Re: Leak through front bearing

I sincerely doubt if the bearing is sealed. More likely, it's just shielded, especially if the "seal" is a solid metal plate. If so, it's perfectly normal for some fuel to leak from the front bearing. That's how it gets lubricated.

You're right - the bearing has metal on each side, so I should have said shielded. However, if leaking is inevitable, wouldn't this cause problems with ingress of air?

Russ.

Were you to remove the ball bearings and reinsert the crankshaft, you'd probably find that it fits pretty snugly without them. An oil film around the crank usually suffices as a seal against excessive air ingress. If you really think that it's a problem, you can change to a sealed bearing, but it's not likely needed. The sealed bearing has the disadvantage of holding polluted and/or nitrated fuel inside the engine, so if you store it nose-down, the front bearing may corrode.

On an engine that uses the crankcase as the power source for a fuel pump, crankcase sealing is critical. You'll notice that all the engines fitted with Walbro or Zama carbs have sealed bearings, but it's not usually needed on engines wherein the crankcase only needs to pump fuel/air mixture into the cylinder.

Geoff Sanders

I got hold of a dead ASP .40 from someone else in the club that apart from a snapped-off carby was pretty much pristine - it had only done a few flights before meeting with an untimely end. I took the front bearing from that, - it appears to leak a little less, but as you said, the crankshaft is a very close fit, and there is also channel from the carby hole forward to the bearing.

Anyway, having put new o-rings on the carby and taken the piston, liner and head from the less used engine, it runs well now, hitting about 13,500 RPM with a 10x6 prop on 5% nitro. It's a little ugly opening up from idle, but it will do fine for the trainer that it's in.

Thanks for the advice.

Russ.