ET: Engine (Twin) Dynamo Fitting

[Simon Dinsdale]

The Miller is not intended to seal there. The intention was that the prmary case can breath through the gap and if you seal it with silicone then it can cause condensation inside the primary case area. ET164 is pressed into the crankcase to act as a top hat so any oil running down the back area is diverted away and cannot reach the hole on the crankcase. PD28 is an oil thrower is so any airborne oil is spun off before it reaches the open gap. The important bit is the PD28 oil thrower and ET164 top hat should have minimum running clearance to work and if the gap between them is too large then oil gets out and its not unusual for owners to remove ET164 top hat from the crankcase because it was catching the oil thrower.
I have 3 twins on the road and one I have done over 50,000 miles and all have this gap and none of them leak oil at the dynamo area.
When I first got the Rapide on the road 30 years ago as a novice I followed the advice of the time and initially sealed the dynamo to the cases with sealant and got condensation and milky oil in the primary chaincase which is not good for primary chain life. I removed the sealant and set the dynamo drive up correctly and never had a problem since. I will admit all my bikes now have a proper oil seal on the driveside mainshaft so the engine doesn't breath into the primary case which may help.

 

[john998]

Hello Simon, Thanks for that, It just goes to show you never stop learning even after the best part of 60 years. It stops me worrying and removes one problem. Next time I pressure wash the engine will avoid that area.
Thanks again. John

 

[oexing]

Certainly the primary is not meant to be an engine breather case. Standard was an effective oil scroll plus its close fit spacer on the crank mainshaft. So only minimum breathing from crankcase was going in the primary - provided the timed breather in timing side was correctly set. Any gap behind trhe dynamo sprocket was just for venting air in primary from temperature changes cold-warm, so minimum air to exit via this gap only. Condensation in this space is a factor when there is a lot of crankcase air finding its way through the oil scroll into the primary - plus lots of short trips when any moisture got no time for drying out at higher temperatures. I found this a while ago in a Horex single primary after having lots of short trips for setting up the carb using lambda sensor in ex pipe.
So get the engine breathing right, refrain from short trips and condensation in primary should not be found. The oil slinger is not too bad , no real seals were suitable as the through bore for the dynamo could not be a tight fit in order to set up tooth mesh of dyamo sprocket into triplex chain - or so I guess . You can bin the oil slinger for a v-ring as it does not care for close fits, you can move the dynamo about for setting mesh in triplex chain. Just the v-ring lip needs to sit on the slightly polished side face around the dynamo through bore , a bit of abrasive on a flat piece for smoothing the rough alu part is allright. Maybe the v-ring can be mounted on the PD 16 or PD 33 drive adaptors , cannot say, don´t have these on my engines.
My thinking for breathing the primary is to use the gearbox filler tube as the lid has a breather job anyway for the gearbox. I drilled into this filler tube in an upward angle so hopefully not much oil from primary will end in the gearbox. Maybe some shield around the drilling can prevent oil collecting and end in the gearbox.

Vic