[terwog]

Thanks Rick...that all helps......terry.

[Rick Parkington]

Ah ok, well if it has a scavenge pump it will be recirculating. The Model 9 was total loss up until 1929 but the 90 and some others were circulating before that so you can use the oil control ring.   
Looking at my heads, for convenience I measured them from a steel rule laid across the surface and the drop to the centre of the dome came out at around 1 5/8" so allowing for the rebate to the mating face and the steel rule thickness I'd say that's about the same as yours. My heads don't have valves in them but the single port 1929 port diameters (ie just beneath the valve seat) are both 1.5" whereas 1932 and 1930 2 port 9 heads are 1 9/16". I think a 90 inlet valve is 1.75" od but may be wrong on that.
So yes I reckon you will get away with the Torsk piston, as you say Umbel seems the correct thing and is pretty close in dimensions.
Sounds like it'll be a cracking bike! 
Cheers Rick

[terwog]

Hi rick...my bike has return gears in the oil pump with a dry sump engine. I really don't know what bikes were total loss and what bikes were dry sump with a return oil line as I am new to these machines. The bike I am building is a sprint. A model 11 so it would have been born with some compression I would think.

If your combustion chamber shape and valve size are the same as mine on the '29 and '32 I would think this piston would be correct for my application. The dome vs the cone would be the largest difference I think...the 1/32 in compression height is not much ....the longer skirt is ok with me as it looks like sunbeam adopted that length in late 1925 for all the 500'sas shown on the hepolite page. Less rock in the piston but the shorter piston would have been a race style as we know it today.

Thanks for your help, terry.
My oil pump looks like the one shown here.

[Rick Parkington]

OK Terry, I'll have a look tomorrow. The piston in my model 9 is slightly domed I think but the one on my 600cc 9A is dead flat; never thought of it before. I did a ninetyised 29 model 9 last year and that had a higher dome so there are certainly some variations.
Generally I would say leave off the oil control ring on a total loss engine. It's important to remember that total loss is effectively wet sump with a tiny capacity. The oil gets very hot and dirty so the more you can leak/burn and replenish from the pump the better within reason. Without oil control rings they generally don't smoke too badly until the sump level gets too high.
As an aside, I ran out of oil miles from anywhere on my Rex Acme a few years back. It was nearly ten miles before  reached somewhere I could buy oil and I was amazed I got away with it until I realised that it was an advantage of total loss. Unlike a circulating system there is no scavenge pump draining the sump so once the oil level gets low enough not to be thrown up the bore in any real quantity, the burning stops and the level will remain the same - leaving, hopefully, just enough to keep the bottom end lubricated, so as long as you go easy it is likely that the worst that will happen is a nipped piston.
Cheers Rick

[terwog]

I meant to say "both heads" not engines.  I don't have any onther pistons or anything to compare....terry.

[terwog]

You are absolutely correct rick.....my piston is a low compression piston.....quite flat. At twenty over it is not original. I wonder what the bike was born with.? I am using a twin port head. I have a single port as well. The combustion depth from the contact face is 1.5" on both engines...I measured 0.014" deeper than that on the twin port....  If you could measure your depth and valve size for the 1929 I would appreciate it. My piston seems like a very flat piston for what looks like quite a deep combustion chamber.

The piston quoted for a 1925 twin port high compression engine is a 2661 umbel which is much closer to the torsk piston in dimensions.  This is the something like piston that I think the bike would have been fitted with initially as this has never been a road bike.

I think the new piston has an oil ring...would this be used if I purchased it ?...thanks terry.

[Rick Parkington]

Have you not misread the comp height, Terry? My 1938 Hepolite catalogue says the 'Talc' piston is 1.5", the  'Torsk' is 1 27/32" so that's 5/16" higher.
It is listed as high comp for a Model 90, I don't know how much the 1925 combustion chamber differs from 1931 - I think the valves are smaller on the earlier motor - but it might still fit but giving a much higher comp ratio. If your head is off,  I have 1929 and '32 heads I can measure for depth to compare if it's any help.
Cheers Rick       

[terwog]

Hi all.....I have a 1925 500cc sunbeam with a 0.020" over piston fitted. Std being 80mm...the piston that came out is hepolite Part number 3906 talc from the their catalogue. The bore is not worn too badly but the wrist pin is loose in the piston and the bore must be near its limit for that size. I am not sure what these old engines should be bored to....0.004" skirt clearance (or should there be a couple of thou. extra on these engines?) that would seem to be fairly loose for a new piston.

I need a new 0.040" over piston and see that F.W. Thornton offers this ...hepolite part number 3291 Torsk....it is specified as a 1931 piston ....I have been told it has a 7/8" pin and using the dimensions on the hepolite page is a longer piston than originally fitted to my bike but the compression height seems correct....the short 2-1/2" pistons were only specified for 1925 then they seem to go to a longer piston.....the 3291 is also quoted as a dome top not cone top like my piston. Does anyone know why this 3291 piston would not work for my application?    I have not asked Thornton re: the ring configuration yet....is this a concern or should the rings be correct? ......Thanks , terry.