How to secure a Piston Rod in the Piston

I know this should be simple, but I don't know the answer.

The piston rod in the attached photo has sheared at the pin where it is articulated. How to solve that is tomorrow's problem.

Having extracted the pin from the piston I realise that the 3/16" piston rod has been reduced to 1/8" to fit through the hole in the piston. The hole in the other side of the piston is 5/32" so I've assumed that I now need to hammer the piston rod to expand it, rivet like, to secure it in the piston.

What I don't know is how best to secure the piston rod while I hammer it - I don't want to spoil the surface of the piston rod as it needs to be a good fit through the gland housing. Additionally, I don't think I should be trimming the rod to size at this stage as the hammering is then as likely to flare the bottom end as well as the top.

I've also included a drawing of the two parts.

PS. Does anyone know the maker of this model? It has a W on the valve cover.

Grateful for any advice.

Avon,

Is there any chance of increasing the diameter of the 1/8 dimension of the rod where it is inserted into the piston by heavy knurling to improve the centreing of the two components. You will need a knurl with opposed rollers to do that successfully so that they squeeze the job..

After that and rather than hammering over the end like a rivet, may I suggest you centre punch in at least three positions to swell the rod into a tight fit in the piston? Quite why the joint was made that way with such a poor fit I don't understand.

Regards Brian

Edited By Brian Wood on 12/06/2020 19:41:42

If the rod was riveted into the piston, how did you get it out? Just curious!

Usually on a small engine like that the piston is screwed onto the rod, or possibly pressed on. I doubt if it would originally have been riveted. and if it was the hole in the piston would not need to be stepped like that, as riveting would produce a very small expansion even if you could do it safely.

I would thread the piston rod, make a new piston to suit, and finish turn it in place on the rod, to fit the cylinder.

.

HI Guys,

I had to use a punch to drive the rod out of the piston. In truth I was surprised that there are two different diameters in the piston and it was that that led me to believe that it had been riveted (ie the end hammered to widen it out to fill the gap).

I think the idea of tapping it is probably the best. I can probably reuse the original piston as well. There's nothing wrong with it fundamentally and my slowly developing lathe skills are probably not yet at that level - that said I could give it a go just as a training exercise!

I'll let you know how I get on.

well that wasn't a success. Lesson to self: 'be more careful when tapping a hole to make sure that the tap is square to the job'! I have a beautifully fitted piston rod with a piston that now wobbles!

So it looks like I shall be taking Redsetter's advice and making a new piston. Therefore, a question: What material should I make it out of? The original is clearly steel, but I don't know what grade - does it matter?:

I rough the piston with a screw thread through, then ring in a stainless steel rod, threaded of course. Then chucking the rod finish the piston. Piston made from brass or phosphor bronze or stainless, ordinary steel will rust.

Brass or bronze for the piston - I doubt if the steel one is original. Brass is OK for a little engine.

Make the piston first - turn slightly oversize, face, drill and tap from the tailstock, put the groove in, then part off. Then if necessary, reverse in chuck and face other end.

Now chuck the piston rod and set it to run true - this is important. Thread using a tailstock die holder. Without removing the rod from the chuck, screw the piston on with a drop of Loctite, and it should be running true. Now taking light cuts with a very sharp tool, turn the piston down to be a tight sliding fit in the cylinder.

It is of course advisable to check that the cylinder bore is circular and parallel before starting!

Hope that helps.

Edited By Redsetter on 13/06/2020 17:16:06

Excellent, many thanks for the prompt replies - now off to source some material and learn how to sharpen my tools properly!!

I had used the talistock to support the die, so reasonably confident the error was in not being so careful when tapping the original piston.

I'm learning!!