G5 Rear engine plates

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
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Roderick
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Joined: Mon Jan 01, 1990 12:00 am
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G5 Rear engine plates

Post by Roderick »

Has anyone experienced difficulty fitting the rear engine plates on these models?

When I offer mine up to the engine & frame, with the stud at the engine inserted they seem to be about 1/8" too short to line up with the 2 holes in the frame downtube (under the seat nose). I know I'm rebuilding a basket case but according to the spares list I have access to, the rear engine plates (042388 Left & 042474 Right) are the same for the 250 & the 350 cc Lightweights so it doesn't seem to be a question of the wrong part.

None of the studs in the engine cradle have been tightened up yet.

Is this a standard problem & is there a wheeze for the best way to deal with it?

Rod
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Ozmadman
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Re: G5 Rear engine plates

Post by Ozmadman »

Are you talking about the two plates that support/provide adjustment for the gearbox? as those two numbers you mentioned are those. Have you tightened everything else engine wise up first as it is possible that the front mounting (down tube) has spread open a fraction putting everything out of line. I would tighten everything up first including the bolts through the bottom rail as well. But, mine were still a very tight fit and I even had to remove the layer of paint where they go round the gearbox as this wasn't helping either. Finally got the studs in the frame with a bit of gentle persuasion with a rubber mallet.
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Paul
1960 Model 8
1974 Yamaha RD250B US Model 6 speed
Rangy65
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Re: G5 Rear engine plates

Post by Rangy65 »

Hi Rod, yes i encountered the same problem. The rear engine/gearbox mounting plates were about 3mm short. I thought that the frame may have ' spread' a little. I fitted the frame to the engine using only the head steady and top front engine mounting bolt. Then i tried to pull in the lower frame tube with a ratchet strap. I could not get this to grip well enough or get enough leverage to do the job. So i opted for opening out the holes on the mounting plates just enough to allow the studs through. I have two G5's and they both have the same issue. Did the factory have a jig? What was also an issue is that with the plates fitted you can't 'slide' the gear box in, so you need to have the left plate fitted first then the gear box. When i fitted the gear box the plates sat even further away from the rear down tube. Took it to bits many times. In the end i used a dremel sanding drum on the radius of each bracket and just reshaped them a little so there was a more snug fit against the gear box.

I guess its just a case of a bit of 'fitting' :headbang:

Cheers
Steve
Roderick
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Re: G5 Rear engine plates

Post by Roderick »

Well, it's certainly a big relief to know others have struggled with these plates too & that it's not just me. I'll proceed along the lines suggested & report back.

Thanks,
Rod
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clive
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Re: G5 Rear engine plates

Post by clive »

My experience is not with a lightweight but may help. I recently tried to put the rear engine plates on a model 18 with the front engine plates and bottom half of the engine already in place. As with yours the rear studs failed to line up with about half the thickness of the stud out of place. After pondering for a while I remembered that when disassembling a bike to get the engine out to work on it, the second you remove the lowest engine stud which also holds the lower frame rails in place the frame springs apart by the same sort of distance.
So I am sure that your problem is flex in the top rail. I got mine to fit by two appoaches, firstly I supported the engine on a small jack and lifted it so that the frame was no longer carrying any of the engine weight, then I loosened off all the studs through the frame and front engine plates and finally i tweaked it all by using a ratchet strap on the downtube below the seat. The holes then satisfactorily lined up, no adjustments to the engine plates needed.

I suspect in the factory the engine gearbox and frame were assembled and supported on the bench, so preventing this problem, and the wheels and forks fitted subsequently.
clive
if it ain't broke don't fix
Roderick
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Joined: Mon Jan 01, 1990 12:00 am
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Re: G5 Rear engine plates

Post by Roderick »

Well, I got the gearbox in & the plates fitted yesterday & in the end they went together relatively easily. Huge relief on my part!

The main problem turned out to be the gearbox straps which had the wrong bend & were touching the frame cradle underneath the gearbox & pushing the latter up against the rear engine plates. Not much but enough to bring the job to a halt. I happened, fortunately, to have an unmolested spare gearbox strap to hand. When this was offered up to the gearbox & frame, it was possible to slid a sheet of kitchen paper towel between the bottom of the gearbox with this strap on & the top of the frame cradle underneath. This was not the case when my painted straps were fitted. I used this original strap to draw a template on a piece of cardboard. The painted straps I wanted to use were bent to match the template. The lower end of the these straps have a sort of slight swan neck, i.e. bend back slightly on themselves, & without it they will push against the frame cradle underneath the gearbox. I did the the paper towel test again with the painted & re-bent straps fitted & there was clearance.

The 3 studs through the rear engine plates then went in relatively easily. The one at the engine end was inserted first which is no problem. For the other two, I fetched out a tapering punch which reduces in diameter from 9/16" to 1/8" over 5&1/2". This went into the lower of the two holes for the rear studs & on the left hand side of the bike. A few light taps & its shoulders pushed the plate on this side far enough back for the hole in the plate for the top stud line up with hole for it in the frame lug & that stud to slid easily in. I then had to do the same thing on the far (right) side for this stud to be able to emerge on that side. With the top stud in, the holes for the bottom one then lined up & that stud just slid in too.

With that done, I was able to fit the wheel & handlebars & suddenly this pile of bits is a MOTORBIKE, not a finished motorcycle, of course, but definitely a very different animal altogether from what it was the day before.

Big thanks to all those who helped with their advice. It was all relevant.

Now onto the rear engine plate cover. The one I've got doesn't seem to fit.

Rod
Rangy65
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Re: G5 Rear engine plates

Post by Rangy65 »

Hi Rod, AMC classic spares provide a very good 'new' cover however its for the G2 250 so you have to modify it by cutting away apart of the metal work to fit. The reason for this is because of the different gearbox configurations in relation to single and duplex primary drives & accosiated differing lengths of the gearbox mainshafts.

The 1960 parts books lists the cover as Cowling,rear engine plate and the part numbers are 042404 G2 - 042643 G2CS and finally 044256 for the G5.

I have an original and a modified AMC classic item which after i trimmed it a bit it fits well. I am assuming that you may have the G2 or CS cowling/cover.

All the best

Steve
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Ozmadman
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Re: G5 Rear engine plates

Post by Ozmadman »

That's Interesting Steve about the different cowls, never gave it much though as I presumed they are all the same apart from the CSR which I thought may be chrome?. Can't fathom out though why there should be any differences as it just drops on the top where the R/H side fits neatly round behind the RH side engine cover and the L/H side fits to the top of the chain guard and just tucks behind the inner primary drive plate where the gearbox output sprocket is. Everything is in the same place casing wise and surely any slight difference in output shaft length/Primary chain are all happening on the other side of that plate behind the primary casing???
Paul
1960 Model 8
1974 Yamaha RD250B US Model 6 speed
Rangy65
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Joined: Tue May 07, 2013 11:35 pm
Location: LINCOLNSHIRE UK

Re: G5 Rear engine plates

Post by Rangy65 »

hi Paul,having not had the two ffty to directly compare with the G5 its difficult to fathom out!! Steve at AMC advised me originally on the new Cowls and how to modify them for the 350.

I guess its just one of those small differances that any one restoring a lightweight needs to be aware of the not so interchangability of parts :headbang:

Hows things by the way?

Cheers
Steve
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