Bushing Material 1

[Beezermike]

I run a couple of old BSA motorcycles and they use a plain bearing bush on one side of the crankshaft as a main bearing (the other side is a roller bearing). Since BSA went to the wall in 1973, the replacement bearings currently available are rubbish - little better than brass!
Can anyone advise what material (presumably some sort of bronze but I'm open to suggestions) would be best to have some bearings made. Rough sizes are ID 1.5" OD 1.75" approximate developed power of the engine is in the region of 50HP at 7000 rpm (all approximate) and the main bearings are about 8" apart.
I'd be very grateful for any advice.
Thanks.

 

[Metalguy]

I'd use "Oilite" bushings. They have some lube soaked into their porous material. I almost bought a new "Lightning Rocket" back around 1965. I knew about that bushing. Glad I didn't buy one!

 

[kenre]

Is the bushing pressure lubricated or splash fed?

Ken

 

[Beezermike]

Thanks Metalguy, where can I get "oilite" and what is it? I presume it is easily machinable or would I need to provide the manufacturers with finished product dimensions (as you will know, these bushings require reaming to fit after being pressed into the crankcases).

 

[Beezermike]

Thanks Kenre for your involvement. The bushings are pressure lubricated and the oil pump delivers the oil charge into a gallery surrounding the bush. The oil is then fed into the bush by way of drillings in the bush. The oil is then fed into the crankshaft from the main bearing bush, so part of the problem is obtaining a satisfactory material which has high wear resistance (but will not cause wear of the crank journal) to help reduce the tendency of oil pressure reduction at the crankpin bearings because of high leakage rates at the main bearing bush as well as obviating vibrational problems associated with crankshaft movement in "out of tolerance" bearings.

 

[kenre]

Thanks for the description Beezermike.

Oilite Would not be suitable, too soft.

Since it is pressure fed i would use Phosphor Bronze. Be sure to have a generous radius on the oil drillings. There maybe better choices in bronze material that someone can chime in on.
I have used Phos bronze for bigend bushings in model r/c racing boat nitro engines successfuly, 2 stroke so no pressure feed at all. They survive 20,000 Plus rpm fine on 40% nitromethane.

 

[Beezermike]

Thanks Kenre. I was just wondering if there was anything better (more modern) than phosphor bronze but I agree, it is a suitable material. The original bush was some sort of bronze coated steel backed shell pressed into a steel holder that, in turn, was pressed into the crankcase. Most of the items these days are all bronze - or not (they appear to be more of a brass than bronze). I was wondering whether the reproduction all bronze bush might deform under working conditions - ie provide less rigidity than the original steel insert? I just don't know enough about automotive engineering to know what to use!!

 

[patprimmer]

Why not search the bearing manufacturers catalogues and find a modern bearing with about the right journal size, then adjust the crank to suite.

If the crankcase main bearing bore was to small, it can be bored a little. If to big, an insert can be fabricated. Similar mods are made to 400 CI SBC engines to run 350 cranks.



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[kenre]

I would certainly go with patprimmers idea!

Ken

 

[Pressed]

That is a Garlock Bushing and is very special.

http://www.ggbearings.com/

And I wouldn't do any kind of reaming on it. It is a steel ring with a PM bronze interior followed by a layer of PTFE. But, there are other kinds now and other manufactures.

Your bike might not use that exact combo of material on it's bushing. You might find an exact match there or elsewhere. I would not think they had a special compound created just for them. Special size, that is a different story.

_______________________________________
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[Beezermike]

Thanks Patprimmer, the notion is a sound one, but the main problem is that these engines go back over 50 years and many of the cranks are on their last regrinds so altering the crank journal is just not an option. In fact, what I say to those that ask me, is to recommend they find a machinist who can just clean up the crank journal then manufacture a bush to suit rather than try to get down to the next size as stated by BSA! It helps the engines last a bit longer!

 

[Beezermike]

Thanks Pressed, I will look closely at the Garlock link - it seems phosphor bronze is probably the material to stick with!
Thanks to all who have contributed - I'm very grateful for your input.

