Large Prop Bushing

[Andy J]

Procured several large wooden props in the region of 20 inch diameter but unfortunately I find all have been drilled out well above that required to match my engine.  By chance I do have some copper bar slightly oversized to the diameter of the prop hole.  Therefore am I asking for trouble if I attempt to sleeve the prop hub to get it to match the engine shaft?

 

As a test I did clean up the copper bar to an approximate diameter and did note that the prop hole would need to be reamed out as there are slight differences in the prop hole diameter front and back which adds to the complexity. Therefore wondered if I did machine up the bar to form a sleeve then this could be possibly fitted by heating it up to red hot and knocking it into place.

[Alan Hilton]

I have made aluminium bushes to reduce prop hole diameter loads of times with no problems .Just make sure the fits are good .I wouldn’t heat things . The crank shaft between the prop driver and the thread is usually a bit bigger than the threaded bit .I make the bush to fit the unthreaded part .only

 

[Outrunner]

I would avoid using copper as it's a pain in the backside to machine. Try brass, aluminium or steel for much better results.

[Paul De Tourtoulon]

Aluminum or even plastic / pvc.

[Nigel R]

Material is probably unimportant as long as the prop is accurately centred.

 

[Eric Robson]

Andy, don't ever think of heating up material to fit on the shaft, there will come a time when it needs to come off to replace bearings etc APC electric props come with plastic inserts to fit various size shafts, if you know anyone who uses these they may have some spare. I have made bushes from plastic and they have been fine just remember to make them shorter than the thickness of the prop as wooden props compress and need regular checks for tightness.

Edited February 24, 2023 by Eric Robson

[Andy J]

On 24/02/2023 at 08:02, Nigel R said:

Material is probably unimportant as long as the prop is accurately centred.

 

Any tips as to how to verify if the prop hub hole is accurately centred?

[Nigel R]

Nothing useful beyond making sure the bushing is as accurate as you can make it.

 

I guess putting it on a prop balancer would show you're close enough?

[Eric Robson]

What size is the prop hole and what size is the shaft?  If the shaft is 8mm and the prop is 10 which I find with my petrol engines,  up to 20cc are 8mm and 20 to 35cc are 10mm shafts. If that is the case with your props stock 10mm Ali. tube  1mm thick would do B&Q stock Ali tube 10 mm diam but I am not sure of the inside diameter 

[Andy J]

Props I purchased have a range of holes sizes.

An 18x8 is the worst at a  loose fit over the shaft of an 1/2 inch drill which needs to go down to 3/8. Also the hub has several 3mm retainng bolt holes similar to that required for a DLE engine which will need to be plugged.

The 19 and 20 inch props are better being 12mm shaft holes which again needs to go down to 3/8th. 

Could not find a tube that matched all the props so have purchased a length of 12.7mm aluminium bar just to try making a couple of bushes just as proof of principal.

Think given the need to also plug the screw holes on the 18inch prop the best option is to bin it.

Would have returned all three items, but given I paid very little for then it is not cost effective. So will use then as a bit of an experiment as to what can done with my mini lathe which gets minal use.

 

[Eric Robson]

Provided your mini lathe has better bearings than mine you should be OK. Use a centre drill to start the bore, less chance of drill wander.

[Andy J]

Yes do have centre drills Eric. Also believe it is better to open up the hole slowly to the required dimension using a succession of larger drills than the last is it not?

[Outrunner]

If you want to ensure the hole is true, best practice is to drill undersize and bore to finished dimension.

[Andy J]

Well the 12.7mm aluminium bar arrived yesterday afternoon so knocked up the first of the two bushes yesterday.  Not a big job but made the mistake of drilling out the bar to 3/8ths knowing the engine shaft was a 3/8 UNC thread. Unfortunately forgot that the end of the shaft has a slightly bigger diameter so whilst the bush does fit over the thread, that's as far as it will go! Back to the lathe it will have to go once I have measured up the engine shaft.

 

Do like this turning lark but yet to understand the relationship of the feed mechanism against the amount of metal it removes for each turn of the wheel as I find it difficult to see the markings on the tail stock and slides.  Did purchase a magnifying lamp which helps but need to improve the marking on the tail stock for drilling precise depths.

[Mike T]

On a small lathe I wouldn't (don't) rely on the markings.  Get it in the ball park with a cheap digital caliper, then creep up in it with trial fits.

[Andy J]

Well opened up the bush with a rat tail file and it now fits perfectly. Prop is near a perfect balance with the bush fitted  so will now have a go at the second one.