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Discussion Starter · #1 · (Edited)
I've been refurbishing an old Bob Cat 21" self propelled mower. It has a 4 hp vertical shaft Briggs motor that is equipped with an auxiliary geared driveshaft at 90 degrees to the main shaft. The gearing is in a sump / mount ring casting that bolts up to the standard 4 hp block.

I'm trying to replace the motor and would like to try bolting the new motor to the original sump so as to use the rare gearing mechanism, which works fine. The two motors are as close as I can find and use the same crankcase gasket, so unless there are funky camshaft or other internal changes and whatnot, it might be a sensible swap.

Here's the thing. When I tried to remove the sump / mount ring casting from the crankcase on the old motor, it looks like a bolt may be located inside a small access panel on the underside and is behind the auxiliary shaft. I wonder if it's a bolt for some other use, as I don't see it on the IPL. Is there anything else I should be looking for that might be holding these two parts together?

Thanks for any thoughts.
 

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I wish had an answer for you but I haven't had the pleasure of digging into one of those. Maybe someone will chime in who has.
 

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I'm pretty sure that auxilary shaft is driven off the cam or crank for sure, not sure which. Also, the block surface may be different depending on where the shaft protrudes from the assembly. Never even seen an engine like this so I'm basing my ideas off videos I've seen or general operational speculation.

EDIT: Just looked at the PDF you attached. Definitely driven off the cam. If you look at the diagram circled, and the one above it, you see the cam for the dual shaft motor has a spiral gear molded into the end. So you'll have to reuse the old camshaft as well.
 

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Discussion Starter · #4 · (Edited)
Just looked at the PDF you attached. Definitely driven off the cam. If you look at the diagram circled, and the one above it, you see the cam for the dual shaft motor has a spiral gear molded into the end. So you'll have to reuse the old camshaft as well.
Yep. That's how I was looking at it too. I'm just wondering about splitting it apart; it may be nothing more than the crankshaft oil seal binding (hey - it never leaked...), but I'm a newbie to all this and don't want to force it.

Part of me wants to just toss the motor on and go mow something already, but the rarity of this little machine (as well as its off-the-chart cool factor) is mostly in the really oddball yet indestructible self propelled mechanism. Gotta have that working!

Here are some pics of the unit with the old engine. The new motor looks like the old one, only . . . well, newer.

PHOTOS: 1978 Bob Cat 21" self propelled C/I mower
 

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What's wrong with the original engine?
 

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Discussion Starter · #6 ·
What's wrong with the original engine?
Needs new rings, cylinder honing. The valves were stuck and are now free, but the end one may be broken and the valve seals are shot; there's an almighty clank when I rotate the crankshaft. The recoil starter was gummed up and I got that working freely. I Haven't taken off the jug yet. I think it also might need a new coil, but maybe not. "Other than that, Mrs. Lincoln, how did you enjoy the play?"

I think I can fix the old motor, but I told myself that a year ago and its still sitting there, and I've found it difficult to get parts. I took the motor to two different mechanics and neither one wanted to mess with it. The new motor has arrived and could be used for this or something else, but I'd like to mow something this summer with it one way or another.
 

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Just rebuild the thing. Its fun! Whats the model number on the engine?

I doubt it needs a new coil. Take the flywheel off and try cleaning and adjusting the points. If that doesn't work, and you can get a Magnetron coil, rip the points out and put a Magnetron coil on.
 
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