New build engine woes

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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New build engine woes

Postby Chessiegolf » Fri May 31, 2019 9:35 am

Here's hoping someone far more knowlegable than me can help.....
I have just taken reposession of a rebuilt 1960 G12.
The work done includes crank regrind, new main bearings, balanced crank and conrods, rebore, oversize new pistons and rings, new valve guides and valves.
The engine started and ran ok and returned oil to the tank, however it quickly leaked oil from the crankcase join. The rebuilder also said that oil seeped from under the heads of the new throat bolts so he put fibre washers on them.
I rode the bike home carefully (12 miles) but when I stopped I could see oil all over the rear right hand side: tyre, silencer, swinging arm, as well as a pool steadily forming under the engine.
The right hand cylinder smoked a bit at start up which the rebuilder said would probably disappear as the engine bedded in.
I can't yet see exactly where all the oil is coming from but will try cleaning it all up and running the engine while having a good look around.
What could possibly be so wrong that a newly built engine throws out so much oil?
I do not have much in the way of workshop facilities, or knowledge, though did think I could try th following:
* check that the feed and scavenger pumps haven't been reversed
* remove the sump plug and check for excess oil in the sump following a warm-up run
* check for spark plug colour differences (the smoking cylinder) - a new carb has been fitted and jetted according to the manual

I would be grateful for suggestions from anyone who has experience here.

Thanks

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Re: New build engine woes

Postby bob » Fri May 31, 2019 10:43 am

Hiya
Sounds to me like it needs to go back to the engine builder , I've had mine apart a couple of times and never had any leak problems .
You shouldn't need to be looking at it yourself .

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Re: New build engine woes

Postby Rob Harknett » Fri May 31, 2019 11:24 am

I have a G12CSR that was supposed to have had a pro rebuild, it pumped more oil out from the front of the crank case, than it did round the engine. There is quite an art in getting the crank cases to seal, so I understood. ( old prewar AJS of Wolverhampton stamped crank cases with matching numbers, when they built up engines ) I got John Bolton to rebuild my G12 CSR engine. He also discover, although it had new big end shells, they were chipped. Also the crank was oval. DIY may be cheaper, but there are some things you may need to leave to the more knowable. I would never touch a bottom end. To end up paying the price to be a learner. Sooner pay to have a good job done. I do not feel one bit ashamed of doing that.

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Re: New build engine woes

Postby Mick D » Fri May 31, 2019 12:39 pm

Hi

Definitely take it back to the builder, if he's reputable he will be pleased you did and sort the problem, if he baulks at the return demand a refund.

It sounds like the crank case breather is not working but it shouldn't be down to you to resolve. Give the engine a good clean, fire it up on the centre stand and observe where the oil originates - and report that to the builder, (take photos if possible).

Don't get disheartened, it will be a good bike one day :)

Regards Mick

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Re: New build engine woes

Postby MalcW » Fri May 31, 2019 3:12 pm

Firstly, I agree that the location of the leak should be identified, and then return the bike to the builder. You could also take photos of the oil on the rear tyre to emphasis the risk factor.

Just a thought, but since the oil seems to be on the RH side, could at least some of it be from the dynamo/timing case seal (if there is a dynamo), or the magneto/distributor joint, or the oil pipe unions on the engine?

Also, what are 'throat bolts'?

Malc

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Re: New build engine woes

Postby Chessiegolf » Fri May 31, 2019 4:19 pm

Thanks for the responses so far.
I do intend to take it back, in fact I've already spoken to the rebuilder about that, however I thought I'd try a bit of investigative work first so that I could identify the probable cause to prevent him attempting to charge for work he may claim was not his responsibility. He disassembled the engine and rebuilt it, the crank was reground and balanced by another engineering firm, so he will probably take a look but that doesn't mean he will be particularly keen on expending time and money on something which is not going to earn any more cash!
The throat bolts are the three 5/16" bolts which are listed as crankcase clamping throat bolts (part no: 014292).
Where do I find the crankcase breather?
Unfortunately I'm now away from home until Monday so can't get on with looking for the cause but thought in the meantime I'd ask the knowledgeable people here for their ideas.
Thanks for your help

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Re: New build engine woes

Postby 1608 » Fri May 31, 2019 4:32 pm

The usual places for the twins to leak oil are the dynamo to the crankcase, btn barrel and crankcase mouth ( there is an oil channel near this point) and famously the crankcase halves where they meet at the oil filter tunnel. I had a leak on the oil feed union where the short piece of tubing is soldered into the brass banjo, just needed cleaning and re-soldering. I agree with the above, if it was a supposedly professional builder then return it for remedying. Its not likely that the oil pump's in and out have been reversed, but the flow and return pipes maybe.

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Re: New build engine woes

Postby bob121 » Fri May 31, 2019 6:30 pm

Just to confirm, he handed the bike over with it already leaking, or did start leaking after you took collection. If he's handed it over already leaking, do you really want him touching it again?

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Re: New build engine woes

Postby Duncan » Fri May 31, 2019 6:36 pm

It is important with the twin engines to ensure that the joint faces are good , there should be a (very) thin paper washer around the filter gallery between the crankcases or alternatively a full face gasket can be made. Ask if one was fitted and what jointing compound was used on the joint faces.

The top front crankcase bolt has an oil gallery around it that feeds the rockers and cam followers via the oil gallery between the crank case halves a dowty washer should be used here a fibre washer will not really be up to the job as it may not maintain the tension on the throat bolt.

As mentioned previously the Dynamo is another culprit, do not be tempted to over tighten the draw stud as it will strip.

Don't get too despondent a little oil goes a long way and it may be an easy fix.

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Re: New build engine woes

Postby Duncan » Fri May 31, 2019 6:39 pm

Another possibility is an overfull oil tank that will blow oil out of the oil tank breather.

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