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A question i can not answer.

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K2-K6:
I'd say that was quite plausible flatfour but I don't know the sprocket sizes used as I've not worked on one with a chain guide.

The problem I think is related to chain tension and any pitch lengthening through wear. I also think it would be at risk with the throttle closed. When the front sprocket is pulling torque,  then the chain run from the wheel onto the front sprocket is reasonably contained with that run in tension. The slack all being in the bottom run will have a much wider acceptance of error in running onto the much larger rear sprocket.

If the chain, sprockets and tension are all ok, I can't see you'd get any problems.

If you've got a bit of wear in the chain so the pitch is lengthened and close the throttle, the bottom run goes tight with all the slack you've got coming onto that front sprocket. Ultimately, I think the chain pin tries to ride onto the top of the sprocket teeth, given the above conditions. It's this I believe trashes the cases.  As you point out flatfour, if you simply increase the sprocket size even by one tooth it's much harder to get the chain to ride over the sprocket.

You get exactly this effect on motocrossers, chains wear alot in that environment, they use quite alot of slack to cope with change in suspension geometry and when you, for example, land it from a big jump, one hell of alot of slack comes toward that front sprocket. They routinely still use a blade type guide round the outside of the front sprocket to avoid this problem.

Car cam belts are also interesting as one tooth number increase on the crank has to have two on the camshaft to keep timing correct so you are limited by space at the cylinder head.
VW PD engines brought this sort of issue to prominence in their design. The inlet cam is used to pump individual pressurised injectors for each cylinder, the effort on these is so high that it lengthens the pitch on the pull side going to the crank pulley. This comes in pulses so the pitch would fluctuate with belt elasticity, they resolved it by altering tooth form on the crank pully to (not change the pitch) but make the pulley slightly egg shaped and accept the varied pitch as it comes in to prevent a belt tooth meeting the top of a pulley tooth. If they met it would overstress the belt or more seriously move one tooth out.

I don't think a chain in good nick that fits the sprockets ok would ever ride up onto the top of a sprocket given the suspension geometry  change on a road bike is quite small.

Their original concern must have been if owners didn't maintain the chains within reasonable boundaries, and possibly moved it further out of reach with a sprocket spec change.

BigAl (Alan):
The early chain guard/splash shield fitment, it's all in Mark's blue book Graham

steff750:
 ;) im with k2-k6 on this one
 i have always been led to believe that the cases only got smashed on the engine over-run when the throttle is snapped shut and the chain links bunching up on top of the front sprocket ,rather than the chain snapping ,all the chains ive seen snap just run off the sprockets and onto the road?

Erling:
Can only agree with K2-K6 and steff750.
There was no chain guard on my CB750 in '69.
When i had the chance to have one in hand I could not fine room for it!
Friend borrowed my Honda for a trip to Italy, on the condition he fitted a new Honda chain, and tightened it every time he refuelled.Say 300km.
Was absolute certain he could not run the life out of my Honda.
Had done the trip to Italy several times already. Had got a BMW750. Pleased me more to ride the long trips on a flat twin.
Can't remember why I forgot to check him by refuelling in Hamburg, but passing Hildesheim another 200km oil cloud came out of the Honda.
Cost him 21 litres of oil to do the remaining 1000km to reach Monza racetrack.
A German I had met in the isle of Man helped cover the hole with resin, which is still in place!
The loose bits were removed from the bottom pan when home again.
11 years later '82, the Honda engine had been in a Seeley frame some 3 years.
On the way to Misano Italy in mid Germany entering the motorway from refuelling no engine connection changing to 3rd!
Looking back chain laid snaking on the road. Link had apparently opened.
I always carried an extra chain on top of there luggage.
Had still not reached the age of O ring chains.
No damage to engine from a "loose" chain like Steff750 says.
An O ring chain came into use, when a CBX 750 engine was shoehorned into the Seeley frame  4 years later.
Erling.
Come to think of it the story goes the Americans testing the prototypes had this very problem of engine damage from a loose chain on the overrun.

Trigger:
interesting comments. I did try something today which I have never done before, I tried fitting a 18 tooth sprocket to a engine with a chain guard. Yes, it will go on but, not enough clearance between the sprocket to chain guard for a chain. Put a 17 tooth on with chain and there is clearance.

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