Best Way?

[frankfast]

What is the best way to enlarge the small end bushing on the connecting rod? The wrist pin on the new piston I purchased is too tight. I have an abrasive drum for a dremel that will fit but am concerned that the bushing will not be uniformly round unless I'm very careful. Is a reamer the proper tool and where would I get one the proper size?

[Duccout]

Hi,

I am not an expert, but I believe that you must not use any type of abrasive on bushes. Apparently, particles of abrasive will embed themselves in the soft bush material and will wear the shaft that the bush turns on. I believe that a reamer or a hone is the only way to go. I think that you will need an adjustable reamer that will allow you to gradually ease the bush out until the pin fits. There are plenty for sale on eBay. Alternately, take the con rod or crank to an engineering shop who will do the job for you.

Colin

[ducwiz]

No no - don't use this abrasive stuff! The one and only tool to produce the correct clearance is a reamer. Best would be an expanding type:

It can be adjusted until the best fit/closest tolerance of pin and bush is achieved.

Hans

[frankfast]

The pin will go in the bushing but it's too tight. Will not rely on someone else to do the work so I have to find an adjustable reamer to do the job.

[Jordan]

Dear friends, I beg to differ about using abrasives for this job.
I have done it many times, using a brake cylinder hone from an auto accessory shop.

As long as it is only a small amount of material to be removed, there should be no problems.
Use kerosene or WD40 to lubricate, and when finished wash well.
Bronze is not so soft that particles can be embedded in it.
We don't worry about using grinding paste on our bronze valve seats, after all.
I don't hone until it's an easy sliding fit, but a little resistance is good initially, as it will bed in to a sliding fit after some use.

I had tried an adjustable reamer, but found that a handheld reamer couldn't be relied upon to make a really round hole.
It looked alright, but when noisy play developed shortly afterwards, when removed it could be seen that it was more like a multi-sided, not quite round hole.
A reamer in a machine tool chuck is a different story, and should give good results. Hand held is the problem.

[ducwiz]

Hi Jordan,

Bronze is not so soft that particles can be embedded in it.

My experience is different. Besides those from Ducati singles, I had motors of many "Regina" models from the german HOREX company on the bench. Their gearbox layshaft runs in bronze bushes. During service work, I measured the OD of its (hardened) shaft journals, as well as the ID of the corresponding bushes. The bushs diameters had stayed mostly close to the initial values, whereas the journals showed a significant reduction due to wear. For me, this prooves the embedding of abrasive particles into the surface of the bushes.

I had tried an adjustable reamer, but found that a handheld reamer couldn't be relied upon to make a really round hole

This is why one should use a reamer with "helically twisted" cutting edges.
Proper lubrication always helps to improve the surface quality. And the reamer should be in a rather brand-new state.

cheers Hans

[Jordan]

Hi Hans,

I cannot account for your evident steel shaft wear, as I have not had that happen with the hone technique on small end bushes.
A fixed sized reamer may not be the exact needed size, whereas an adjustable one can provide that.

I asked a mechanic how he deals with this problem.
He said he also doesn't always get a nice, round hole in the bush, rather a "6-sided circle".
So, he stops reaming (with an adj. reamer) when the pin is still quite stiff to push into the bush.
Then it becomes the correct size after some running in causes the high spots to disappear.
It's not a method I would be confident to use, but I can see it could work if one were good at guessing.

The brake hone method was taught to me by a respected mechanical engineer and vintage motorcycle racer of a Chater-Lea.
I tried it once and it worked, and is the way I continue to do it.

If I could learn how to reliably ream an accurate circular hole by hand, I could change my ways.
Perhaps an helical reamer is the answer.

[frankfast]

Do you think removing approx. .009"from the small end bush on the rod can be achieved using a hone?

[Duccout]

If the pin will go into the bush, then the fit is very close, and just needs the slightest easing. I would purchase an adjustable reamer and adjust it so that it does not take off any material, but is a good fit in the bush, then using a lubricant, gently ease it through the bush, carefully rotating the reamer. This will remove the tiniest amount, and the pin should be the right fit.

Colin

[Jordan]

Do you think removing approx. .009"from the small end bush on the rod can be achieved using a hone?

That's a lot.
I have only done .001" or so.

When making your own bush you don't need to have so much material to remove.
I make the bush ID to be as the finished size, and the OD .001" oversized.
Then when pressed into a bore, the ID becomes smaller by about .001"

If I had a lot to remove from the ID, I would consider removing the bush and making the ID larger in the lathe and collet.