Technical Shaving piston tops ... mill or lathe ??

.-40 over 390 block, 390 forged pistons, 428 crank.... need around .083 shaved off piston tops

Plenty of top ring land will be left and piston thickness

Whats ya favorite way... mill or lathe or big giant file ;-)


Ricky.

 

Mill or Lathe, either one can do the job.
Lathe is probably the fastest, set up and cut time is quicker.

I've done that same cut on 400 SBC .083" dish forged pistons, using the dished piston on a longer rod and basically cutting the rim off the circular dish to make a flat top. Lathe was quick and easy.

 

If you ca fund a lathe chuck that big and fairly deep, that is the best way to go. You should make up a slug so the skirts don't collapse.

I have seen them milled in a machine vise with a fixture made up, mainly for valve relief cuts. You could also bore a set of soft jaws and drop them in.

 

I would use the mill. Grab the piston in the vise by the flats near the pin.

Stupid easy with a fly cutter, shell mill or multi-pass with an end mill.

 

Lathe would repeat better if a ring was inserted into ring groove and butted up against jaws.
What I have done in the past for stuff turned I didnt want to scuff up was take a length of heavyish wall pipe/tubing and turn OD and ID concentric. With ID turned to piston diameter.
Then saw cut lenghtwise and you now have a split 'bushing' