Motoscrubs.com

As those of you who have read my occasional waffle will already know, I'm refurbishing my '74 350 on a tight budget.

One decision I made last year was to leave the conrod/crankshaft as is. I spoke to Nigel Lacey about it, and he was sympathetic, reckoning that if the big end was within lmits, and I only run the bike on the road, then carry on and see how it goes - given the cost of replacing the whole shebang.

What I did decide to replace was the worn small end bush and the gudgeon pin. The bush is a standard size, 18x22, and could be cut and lapped to a suitable width for the piston, then drilled where the oil holes need to be. The gudegeon pin, to my delight, comes from an old MZ - exactly the right size, and cheaper from Germany than Italy.

It may be that I was simply a numpty, but after about 1500 very happy miles I could hear a slight rattle from the top end at just off idle, so I thanked Dr T for his foresight in makling the top end of our engines so accessible, and took off the head and barrel. This revealed that the small end bush had worn enough to allow the piston to rotate slightly. Big end still fine, but little end in need of work.

Upon reflection, it could be that a commonplace "oil-filled plain bronze bush" costing only a few quid simply isn't up to spec for the stressful location of the small end. Maybe Italian (or in my case, Spanish) bushes from the last century were of better quality than our modern equivalents.

Thinking further, I delved into the internet and books like Phil Irving's Motrcycle Engineering, and wondered if I could use something else as a bearing. Very soon, I discovered that lots of engines use needle roller small end bearings, and I''ve ordered the right size and spec made by Wossner, from a very helpful bunch called Rebuild Racing https://www.rebuildracing.co.uk.

Do any of you wise owls have experience of needle rollers at the small end? I confess I'm going to have a go at it anyway, and if all proves well, then that's a useful addition to the encyclopedia of Ducati singles knowledge.

Fingers crossed,

Pete.

Hi Pete,

I too have replaced a small-end bush in situ, including reaming it to size, and to my amazement, it all worked!

Now, on to my observations: normally the bronze bush is fine (AFAIK) I have run various Ducati Twins for thousands of miles and never seen a worn small-end, and only one of my Ducati Singles has ever shown wear there. Many, many Ducatis have been raced with the bronze bush and never given trouble, in spite of the revs that they are pushed to, so I think that there is another reason for the wear in your small-end.

Hopefully Nigel or Eldert will give their opinion and may prove me wrong. There are different grades of phosphor bronze, so maybe, as you mention, modern replacements are made from the wrong material? I await some opinions and experiences.

Cheers,

Colin

Hi Pete

i would not use a needle bearing in a 4 stroke engine . Yamaha tried in the XS 650 series and it was not a succes .

a needle bearing should run on a hardened track and i doubt the the smallend is hardened

i made up a lot of small end bushings and used Ampco18 material and never had one fail .

Regards Eldert

Thanks for encouragement and warnings.

According to the manufacturers, "AMPCO® 18 is an aluminium bronze alloy (specifically Al-Fe-Cu) known for high strength and good "running" properties against steel. It is significantly more durable than standard sintered bronze or Oilite materials".

I confess ignorance when I made a bush out of what must be "standard sintered Oilite" - which succumbed to the stress.

The difficulty is getting hold of a small length of Ampco 18 tube which I can cut to fit, and which I can then either work by hand or take to a machine shop... Not always easy for a one-off job.

Still with fingers crossed!

Pete.

Hi Pete,

classicducati.co.uk has original small end bushes in stock for £12; that might be the easiest route for you. I fitted one of these to one of my Spanish engines, but I had to turn down the O/D as it was larger than the Spanish bush (I don't know if they are all the same) but the I/D is 18mm, although it needs reaming after fitting.

Cheers,

Colin

Pete
Oilite (which is sintered) or even leaded bronze is too soft for this application. I've used PB102 (wrought phosphor bronze) without issue, that may be more available in smaller lengths. You should even get away with pb1 (continuously cast phosphor bronze), easier to machine, but not as hard. I have seen split plated bushs used for small ends, usually steel with a bronze coating on the inside.

I wouldn't use a needle roller, firstly it would need to be wider than the rod, otherwise it's likely to drift out as there is more width inside the piston than the rod width and as Eldert points out the rod's not likely to be hard there (or dimensional or surface finish suitable). Design wise needle rollers are not normally found in Fourstroke engines,

Regards Nigel

In my work life, I have used Oilite bushes but only for control levers etc which don't normally get any lubrication and are relatively lightly loaded.