Magneto chain tension

[martinco5]

I’ve refitted the magneto to a G80CS engine but can anyone tell me how much slack, if any, there should be in the chain driving the magneto. I’m concerned at having the chain too tight causing ware in the magneto bearings. 8 inches between extreme ends of chain, 5.5 inches between the centre of the sprocket on the camshaft and the centre of the sprocket on the magneto.

Many thanks

Martin

[SPRIDDLER]

can anyone tell me how much slack, if any, there should be in the chain driving the magneto.

From the Owners Manual...........

Mag chain.JPG

[Jdraper]

Adjusting tension is all well and good, but sprocket alignment is also important, and difficult to adjust. A while back, I realised there was supposed to be a rubber sealing block between the magneto & the chaincase. I bought one from spares and this is such a tight fit I had to move the mag back a bit on the platform, moving the sprocket out of alignment. Looks like I will have to ditch the new part & fabricate something with more give..

[clive]

Adjusting tension is all well and good, but sprocket alignment is also important, and difficult to adjust. A while back, I realised there was supposed to be a rubber sealing block between the magneto & the chaincase. I bought one from spares and this is such a tight fit I had to move the mag back a bit on the platform, moving the sprocket out of alignment. Looks like I will have to ditch the new part & fabricate something with more give..

I too have bought the club seal for the SR1 magneto and found it too thick and solid to fit without putting the chain out of alignment. A number of issues come to mind. Firstly for a 1955 18S the parts list does not include any rubber seal. Secondly the correct mag plate does not allow any adjustment of the chain alignment.
I persevered with fitting a seal despite this and did as you suggest with softer material cutting it across the radius so it could be pushed into place without removing the magneto. I did wonder if the reason the club seal not fit on my bike was because my SR1 mag is almost certainly from a stationary engine and may have a different shape at the mating face to the timing cover.

[Jdraper]

Good to see it wasn't just me with the rubber block. I have had another look at the parts list for my '55 - no rubber block - no harm in checking! Wasn't aware of the plate issue, but the two platforms I have - one that was on the bike when I got it (42 years ago) and one that came with another mag both have elongated holes. Was this another '55 peculiarity? I'm getting fed up with these...

PS for chain lube, I use a modern anti-fling chain lube which seems to work really well and keeps the castings pretty clean.

[martinco5]

Thanks to you all who responded and apologies it has taken me so long to thank you.

[56G80S]

+1 about the mag to chaincase seal and I did as Clive mentioned - probably after reading Clive posting this solution elsewhere.

In common with JDraper, I have more than one mag and used an ex-stationary engine one while the original was away for repair. I did mark the underside of the mounting platform and then elongate the holes to achieve mag sprocket alignment with the camwheel sprocket although it's a pain doing so.

I recall Biscuit (or ITMA's) advice that more harm arises from a tight chain than does from a slightly slacker one and to find any tighter spot first and work from that.

JohnnyB

[sno335]

Just set the free play on my 54 G80.Because I fitted a NOS advance/retard unit I had to allow more than the recommended 1/4" or the springs wouldn't return the Bob weights to full retard. ( Inspite of plenty of oil on the mechanism).I think once the tension is set if I set the timing with the chain pulled tight on the driven side there shouldn't be a problem.
John

[Andy51]

Re the rubber sealing ring, it is important to keep water out of the mag chaincase. I went without one for many years, but a starting problem caused me to look inside and the Lucas ATD was seized through rust ('54 18S). I too found the rubber ring too large so I used a softer one that came with a replacement toilet cistern flushing mech, fitted well and compressed easily - I suppose my bike is no longer bog standard?!

[Expat]