response. I would say the JP piston is out because of its weight.
I already watched if coating my original (and first) piston is an option and got the information that this is possible up to about 1 thou without boring the barrel (just slightly honing). I take that as a fall-back position.
I got the following message from Paul Hutton. It doesn't look good for the domed piston without head chamfer.
Quote... I can tell you that the diameter of the top land, adjacent to where the dome meets the body of the piston will be 80.46mm so the interference with the head, if there is any, will be a maximum of 0.27mm.
I have looked at the three heads I have on my shelf. All are twin-port M9s, and two are from 1929/30. These would have had the 4440 piston. I measured the diameter across the chamfer, and in each case found it to be 80.20/80.30 mm. Such a chamfer would not accept a +0.040 piston if the piston edge went up beyond the top of the barrel. I am certain that JML would not have designed a cylinder head/piston combination that would not accept an oversize piston. Indeed, the Hepolite drawing I have lists dimensions for all oversizes up to +0.040" (1.016mm).