Piston Pin Fit

[Joe Flanagan]

I know the manual says, "a tight two-thumb press." That is not what I have. It seems I have a tight two-thumb press in one hole of the piston but have to tap pretty hard (drive it, actually) to get it through the other side. These pistons and pins were checked by my machinist eight years ago when the engine was built and also by the one who did the recent honing. It just seems so far away from the two-thumb fit that I have to ask what you guys think.

PS: I did have the engine running with these pistons and pins.

[aero3113]

Before I took my new pistons and pins over to the machine shop I checked the fit and the pins slid in with little pressure. Did the pistons from Vintage Power Wagons come with pins? Are you not using the pistons from VPW now?

[Joe Flanagan]

Yes, pins and pistons both came from VPW.

[aero3113]

You say they were checked 8 years ago when the engine was built? Didn't you recently rebuild your engine?

[Joe Flanagan]

I'm rebuilding it now. I was just putting the pistons together.

[aero3113]

I thought you had your engine back together then had to take it apart because of a issue you had when it was running(I may be thinking of someone else)? Do you have this problem with all of your pistons?

[Don Coatney]

Does the piston rod pivot easily with the pin in place? Are you able to get the pin keepers in OK?

[Joe Flanagan]

Don: If you're asking whether the pins pivot in the two holes in the piston skirt, no they do not. But this is where the manual says it should be a two-thumb press. The rod itself pivots easily at the piston pin bushing.

Keith, you're right. I'm the one who had my engine running for four months before it developed a knock on acceleration. I isolated the sound to No. 6 but no sign of malfunction or failure was ever discovered.

Finally, I just want to say that I jumped the gun a little on my post. One piston has this problem where the pin seems too tight in the piston skirt. The others all go in with a firm two-thumb press per the manual.

Also, the lock pins go in fine.

[aero3113]

Check for any burrs in the hole of that piston?

[Joe Flanagan]

Doing that now.

[Joe Flanagan]

Sure enough. The one that was really tight had some irregularity around the edge of one of the holes in the piston skirt. I'm sure it was caused by my carelessly tapping out the pin when I disassembled the engine. I filed and sanded it a bit and now the fit is good. I think I'm all set. By the way, I did ask my machinist if the fit between the pin and the skirt itself should be a fairly tight friction fit and he said that it should be.

[aero3113]

Is there any chance that is the #6 piston? I'm sure its nothing to worry about but you never know.

[1941Rick]

Knock when accelerating....how is your timing chain?

[Joe Flanagan]

The tight pin was number one.

I had re-used my original timing chain. On the advice from some of the guys I bought a new one and will be installing it as I continue the rebuild. Do you think that could have been related to my knock? The knock went away when I pulled the spark plug wire from cylinder 6. Returned when I replaced the wire.

[DutchEdwin]

I know the manual says, "a tight two-thumb press." That is not what I have. It seems I have a tight two-thumb press in one hole of the piston but have to tap pretty hard (drive it, actually) to get it through the other side. PS: I did have the engine running with these pistons and pins.

Hi Joe,

I know it's a late response but.....anyway like to share this information.

At what temperature did you put the pin in?

The expansion of aluminium is different from steel. factor 23x10e-6 for alu, 10x10e-6 for steel (at the celsius scale that is).

This means when you try to put it together in mid winter at freezing temp in your garage it will not fit. The hole in the piston has become much smaller then the pin outer diameter. Do put it together indoors at 20C /70F (let the parts warm up first for one or two hours) and you will have no problem.

Trust me, over the years aluminium or steel do not shrink or grow when there is no corrosion.

To your engine, it they fitted right then, no problem for that. Will still be ok on the fit.

Good luck.

Edwin

[Joe Flanagan]

Edwin,

It turns out I was a bit careless when removing a couple of the pins when I disassembled the engine. There were slight burrs around the edges of the holes of a couple of the pistons that made it hard to push the pins through. I cleaned these up, installed the pins at room temperature, and they were pretty much a two-thumb press fit as recommended by the manual. Thanks for the advice.