Oil scavenge problem

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
Kneedy
Posts: 9
Joined: Fri Sep 25, 2015 11:18 am
Location: SURREY UK

Oil scavenge problem

Post by Kneedy »

Hi, this is my first post on here, I hope you guys can help me.
I have recently completed the 2 year long restoration of my first Matchless, a 1956 G80S. It had been laid up 36 years ago by an old friend of mine (now deceased) following a catastrophic engine failure caused by the oil pump guide pin being incorrectly installed, preventing the pump plunger from turning. The engine had then been started, resulting in the destruction of the plunger, timing side crankshaft axle and bush and severely damaging the crankcase.

I sourced a 2nd hand crankcase half from a fellow AMOC member, fitted new big end, crank axle and plunger. The case was a good match and with new bearings and bushes everything spun nicely and I carried on with the rest of the build.

I started the bike for the first time a couple of weeks ago, all seemed well though it took around 3 minutes (in short bursts) to get oil returning to the tank. After about 5 minutes continuous running increasing amounts of oil started blowing from the crankcase breather. shut down, drained the crankcase (it doesn't wet sump when left stationary for a week or more), and tried again. Same thing but I let it run on until it became apparent it had filled the primary chaincase and was losing oil from the input shaft aperture. I drained around 750ml from the crankcase this time, plus emptying the chaincase.
I have checked the oil circuit again, thought I'd found the problem with a small air leak on the scavenge side with one of the drilling blanking screws only finger tight (my mistake) but sorting that has not cured the problem.

The replacement case showed signs of having been modified, maybe for competition use (drilled for sump plug wiring, grub screws fitted to retain cam follower guides) and I am wondering if someone has opened up the oilways which to my mind would reduce pressure available to the scavenge side (where the oil pump capability remains the same).
I checked it over thoroughly at the time but could well have missed something.


Thanks for reading this far, the bike looks lovely but is no good as it is, so before I pull the motor out again any advice would be most welcome
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clive
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Joined: Mon Jan 01, 1990 12:00 am
Location: LONDON UK

Re: Oil scavenge problem

Post by clive »

3 minutes to get oil returning to the tank after a rebuild is not excessive. You are getting an oil return but not at a fast enough rate to prevent the sump filling. Apart from an air leak on the return side (by the way it should not be screws on the back pump plate but it should be the little BA bolts, otherwise its a pig to tighten them up once the engine is in the frame. If you have got screws there and they were loose i wonder if you have actually prevented an air leak.) there are a couple of possible causes.
That the pump guide pin has been put into the wrong way round. The pin has a recessed section at one end and this must go into the holder not stick out into the pump. If the wrong way round it would reduce the effectiveness of the pump, but both supply and return so maybe not your problem.
Secondly that there is something partly reducing the capacity of the sump return drilling. Usually its a bit of broken piston ring. The drilling starts just above and in front of the sump plug if I remember correctly. You may well not have seen any blockage unless you checked by pushing something through the drilling to check it was clear (better to use air line if you have one).
clive
if it ain't broke don't fix
Kneedy
Posts: 9
Joined: Fri Sep 25, 2015 11:18 am
Location: SURREY UK

Re: Oil scavenge problem

Post by Kneedy »

Thanks for the reply, the oil pump end plates are indeed held by BA bolts, the screws I referred to are the ones used to plug the end of the oilway drillings during manufacture. I will remove the oil pump guide pin to check the recess as you suggest, I was not aware of that.
I have had the pump front end plate off and watched the plunger move whilst turning the engine over and its travel equates roughly to that generated by the helical groove, (using the old damaged one as a guide).

