G5 CRANKCASE BREATHER OIL LOSS

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
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G5 CRANKCASE BREATHER OIL LOSS

Postby chesp » Thu May 30, 2019 5:05 pm

Hi Everyone,
This is my first time on the forum with a problem of oil leakage from the crankcase breather. I have owned the G5 for 44 years but it has been in bits for 40 of those years.I have recently completed the rebuild over the past year and it is complete and running. On the first road test I noticed on my return, oil had been discharged from the crankcase breather, approx 1.5 pints lost over 20 miles.
I have checked externally the tightness of lock bolt for the breather stator ring and that was still in place (the later type of breather is on this engine, separate lock bolt and breather hole in crankcase). I have also changed the felt oil filter for the wire mesh type in case oil flow was restricted back into the integral oil tank. Unfortunately this has not solved the problem. I know that a small amount of oil escapes through the breather oil but not to the extent I am experiencing.
If any one has experienced this problem, I will look forward and appreciate any suggestion to what is the cause and remedy is to this problem.

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Re: G5 CRANKCASE BREATHER OIL LOSS

Postby clanger9 » Thu May 30, 2019 9:07 pm

Daft question, but is the oil level in the tank too high?
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Re: G5 CRANKCASE BREATHER OIL LOSS

Postby chesp » Fri May 31, 2019 10:12 am

No. Oil was measured in at 2 1/2 pints and top level sits roughly 1/2" below the oil return hole in crankcase.
I have also ensured that the small breather hole between the crankcase and the oil tank is clear.

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Re: G5 CRANKCASE BREATHER OIL LOSS

Postby clanger9 » Fri May 31, 2019 4:13 pm

Hmm, there are a few things you can look for if it's not scavenging properly:
- Is the "plug" end of the oil pump properly sealed? The big screw plug can be tricky to get all the way home. Needs the gearbox out to check this.
- Is there a blockage at the scavenge pickup? This is just above the rear sump plug, so you can remove the plug and have a look inside.
- Is the return oilway clear at the bottom of the filter housing. There's a cup washer down there for the felt filter that isn't needed with the mesh filter.
- Is the horizontal oilway from the top of the filter housing clear at the crankcase joint? If you used the dreaded silicone to seal the flange then it's possible for a blob of sealer to get lodged here. Maybe give it a poke through the return hole with a pipe cleaner.

Finally (again, apologies if it's a daft question, but after a rebuild anything's possible): you did put the oil pump guide pin in the right way round?
1966 AJS Model 14 250 CSR "cafe racer"
1993 Ducati 750SS

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Re: G5 CRANKCASE BREATHER OIL LOSS

Postby chesp » Fri May 31, 2019 4:54 pm

Thanks for those suggestions. It seems that I am not going to get away without a partial strip down.
I will check those first two items. I have checked the remaining three and the are satisfactory.
Again, thanks for your help I will let you know how i get on.

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Re: G5 CRANKCASE BREATHER OIL LOSS

Postby Ozmadman » Fri May 31, 2019 5:45 pm

You mentioned checking the breather stator bolt but is there definitely a breather rota on the crank. It fits on the crank and runs within the stator ( called a timed breather) I only mention this as mine was completely missing when I rebuilt my engine. If it is missing then the engine is constantly breathing
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Re: G5 CRANKCASE BREATHER OIL LOSS

Postby chesp » Sat Jun 01, 2019 5:22 pm

I am quite certain i fitted the rotor but if all else fails I will be splitting the crankcase for examination. At this moment I am trying to avoid doing that. Thanks for you input

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Re: G5 CRANKCASE BREATHER OIL LOSS

Postby clanger9 » Sat Jun 01, 2019 6:19 pm

Before you take it to bits, it'd be worth taking a close look at the return feed. Is it a healthy "pulse" or is it bubbly/frothy?
Paul posted a good video a while back showing what a healthy return feed should look like.
1966 AJS Model 14 250 CSR "cafe racer"
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Re: G5 CRANKCASE BREATHER OIL LOSS

Postby clanger9 » Sat Jun 01, 2019 6:28 pm

Ah, I just had another thought: you mentioned you've changed the felt oil filter to the earlier mesh type.
However, the later engines have a longer filter to cope with the higher oil pump output. I'm not sure if you can/should mix the early mesh filter with the later crankcase. I guess the early filter it could limit the return flow. Just a thought...
IMG_0215.jpg


Before you dismantle it, I'd double-check it has the correct filter for your crankcases.
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1966 AJS Model 14 250 CSR "cafe racer"
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Re: G5 CRANKCASE BREATHER OIL LOSS

Postby chesp » Sat Jun 01, 2019 7:12 pm

The return is 'pulsing' but not as strong as the one shown in Paul's video. Possibly hinting to an oil flow problem/lack of scavenging in my engine. The wire mesh filter I have fitted is bigger than the felt filters, it is the filter that was in the engine when I dismantled it. The filter occupies the full filter housing in length and diameter. There is no metal cup washer at the bottom but there is a holed rubber sealing washer. The opposite end is attached to the chromed screw plug.

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