Thread: BSA Motor repair.

I have a friends late 40's BSA motor bike engine to repair. It is a single cylinder motor and the bronze bush in the conrod little end needs replacing. I have a new bush which I will fit but I am unsure of how critical the reaming operation is.
I have both a hand and machine reamer of the correct size (3/4") but would be interested in others opinions on which to use and how critical the alignment is.
The bottom end of the motor is fully assembled still and the owner does not want to take it apart as it would mean taking the actual crankshaft apart.
Any comments would be appreciated.
Thanks, Mm.

If no suggestions are made, maybe have a look at other forums. There's bound to be a number of technically orientated English bike forums online. I imagine that having the rod attached to the crank compounds the alignment problem.

Good luck Phillip,

BT

Phillip,
'
You might have luck with some of the links here -

The BSA Club of NSW - links of interest

BT

As this could apply generally to other metalworking, I'll answer the question here.

I also have a BSA single and other bikes, and have renewed small end bushes with the conrod still in the crankcase. This is the method I use:
Turn up a bush of leaded bronze, with the ID equal to the gudgeon pin diameter, and the OD about one thou bigger than the conrod hole. Press in the bush. The ID will decrease a bit due to being a press fit. Use a brake cylinder hone with kero to size the bush to the gudgeon pin. Clean thoroughly when done.
A hand-held reamer is pretty much guaranteed to produce a hole that isn't truly round, and this method avoids that problem. However, if you want to use a reamer, arrange for as little metal to be removed as possible (see above), and stop reaming when the gudgeon pin is still somewhat tight to get into the bush. It'll bed down in service and improve the fit. If you ream until it's a nice easy fit to begin with, it'll only bed down further and you'll end up where you started with a sloppy small end bush. That's because it's almost impossible to avoid getting high and low areas when using a hand-held reamer.
Some people raise an eyebrow at the idea of using abrasives on a bush, but in practice it isn't a problem if it's cleaned out properly.

Jordan

Jordan,

I have never used a cylinder hone or a brake cylinder hone. I don't want to side track Phillip's thread so I was wondering if you could describe the process in a new thread. How difficult is it to ensure a parallel bore?

Bob.

They self centre,they are a floating hone.

So Peter,

Is the trick to ensure that the stones are longer than the length of the bore to ensure even abrasion?

Bob.

In my opinion it is not necassary as the stones are traversed throughout the bore,extending slightly at each end if an open bore.

Originally Posted by Anorak Bob

Jordan,

I have never used a cylinder hone or a brake cylinder hone. I don't want to side track Phillip's thread so I was wondering if you could describe the process in a new thread. How difficult is it to ensure a parallel bore?

Bob.

There's not that much more I can add, so I'll just say it here.
Brake cylinder hones are longer than small end bushes. The stones should be in good condition, preferably new. It seems a rough & ready method, but as very little needs to be removed it does OK. A reciprocating action, moving so that both ends of the hone enter the bush somewhat, helps prevent the stones from waisting. The method was taught to me by a motor engineer, who was well known as a restorer and racer of Vintage motorcycles. The info on reaming was provided by my friend Geoff, a motor mechanic, after I quizzed him on what I did wrong after following instructions from a repair manual. Seems some knowledge is "guild secret", not to be published for consumption by the masses!

Jordan

Peter and Jordan,

Thank you.

Bob.

For completeness, I should have mentioned that it's an adjustable reamer.

Jordan

Thank you Jordan, Peter and Bob for your inputs.
I have just been out to the workshop and checked a few things.
I have a new brake cylinder hone (pictured) so that is OK, and I checked the supplied bush for the bore sizes. The unfitted size is between 1 and 2 thou. under the nominal 3/4". As I understand the process of reaming this would be insufficient material for the reamer to cut properly even if the bore reduces a few more thou. when pushed into the conrod. The books I looked up recommend 10 to 15 thou under nominal for this dia.
I will fit the supplied bush and size it as Jordan recommended. The obvious concern is embedding grit into the bronze (this would also be an issue with brake cylinders as well and this is the intended use for the hone). For cleaning I am thinking of using one of the green kitchen Scotchbrite scouring pads and lots of kero.
Thanks again for the help. Phillip (Mm)

Originally Posted by Metalman

The books I looked up recommend 10 to 15 thou under nominal for this dia.

I'm puzzled by this. That's a lot of metal to remove, and could result in misalignment?
I didn't know about a minimum amount to be removed requirement?

Jordan

I think you may find that the refference to .010 to .015" is relating to machine reamers and not hand reamers.

Peter and Jordan,
Yes you are right, the recommended material removal for a hand reamer is less. It is in the order of 3 to 6 thou. which means I could use a hand reamer for this job. I did not read my reference book (Culley) carefully enough. However I will still use the brake hone method as I think it will allow me to creep up to the correct size and also the alignment is more likely to be correct.
Thanks, Phillip (Mm).