cam ground pistons

i have a set of old flathead pistons that have the first cut on them but were never finished. I'm told that need to be finished cam ground. Any light anyone can shed on this and better yet who can do the work would be appreciated

 

Generally the top and second land have a taper cut on them like \ from the top to bottom. The third land is generally cut back straight and further than the second land like l from top to bottom. The skirt generally has a barrel shape like ) from top to bottom. In addition to this the piston will generally have ovality cut into it around the piston so it will look like an oval from from the top.

The combination of these shapes will require you to find somebody with a lathe with live tooling. You might try an aftermarket piston company like Egge and see if they will finish turn them for you.

 

As the cam grinding on pistons is a specialty production maching operation I can't imagine how a conventional machine shop evironment could readily accomplish that final sizing profile. The purpose of this skirt profile is to be able to ***emble the engine with tigher piston to skirt tolerances and thus reducing/eliminating piston slap/noise during warmup.

Frank

 

Hunt up Jahns Pistons and see if they can't help you here.

pdq67

 

i ended up with several pallets of old piston sets and a lot were "semi" or labeled as requiring finishing . seems it was common practice at one time? less trouble and expense just to get some quality current production pistons . they are not worth the trouble.

 

I hate to say this, and I'm all in favor of using original or vintage parts, but jetmek is correct. It will cost you far more than you want to pay to get them finished. The setup costs are the same whether the machinist does one set or fifty. In addition, the machinist is going to have to figure out what's already been done to them. You will have to determine what finish dimensions you require (although, if you have a piston company machine them, they can help you with that part, at least).

If they are for some type of rare application, where no replacements are available, it might be worth it- but even in that case, a piston company would probably prefer to begin with their own material from scratch.

 

The big question here is what, exactly, is unfinished?
Is it just the final couple 'thou for bore sizing ? Remember that the cam (as in not-round) grinding process is to allow for differential expansion because of the pin bore. As mentioned, this is not an everyday process for anyone except the various piston manufactures.
The other concern is whether or not the ring grooves are finished, both in width and depth. This, again, is kinda specialized.

Hopefully you did not pay too much and can flip them on ebay then buy a finished set.

.

 

cam ground makes me think valve relief depth.

 

You would think that's what it meant, but, no. It refers to the shaping described by brayrod; virtually all modern piston designs have a general "shape" such as he outlined. They are neither round, nor "straight up & down".

If ya wanna look at really weird stuff, there has been some work in the past done with true oval (think "speedway" shape) and square/rectangular pistons.

 

Quite common to have semi's left then they can be sized to the excat fit, I recently bought a set from North western Auto Parts and they did the finish for me, I think it ran me 100 to do six, I doubt EGGE will do the finish work for you but you could try, and Jahns is gone someone else picked them up and they are pretty much outa bussiness, I had Aries forge me th elast set of pistons I needed and stayed away from the cast. Egge seems to have problems with the correct position of the cam or barrel shaping of a piston, I bought aset from them in "03" and with in a couple minutes of warm up they siezed, all our boring and sizing on our end was perfect, come to find out the camming was about the 45 degree mark instead of 90 degres from the wrist pin, they made good, it was their problem but for that applacation they did say forged was best, so the next engine like that that I did we went to Aries.
Check with a local machine shop and see who they recomend to do the finish, my machine shop here sends pistons to Texas someplace to have the tops cermiced and the skirts shot in teflon.