Insipid oil return

[AndrewM]

I had already verified that with the oil pipe feeding up to the head disconnected from the lower plate, no oil came out when the bike was running. Rather than take the front plate off and check for rotation and stroke, I thought I would try extracting the guide pin and do a quick check for wear/damage. Unfortunately this didn't go to plan. There was a little bit of resistance when extracting the guide screw, but not much, but the result is shown in the image below.

Top image is the replacement part from the AMOC Parts webpage, bottom image is what came out of the engine. The good news is that the number of threads are the same so it looks like just the tip that has broken off, but whether it was already broken before I undid it, or broken during the process of extracting it due to it being worn or bent, I don't know.

Anyway, clearly something wasn't right with it, so I'll order a replacement part, and ponder what to do next in the 2-3 weeks it will take to get to me. I'll see if I can get a long slender magnet that I can insert into the hole and see if I can extract the guide tip so that I don't scratch the bore when I remove the plunger.

One major concern is whether it's possible to extract the plunger with the gearbox in place, or does this require a full strip-down, clutch off, gearbox out? Bike is a 1946 Model 16.

[AndrewM]

So it looks like I have been able to avert a disaster, I was very concerned that the broken tip was going to be jammed in the bore preventing the plunger from coming out. I took the oil pump end caps off and very gently rotated the engine, which it did without any problem. I then pushed gently on the plunger from the front at the same time and it slid out with no issues. When I looked for the broken off tip it was nowhere to be seen until I pushed a small screwdriver up the guide screw hole, and it popped into the bore.

The plunger looks to be in good condition as does the bore, so I should be able to reassemble all with a new guide pin in a couple of weeks.It looks like the old pin had been hanging on by a thread for some time, providing some reciprocal motion to the plunger but not enough for the rocker box oil feed hole to match up with its partner in the crankcase, hence no oil getting to the rocker box.

Fixing this should also improve the overall oil flow which I was concerned about at the start of the thread, so good news all around. Thanks for the advice everyone, it was a great help.

[clive]

I would replace the bolt too, the end of the thread looks dodgy. Also if you have been having feed problems to the valve gear make sure the gasket on the front plate is not obscuring the supply hole.

[AndrewM]

Mine is the 1946 model, so it’s a one-piece bolt that has snapped. AMC switched to the hollow bolt and hardened pin the following year, presumably to avoid this problem which would have been exacerbated by the higher loads of the two-start pump.

[SPRIDDLER]

As an aside, looking at the apparently undamaged annular guide pin groove in the plunger and its drive splines I don't think the guide pin has been broken for long or it took much force to snap off the end of it. The splines on the plunger are quite brittle and chip quite easily. Hopefully a new guide pin will sort it out.
Presumably you've confirmed the width of the annular groove which I think should be 3/16", requiring the one piece guide pin. It may be a trick of the photo but it looks to me as if it could be 1/4".

Just for info, two-start plungers (requiring the1/4" guide pin) normally have 2S stamped on one end.

[AndrewM]

Pin is 3/16”, groove in plunger a smidgen more than 3/16”, so not a 2S plunger. The broken end of the bolt is 90% black, not fresh metal, so it has been fractured for some time. There is a tiny patch of bright steel, so maybe the tip was just hanging on by a thread until I extracted it.

The annular groove is in good shape other than a tiny rough patch on one face at a point which corresponds to peak plunger motion towards the rear of the pump, so it looks like something was out of place. The splines on the plunger are all undamaged, just have typical wear that you would expect in an old bike. Very lucky not to have come off worse in my opinion.

[SPRIDDLER]

Pin is 3/16”, groove in plunger a smidgen more than 3/16”, so not a 2S plunger. The broken end of the bolt is 90% black, not fresh metal, so it has been fractured for some time. There is a tiny patch of bright steel, so maybe the tip was just hanging on by a thread until I extracted it.

The annular groove is in good shape other than a tiny rough patch on one face at a point which corresponds to peak plunger motion towards the rear of the pump, so it looks like something was out of place. The splines on the plunger are all undamaged, just have typical wear that you would expect in an old bike. Very lucky not to have come off worse in my opinion.

[56G80S]

Just to reassure you a bit more, the plunger looks to be in decent condition. The one I recently refitted had very minor surface scoring and so did the bore although the plunger splines were good. Nonetheless it is pumping better than it ever did since I bought it in 1976, new timing side axle and bush.

I'm pretty sure you will be fine.

Johnny B

PS I guess that's Perth Australia?

[AndrewM]

Thanks Johnny, yes plunger and bore look good, no scoring or other damage. Good to have a potentially major issue resolved by the replacement of an £8 part.

And yes, Perth Australia. Not the sort of place you want poor oil flow on a hot summers day