Troubleshooting a JAP Special

[Rockburner]

I guess I should also state that I don't want to be grinding material off the crank itself; I could grind down that nut and the crank-pin shaft end to get more clearance; because I don't want to mess with the crank's balance: I've no idea how well balanced it is.... but I don't want to mess with it!

[cardan]

"I fitted the 0.060 thrust washers on the timing side, and a 0.023 on the output side..."

I guess this is part of the mis-matched engine thing. It would be nice if there were no shims between the face of the crank and the bearing inner. Usually this is accomplished by shimming the bearing outer in the crankcase. As it stands, your shims are pretty large and heavy, and there could be quite a "hula hoop" effect as they spin around the main shafts (or the shafts spin inside them), which could cause damage to the shafts and/or the shims. In reality, if you're not going racing, it will probably be ok. But it would worry me. Maybe check with your JAP man?

You mention there is not much end thrust when the engine is vertical: true provided the piston is square and centred. You might be amazed to see the inside of an engine that has been running with a bent con rod!

Leon

[Rockburner]

"I fitted the 0.060 thrust washers on the timing side, and a 0.023 on the output side..."

I guess this is part of the mis-matched engine thing. It would be nice if there were no shims between the face of the crank and the bearing inner. Usually this is accomplished by shimming the bearing outer in the crankcase. As it stands, your shims are pretty large and heavy, and there could be quite a "hula hoop" effect as they spin around the main shafts (or the shafts spin inside them), which could cause damage to the shafts and/or the shims. In reality, if you're not going racing, it will probably be ok. But it would worry me. Maybe check with your JAP man?

You mention there is not much end thrust when the engine is vertical: true provided the piston is square and centred. You might be amazed to see the inside of an engine that has been running with a bent con rod!

Leon

I took a 0.050 and a 0.040 thrust washers out of the engine when I stripped it, and the internals seemed ok. The engine is pretty unstressed and won't be used for racing or anything.

The thrust washers are part of the design on the engine, it's not possible to adjust the main bearing liners, they're pressed into the cases, and the thrust washers/shims allow for the end first to be adjusted. This IS a 1930's design....

But you're right that a call to the expert is in order.

[Rockburner]

In regards to the issue of the crank-pin nut contacting the bearing liner screw......




I've had a word with the JAP Guru... and his advice was to simply take an angle-grinder to the crank-nut and make clearance!

Apparantly it's a common thing, the cases are "not" exactly precision made, and there's a lot that were cast by jobbing engineering firms after JAP stopped production, so exact fitting is not always going to happen. His advice was to figure out the desired shims on the crank (to get the piston and con-rod as central as possible), then see if the nut is striking the liner screw (and the casing), and then get the grinder out! 

He also suggested that if I wanted to confirm the clearance I've created, I can fit slightly narrower shims (eg ten-thou thinner), and see if there's contact.  If not, then I should be good for the lifetime of the engine.  Sounds like a plan to me.

He also pointed out; when I mentioned my concern about affecting the overall balance of the crank; that shaving a 1/16th off 1/3rd of the crank nut would be akin to taking a piss before a weigh-in... totally irrelevant! Which was reassuring.

So - that'll be the next thing on the list!  GRINDING!