Oil Pressure

Started by Alex, May 26, 2022, 05:52:37 PM

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The 500/600 32 have a sophisticated system with their oil backflow construction. I'm always amazed at what the English engineers did back then.
I let close complete cleaning and reassemble the machine first run with Plexiglas cover. The original cover is only installed when everything is running smoothly.

Rick Parkington

Understood Alex, glad it helped. Funnily enough your question has made me rewrite the lubrication notes I made when I built my first Sunbeam in the early 2000s because looking them out to refresh my memory i spotted some basic mistakes largely made due to evaluating it through experience of newer 'old bikes' - a sort of 60s parallel twin view of pre-war oiling. Since then I've had a fair bit to do with total-loss lubricated bikes and come to realise that the older stuff is basic but perfectly effective and was built around the technology of the day - not just the design of the bike but the customer of the time too, with his ball-pein hammer and 'King Dick' adjustable spanner! If these bikes had been complicated to maintain they wouldn't have been any use in their day. Generally they are pretty forgiving and we modern owners make it more complicated than it is - partly because they're now too costly to risk damaging!
Good luck with the fire up!
Cheers R


Hi Rick, thank you very much again for your detailed reply! That is very helpfull.

I did not take the telltale apart honestly as it seemed working fine to me. I was also able to pull it out further when I tried, so things add up. There where no grease nipples on my box, but maybe there have been originaly some but was taken off previously.

I honesly have to say, I have worked on many bikes of all types, just never on a british pre war bike before, I'm fastinated and surprised by the mechanicals sometimes at the same time.
I spend 2 years restoring the bike so I'm maybe a bit over cautious about everything, but in this case what you say makes sense to me and now everything is clear.

Now only one thing left, fuel "on" and go 8)

Best Regards

Rick Parkington

Ah, OK Alex, no the rocker lubrication is a bit more basic than that - generally relying on 'oil mist' - engine breathings going up the tubes. Prior to that, in 1928, rockers were exposed and grease lubricated and I thought the '29 rocker box has grease nipples on the rocker box too, thinking about it, so a squirt in there every now and then is about all you can do. 

Probably worth saying that these engines don't generate oil pressure as such. To get pressure you need flow AND restriction and since a roller big end doesn't offer any resistance to the flow - unlike a shell bearing, which is effectively clamped over the oil holes in the crankshaft - there is no pressure. It's like the tourniquet when having blood pressure taken or TT winners shaking the bottle of champers; there's not much pressure until you block it with your finger and then it sprays all over the crowd. The telltale creates its own pressure by closing off the oil supply with its spring loaded plunger. This has the added benefit of (largely) preventing the oil tank draining into the sump when standing but also means that when the engine starts pumping oil and finds its path blocked by the telltale plunger, pressure builds until it overcomes the spring and opens the plunger, allowing the oil to escape into the big end and crankcase. This releases the pressure until the valve tries to close again and a clever balance is created where the valve closes just enough to restrict flow and build up enough pressure to keep the valve open. It's primitive but pretty foolproof because unlike modern electrical warning devices, the indicator is part of the mechanics of the system - the telltale can only be open if the oil is pumping and if it is open the oil must be getting to the big end.
In your case, if the engine is turning slowly as on the rollers, the pump will still create enough pressure to open the telltale - it'll just take longer - but having done so, once the feed to the big end is open, the pressure at the telltale drops and the balance will be a smaller opening to the crank. That's why I say don't worry about it. The only thing that can go wrong here is down to 'the nut on the seat' as used to be said in the bike trade. When owners rebuild the telltale with extra sealing washers or a too long spring so that the plunger cannot fully open. IT's worth checking you  can pull the plunger a bit further out by hand than it's coming out on the starting rollers, then you know it's just balancing for low revs and can open further when more oil is needed at high revs. I just had a look a the cover of a 1930 ish engine I'm fitting and the big end feed opens almost as soon as the plunger moves; the later system with rocker feed is more complicated because in this case the rocker supply opens first and the big end after so if the telltale isn't right there's a potential for all the oil to go to the rockers - but your 90 is much simpler and if the telltale moves, oil should be getting through ok.
Cheers Rick   


Hello Rick,

thank you very much for your reply! You are right, maybe the starter is turning not fast enough, but I thought if it turns fast enough to start the Motor I guess tihis is about a low idle speed high enough to reach enough oil pressure.
My model dont have a pipe to supply to the rocker box. This is a 1929 modell. I couldnt find a description honestly descriping in the books I have how the oil circuit to the rocker box is going, but I guess the oil is pushed up with pressure in one of the push rods and down in the other one (?),  but I think oil pressure was not enough to push it up so I was a bit scared to start the engine without oil in the rocker box.

Best Regards

Rick Parkington

Hi Alex, 
Not sure what year your 90 is - I can't see a rocker feed take off - but my feeling is that the roller starter won't turn it over fast enough to fully open the telltale. If oil is coming out of the telltale when removed and it is returning to the tank I would start the bike and see what happens when it's actually running. If oil is returning there's obviously some splashing about in there and with a roller bearing engine that will be plenty to run it for the 30 seconds or so it will take to see what happens with the telltale. Also, if at the end oil is still returning it suggests it's also supplying.
The 1932 type enclosed rockerbox models, with a rocker feed take off just above the telltale and a tank mounted gauge, are a bit complicated but in brief the first movement of the telltale on these supplies the rockers and the big end supply only opens with further telltale movement that but yours doesn't seem to be one of those models so I'm not sure what you mean about no oil getting to the head?
Cheers Rick 



Just about starting my M90 Sunbeam, but before I actually do that I wanted to make sure engine lubrication is working.
I turned engine with a rear wheel starter without spark plug to see if tell tale is working (=oil pressure).

The tell tale comes out but I'm not sure if enough far. In the old instructions it only mentiones that it should come out. See the reading in the picture. The TT would need another 3-4mm to come further out to open the oil whole inside fully. I also checked cylinder head = no oil. Oil does flow back into the tank and also when I remove the Tell Tale oil is coming out.

My conclusion = not enough oil pressure. Maybe still air in the system or oil pump not producing enough pressure I guess?

Any experience or tips? I will continue another day to figure it out but maybe in the meantime someone has a good tip for me!