Calling all engineers

**220 Ohms** · 8th November 2010, 17:03

I need some ideas on the best solution to a problem and possible good places to get it done.

Background:
I have been modifying an old scooter engine to accept a 2 speed auto transmission from a related model. All has gone well (and cheap) so far with only minimal engineering required, I finally get to the point of bolting on the sidecover for the last time and find it doesn't meet the crankcase mating surface anymore.

Due to a slight difference in the donor models used the thread on the crankshaft is
hitting the shoulder of the bearing seated in the sidecover and is stopping it from fitting correctly by about 4mm.

So far I can see 3 options

1. Bore the bearing seat deeper allowing the standard bearing to sit deeper in the side cover therefore avoiding the thread while still supporting the end of the shaft.
- Spoke with an engineer here in Wellington, could be costly ($400+) due to the setup required to machine it accurately but is the simplest idea.

2. Source a shorter bearing
- Haven't found any in the size required or similar sizes

3. Space the side cover out
- I have a number of spare sidecovers that could be used to make a spacer but that also requires machining and would affect the other 3 shafts which currently seat fine in the cover

I'm inclined to go with option 1 but need to keep the cost down as the whole project has only cost about $300 to date.

Any ideas or recommendations are welcome

**JMemonic** · 8th November 2010, 17:37

If you dont need to space it to far consider making a gasket out of either some thick gasket paper or some sheet alloy.

**kevfromcoro** · 8th November 2010, 18:05

Originally Posted by JMemonic

If you dont need to space it to far consider making a gasket out of either some thick gasket paper or some sheet alloy.

Yip.. I think hes on to it..
You are going to have to make up a spacer...
Alloy or plastic
Bit of work....
But it may do the trick

**220 Ohms** · 8th November 2010, 18:16

I'll have a look to see how big a spacer I could get away with, it all depends on the other shafts in the transmission.

**NinjaNanna** · 8th November 2010, 18:48

dumb question, can you get away with shortening the crankshaft thread by 4mm?

**Grumph** · 8th November 2010, 18:49

A narrower bearing is the clean solution - quote the dimensions of the existing one please...

A possible solution is subsituting a bronze bush - i'm assuming the bearing in the side case is merely an outrigger to take primary transmission loads ?
A bronze bush running in oil would be perfectly satisfactory and can be quite narrow for the low loads it would see.

**220 Ohms** · 8th November 2010, 20:49

Originally Posted by Grumph

A narrower bearing is the clean solution - quote the dimensions of the existing one please...

A possible solution is subsituting a bronze bush - i'm assuming the bearing in the side case is merely an outrigger to take primary transmission loads ?
A bronze bush running in oil would be perfectly satisfactory and can be quite narrow for the low loads it would see.

The existing bearing is a needle type 12x19x12 (bore x OD x depth, NK1212)

I thought about a bronze bush but don't have much experience with them so didn't know if it would be suitable, good to hear it's a possible option.

You are correct, there is a small centrifugal clutch running off it and it does sit in oil. The crank runs on a pair of roller bearings 20x47x14 in size and the engine only outputs about 3hp.

Would sintered be better than plain?
If I was to go for bronze I would replace more often also.

**Grumph** · 9th November 2010, 05:31

From memory - don't have my bearing catalogue - there are single row ballraces 8mm wide. 19OD is a tad small though...
I'd make up a phosphor bronze bush to suit. Life may surprise you...If the crank is true to reasonable limits there's very little going on outside the main bearings.
Allow .002-.003in clearance on the shaft and make the bush a light drive fit in the cover.
I deal with several larger motors which use an outrigger bronze bush like this and also feed oil into the crank via this bush. The bush condition gets looked at frequently as you'd imagine and they last several race seasons easily.

**220 Ohms** · 9th November 2010, 07:45

Thanks for the advice Grumph
I think I'll try the bronze bush route, this project has been about solving problems the simplest and cheapest way while having a bit of fun plus if it doesn't work out I still have a large amount of spares to rebuild with.
Should any engineering firm be able to turn one out for me?

I'll post up some photos tonight just for amusement