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Kenny
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#854043 07/18/21 8:35 pm
Joined: Jul 2021
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Mercian Offline OP
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Hi all, I’m hoping for some advice. I have a 1947 girder rigid MSS that seems to fill the primary drive case with oil at an alarming rate. It appears to be coming from the crank seal, or lack thereof. I am lead to believe that for later bikes this is actually the way Velocette do the crankcase breather, is this the same for the older bikes? Is there any way to stem the flow? Like adding another breather and/or sealing the crank?
Thanks

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Joined: Jul 2021
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Mercian Offline OP
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Well, the Velo is still throwing oil out of the breathers like a lawn sprinkler. I added a new breather into the timing case under the magneto, that didn’t work. I blocked up the crank breather, that didn’t work. I reckon it’s either breathing really heavily or the scavenge pump isn’t doing enough. It’s just had a rebore and new piston/rings. I suppose it could be extremely knackered valve guides (which I haven’t actually checked) but my money is on the scavenge pump as the return into the tank seems a little sluggish. Is this a known problem with Velos? Can the scavenge pump be removed or tested without stripping the engine?

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Bry Offline
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Welcome to the forum.

Is this a recent problem since you had the rebore done? Could it just be the result of blowby if the rings have not seated.

Is the engine wet sumping when sitting?

You say that you added a breather under the magneto. For this modification, as used on later alloy head engines to work, you also need to add a hole through the timing gallery into the crankcase which really needs the crankcases split to do. The breather hose from the spigot that should be screwed into the hole below the magneto should be connected to the back of the later style oil tank, the iron MSS oil tank does not have this connection. I made this modification to my Venom, including adding the connection to the tank, but also left the breather hole in the crankshaft. But, in my opinion, this modification is not necessary for an iron head engine.

https://www.britbike.com/forums/ubbthreads.php/topics/229196/re-59-velocette-venom#Post229196

https://www.britbike.com/forums/ubbthreads.php/topics/350698/re-59-velocette-venom#Post350698

My iron head MSS is quite sensitive to the oil level in the tank and even a slight overfill will result in excess oil in the PCC. Same for the gearbox oil level. I can easily monitor this as I run the PCC dry (relative term with Velos!) using an O-ring primary chain. The oil mist from the crank breather actually helps keep the engine shock absorber lubricated. I usually drain off a small amount of oil from the PCC every 500 miles or so.

The oil pumps seldom give trouble unless debris enters them. To remove the pump the engine needs to come apart as it is a shrink fit into the crankcase and heat is needed to move them. There is a "filter", actually it's a plug with reduced clearances that is designed to catch any large debris before entering the pump. This can easily be removed and checked. If you are getting good flow back the oil tank - it will be intermittent as the scavenge side of pump is of a higher flow capacity than the feed side - then I wouldn't go removing it at this stage.


1937 Rudge Ulster (project)
1946 Velocette MSS
1955 Triton
1959 Velocette Venom
1966 Triumph T120
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Mercian Offline OP
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Thanks for the reply Bry. I bought the bike in its current condition although it had not been built very well so I’ve done a fair bit of work on it, including honing the barrel after it partially seized on me (which is how I found out it had been relined with a new piston). I don’t therefore know if this is a recent problem as I have no history with it. It doesn’t smoke and has bags of compression so I’ve got no reason to think the engine isn’t ok. It certainly looked good inside.

It has an oil tap to stop wet sumping (a dubious addition) so I don’t think that’s the problem. It tends to have about an egg cup full in the sump if I drain it after a run, not outrageous. The scavenge pump does return oil to the tank but not a massive amount, at least not compared to the other bikes I’ve got (T100 engined Tribsa and a DBD34).

There already is a drilling through the timing cavity into the crank case, it might benefit from another but I’ll try it as is before I strip the engine, again, to add another.

I’ve had the pump out (tight but not a shrink fit) and rebuilt it, nothing obvious, plus blew out as many galleries on the scavenge side as I could find. All seem clear.

I will TIG another union into the oil tank to connect the breather hose to, that should actually solve the symptoms but I’d like to solve the problem if I can.

Last edited by Mercian; 07/21/21 12:54 pm.
Joined: May 2006
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Bry Offline
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The pump needs to be an interference fit into the casing. If the casing bore is scored or damaged around the ports for the pump it can short circuit the oil flow or draw air. I am travelling at the moment otherwise I would have taken a video of my oil return to the tank for you to compare.


1937 Rudge Ulster (project)
1946 Velocette MSS
1955 Triton
1959 Velocette Venom
1966 Triumph T120
Joined: Jul 2021
Posts: 4
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Mercian Offline OP
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The oil pump is fine I think, it returns a good squirt every few seconds and visibly climbs up the clear tube fast enough. I must have looked at it In between squirts, dark tank, no glasses etc.

It just seems to spray oil out the breather pipe worse than any other bike I think I’ve ever had. I’m going to route the breather tube back to the tank and see how that goes, maybe with some kind of oil separator. I’ll probably check the valves and guides at some point to be sure they’re not a cause for concern but I’m relatively happy it’s not got anything actually wrong with it, it’s just a pre war Velocette with an oil fetish.

At least it’s not a Scott. They’re a wonderland of weirdness. Thanks for the advice.


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