Building a G80 Engine from Parts advice

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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rannsachair
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Building a G80 Engine from Parts advice

Post by rannsachair »

I have a couple of G3LS's and fancied a little more oomph so picked up a 52 Model 18 engine.

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I had the casings vapour blasted and one crankcase half was corroded through:


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I am going to put the engine in my 55 G3LS and use Matchless inner and outer timing cases.


I have had the barrel resleeved with new piston rings on a good standard wire wound piston.


My problem is the corroded crankcase - I could try getting it welded up or alternatively I picked up a spare 55 G3LS crankcase set:


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so what is best option:

1. Weld up Model 18 Crankcase?

2. I understand that casings are matched - It would not be a good idea to just swap out the corroded casing with one half of the 55 one?


3. Use the 55 crankcase - then do I make a new bush to fit the smaller shaft on the 52 flywheel? I understand that the flywheels are different - would the rest fit?


4. Look for another earlier set of crankcases?


Any advice appreciated.

Thanks
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Rob Harknett
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Re: Building a G80 Engine from Parts advice

Post by Rob Harknett »

The timing cases seen with the bulge for SR1 AR mag will be 54 onwards 500 or 55 onwards £50
rannsachair
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Re: Building a G80 Engine from Parts advice

Post by rannsachair »

Rob Harknett wrote:The timing cases seen with the bulge for SR1 AR mag will be 54 onwards 500 or 55 onwards £50
I have a spare:
Image
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Rob Harknett
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Re: Building a G80 Engine from Parts advice

Post by Rob Harknett »

Years etc apply to both the AJS & Matchless timing covers.
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clive
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Re: Building a G80 Engine from Parts advice

Post by clive »

I would only change the timing side bush to fit the earlier crank if you are very confident about your machining skills. It needs to be heat fitted into the crankcase and then pinned as well as having the correct cut out for the pump. Thinking about it I am now wondering if the 55 cases have the oil pump bore in a slightly different place to take into account that the later cranks have a different size oil pump drive pin. Probably easier to either get the case welded, looks a relatively simple job for a competent ally welder, or buy some earlier cases. Of course it is difficult to tell if they are mismatched anyway!
On the timing side cover you do have the inner case as well don't you, the Matchless one cannot be fitted to the AJS inner case.
clive
if it ain't broke don't fix
rannsachair
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Posts: 7
Joined: Sun Jun 08, 2014 7:13 pm
Location: Argyll, Scotland

Re: Building a G80 Engine from Parts advice

Post by rannsachair »

clive wrote:I would only change the timing side bush to fit the earlier crank if you are very confident about your machining skills. It needs to be heat fitted into the crankcase and then pinned as well as having the correct cut out for the pump. Thinking about it I am now wondering if the 55 cases have the oil pump bore in a slightly different place to take into account that the later cranks have a different size oil pump drive pin. Probably easier to either get the case welded, looks a relatively simple job for a competent ally welder, or buy some earlier cases. Of course it is difficult to tell if they are mismatched anyway!
On the timing side cover you do have the inner case as well don't you, the Matchless one cannot be fitted to the AJS inner case.
Thanks, that is helpful, I quite like machining as a hobby, I have a small myford:

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And can do limited milling on it:
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I will put the castings on my surface table and make some measurements of the relative position of the oil pump. though sounds like a weld repair is the safest bet. I can weld MIG steel and stainless though I will ask a local welder if he can TIG the alloy.

I am on the lookout for an inner case, though I can pinch the one of my 55 G3LS if I do not get one in time for the rebuild.
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