Expanding a McCoy piston

[Allen Eshleman]

I have been reading several posts here about how to restore a McCoy motor's compression.  I see two recommendations.

1.     Tap the top of the piston, in it's liner, seated at the bottom of the liner with a flat punch.  Question: Is a flat punch the tool that seats finishing nails or does it have a bigger flat head than that?

If I were to do this,  are these real gentle hits?

2.    Baking the piston.  One person said this is rarely successful since the piston has already been run at very hot temperatures.

Which of these would you personally try first?   Is there any more advice out there?   I have three McCoy 19's. 

One is the older shiny case one. It had the best compression and ran well until the head came off in flight.  I've put it back together but it still but it still binds a little.  I haven't tried running it yet.

The other two have the lightning case.  One ran well for me until recently although very fast with no break.  The other was given to me and hasn't yet started for me and has compression.   I'm thinking of playing with that one, using one or both of the above procedures.

[James_Mynes]

I have never attempted tapping a piston to swell it, but if was going to try it, I certainly WOULD NOT use a nail set. The hammer tap focused on that tiny point will punch through the top of the piston in short order. Good luck trying to get compression after that.
I would think a brass punch, about a 1/4" diameter at the business end, would distribute the tap and maybe swell the top of the piston. I thought this method only worked with a dome top piston, though. My Redhead .35 has a baffle, and I'm pretty sure it's flat on top. Doing this to a flat top piston should draw the sides in more???
As far as heating goes, I've heard 500*F for thirty minutes, but again, I've never tried it.
Maybe there are some McCoy gurus that can provide more insight.

[Jim Kraft]

I have been expanding McCoy Red Head pistons for years using a small hammer. For me, I just put the piston on an anvil after I mic it, and then tap the dome with a hammer until it is about .001" larger at the top. If you hit it to hard you will break it. If you expand it to far you will have to lap it back in. I have done about 15 of them and done several 4 or 5 times.  After running in it will have a shiney band around the top of the piston about 1/8" to 3/16" wide. They do expand evenly around the piston even with the dome fence. I do try to not hit the fence, but have by accident without harm.

I have heated the pistons for Super Cyclones with a torch and dropped them in a can of oil with great success. Did not seem to work for me on the McCoy's. Maybe Ty has the answer by just letting them cool on their own. The way I look at it they are not much good anyway, so you might as well try it. I can usually get around two years of quite a bit of flying out of the McCoy's before they need hammered again.


This is an old trick that has been used by many people.

[Jim Kraft]

I can believe that Ty. I got 2 brand new 35's that had almost no compression. They have been running fine for several years after "fixing" them. But, then, I have had some that were great out of the box. Of all of them that I have had I have never had one with a bad bearing fit, and have never had one get worn. If they had just had a decent piston/cylinder they would have been great engines. The design was great.

[Allen Eshleman]

" For me, I just put the piston on an anvil after I mic it, and then tap the dome with a hammer until it is about .001" larger at the top."


For Jim, what do you mean by "mic-ing" it?

I think I will try this.   Do you tap it several times?   

[Tim Wescott]

For Jim, what do you mean by "mic-ing" it?

Measure its diameter with a micrometer.  It's a skill -- almost to the point of "if you have to ask, don't try", except that if you don't try, how will you learn?

I wouldn't try any of these things on an engine I cared about thought I could fix some other way.

'course, I haven't done this, so I may just be scared for no good reason.

[Allen Eshleman]

I have a neighbor or two who could probably do that, who probably has a micrometer.   

Thanks for explaining that to me.

[Jim Kraft]

I do tap it several times right in the center. If it does not expand you need to tap it a little harder. Instead of using a micrometer you could just slide the piston into the cylinder to see if it is tighter. I always start out with fairly light taps as you can always go a little harder. I have kind of learned over time about what it will take, but if you just start out easy you will be fine. You will end up with a little bit of a dent in the piston  which will not hurt anything. I have one engine that has an almost flat top from expanding it but it run fine. Some of the McCoy's were overcompressed anyway and it does lower the compression slightly.

[Jim Kraft]

I just checked Mecoa, and they have pistons, cylinders, and rings for about $45.00. You can probably pickup an engine off of ebay for a lot less and hope for good parts. I never did like dikes rings, but then I guess they work OK.

[Allen Eshleman]

Can this be done to a piston with the raised slot on it?  I have tried it once now with one of those.  It didn't make any difference.  Is it possible that I didn't hit it hard enough.  I hit it gently several times in the piston sleeve with a flat punch.

[Bill Little]

Another fix is to find the series 21 McCoy, the butt ugly square crankscase ones. Swap the cylinder and piston. Very good runners.  I have two of the .40 engines with the series 21 cyl and pistons. Fine runners.

Hi Ty,

The problem with the .40 is that the Series 21 cylinder will not fit in the Red Head crankcase.  The .29 and .35 are drop in replacements.  For some reason, the Series 21 .19 has several changes which will require machining of some sort to make a swap.

No problem on the .40, though, since almost all the Red Head sleeves have the "fences" in the ports for a ring.  Just put a Series 21 piston only in the Red Head and get a ring from Frank!   This works perfectly.  (do make sure the "fences" are in the Red Head cylinder ports)

My OTS Super Clown with the Red Head .29 has the Series 21 cylinder and sleeve in it, and it runs at "least" as good as the stock Red Head, if not better.  The McCoy .40 I used in my Parrott P-47 had a Series 21 piston and ring only in it.  Identical run characteristics as the stock engine.  All the timing stayed the same.

This IS the best alternative to a "worn out" Red Head that I know of short of chroming the engine.  (I don't know who does that anymore)

BIG Bear
RNMM/AMM

[Jim Kraft]

I would say you probably did not hit it hard enough. Doing it in the sleeve works if you put the piston at the bottom of the sleeve and set both on a hard steel surface like an anvil. The botttom of McCoy sleeves is quite a bit larger than the top or past the ports. And it works just fine on pistons with a fence as most older engines have. You should feel a great difference when pushing the piston up through the sleeve if you have grown the top of the piston. In fact it will probably stick at some point. If it gets to tight you will have to lap it back in with super fine compound.

I have a Forster 29 rear valve ignition enigine that has a steel piston with a dome, and I did this to it many years ago and it has run great with super compression for many years.