When I was building the bottom end I flushed out the oilways with white spirit and compressed air, and all appeared well (I have since retired and no longer have access to a fully equipped workshop, have to make do with my garage now!) and while investigating this problem have probed the oilways in situ, using the old damaged case as a guide, as far as I can tell they are clear but can't really get to the scavenge port into the oil pump bore, and that may have an obstruction as you say. I guess I could pull the plunger out from the front and force liquid under pressure up the scavenge drilling and watch what comes into the bore. I'll double check the plunger against the old one while its out too.
I suspect she's going to have to come down again.............
Spanielsam
Posts: 86
Joined: Fri Jul 03, 2015 4:16 pm
Location: LEICS UK

Re: Oil scavenge problem

Post by Spanielsam »

Hello
I had a similar problem with my crankcase filling up with oil even though some oil was returning to the tank
I had some excellent responses to my post which was titled "G80 wont start" , i dont know how to link that post but there was some great advice regarding the oil galleries including one that i had not realised was there and was partially blocked on my bike
......
Kneedy
Posts: 9
Joined: Fri Sep 25, 2015 11:18 am
Location: SURREY UK

Re: Oil scavenge problem

Post by Kneedy »

Thanks I'll have a look at your previous post Spanielsam. I've just realised the oil pump plunger won't come out of the front of the engine as its a bigger diameter at the back! If I find the problem I'll post it on here, but it may take a while.
StephenG80
Posts: 264
Joined: Sun Apr 30, 2017 10:42 pm
Location: ARGYLL UK

Re: Oil scavenge problem

Post by StephenG80 »

Spanielsam wrote:Hello
I had a similar problem with my crankcase filling up with oil even though some oil was returning to the tank
I had some excellent responses to my post which was titled "G80 wont start" , i dont know how to link that post but there was some great advice regarding the oil galleries including one that i had not realised was there and was partially blocked on my bike
You'll find that thread here:-) http://www.jampot.com/forum/viewtopic.p ... wont+start
:beer:
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les ward
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Joined: Mon Jan 01, 1990 12:00 am
Location: WILTSHIRE UK

Re: Oil scavenge problem

Post by les ward »

I knew someone witk similar problems to yours. He had fitted a NOS plunger, but it was from a Lightweight. The plunger was stamped G2 (very small) on the end. Might be worth trying a different plunger.
56G80S
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Joined: Wed Oct 12, 2005 10:45 pm
Location: N YORKS UK

Re: Oil scavenge problem

Post by 56G80S »

This may be entirely a red herring, particularly as you clearly know what you are doing, but whne you replaced the timing side axle did you ensure that the there awas a match between the axle and the and the plunger - single start versus two start?

You also probably know that it is important if you are inspecting / replacing the plunger to keep the engine still or it's a nuisance trying to "re-register" the guide pin into the plunger groove? Othyerwise its remove and end cap and fiddle about.

Good luck with this.

Johnny B
Kneedy
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Joined: Fri Sep 25, 2015 11:18 am
Location: SURREY UK

Re: Oil scavenge problem

Post by Kneedy »

Many thanks for your replies, having read Spanielsams original post which mirrored the symptoms I have, most helpful, I removed the oil pump plunger and end plates and was able to inspect the oilways with a pencil torch and mirror. I squirted WD40 through them and watched the resultant flow, all appeared well. (the new style WD can with ratcheting nozzle worked well for this). The guide pin was the correct way round.

I compared the new plunger with the damaged original (marked 2S for two start) and they were identical, apart from the damage, however I noticed there was considerable witness marking on the new one where the teeth had been in mesh, which I would not expect after a total of about 30 minutes running, mostly at tick over.

Having now run the engine up and the problem being exactly the same, I think you are right, Johnny B, and I fitted a single start axle. According to this months Jampot mag (good timing!) this would result in the pump running at half speed, thus not scavenging enough. The mismatch might explain the witness marks I guess.

I will split the cases to check, which will give me the chance to double check the replacement crankcase half, I'm doubting everything now!

As the old saying goes, if a job's worth doing it's worth doing twice...………... :headbang:

Thanks again, I'll post what I find
Mick D
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Location: Aberdeenshire

Re: Oil scavenge problem

Post by Mick D »

Hi

Whilst a mismatch between the driving gear and plunger could account for the witnessing I doubt that it is the cause of the poor scavenge - if the plunger is being driven at half speed both the pressure and scavenge pumps will operate at half flow, no reason for the sump to fill. When you strip the engine to check the drive gear you need to find the cause of the poor scavenging.

Regards Mick
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