 

[kenre]

Is there a particular reason the bushes wear?
the design of the engine? which would be hard to solve.
Making sure the oil pressure is adequate may help the bushes life.

Ken

 

[Beezermike]

Thanks Ken. I imagine with vertically split crancases there will be a whole range of issues but suffice to say many "home mechanics" do not get the cases aligned satisfactorily, I doubt I could get them much better and know a little bit about it!! In a nutshell, it's not a good design but we love our old motorbikes and I'm just exploring the possibilities of modern bearing materials. I've searched high and low through manufacturers and metallurgists and one machinist I spoke with mentioned a material I have been unable to locate - I'm afraid I can't remember its name but I would if I saw it!! Some sort of aluminium bronze I believe. It could be that he was pulling the wool!
I have had a pressure block made to test the oil pressure blow off at the relief valve and I am also experimenting with synthetic oils but as these bikes were not equipped with cartridge filters in the supply line, I've had to resort to fitting one in the return line. This is never as successful as in the pressure side of the supply line but is better than nothing and is a necessity with oils laden with a modern additive pack.
Again, thank you for your input and please don't stop questioning my methods or motives - it's all too easy to become blinkered without anyone to bounce ideas off!
Many thanks.
Mike.

 

[kenre]

Hmm, some sort of Aluminium bronze....?

Could have been AB1, AB2, or CMA1, which is a manganese aluminium bronze. Have a search through the material specs which there are many on the net and see if they can help jog your memory.



Testing the the oil relief valve pressure is a good start, but more important would the running oil pressure, especially when hot. The valve only regulates max pressure, and stays shut at anything below.

Hope this is of help

Ken

 

[Beezermike]

Thanks for the useful link Ken, CMA1 rings a bell!! I agree entirely with the running oil pressure notion, but as the oil gallery is a fixed volume and the pump speed increases with engine speed, an oil pump would have to be really knackered not to maintain running pressure (on these engines it is in the region of 40 psi) even with the oil at running temperature. Having said that, I have seen a number of well worn oil pumps!!
Thanks for your help.

 

[kenre]

A worn pump certainly wont build oil pressure, but it is the clearances between bearings and shafts that detirmine the pressure.

I would have placed my money on it not being CMA1 !
not that i am a betting man!

I cast r/c racing boat props using CMA1. is was designed especially for props, being quite fluid when molten so it fills molds well. Very slightly stronger than AB2, but just as difficuilt to machine!! never heard of it used as a bearing material, nor seen it available in any forms other than ingots, but not to say it isnt available!!

ahh the fun of playing with old english bikes!! not me, but have a mate who loves his Ajs

Ken

 

[Beezermike]

Hi Ken,
yes, worn bearings do have a tendency to reduce oil pressure but that's why I'm after the best material I can find to help reduce the wear. When I rebuild an engine I know the tolerances are good - by the way, my enquiry relates entirely to engines I rebuild and I do not mean to suggest that I'm looking for the panacea of all ills with everyone else's engine!
Anyway, it rather seems a phosphor bronze bearing is as good as I'm likely to find! I don't mind experimenting with materials but it's the time involved in extrapolating the results!!
Cheers,
Mike.

 

[Tmoose]

The location of the oil hole relative to loading can really help (or kill) a bearing.
Don't cling desparately to the journal size. The crank journals must be round withing 0.0005 inch, straight and concentric, around 10 uinch finish, and be directionally polished to avoid the "petting a porcupine" effect busting thru the oil film in times of trouble.
A full circumferential groove in either the crank journal or bearing bore reduces the load capacity a lot.

Are the rod journals fed oil thru a small journal outboard of the main ?

 

[Beezermike]

Hi Tmoose,
Thanks for that. Yes, the main journal has an oil groove and the big ends are fed from a drill way from the main journal - but that's the engine design and short of fitting a race bearing in way of the timing side journal and a quill to take the oil directly into the big ends, there's not a lot of scope for change! I'm not so much after radically altering the engine, just want to keep the old ones running for as long as possible on the best possible materials.
Thanks,
Mike.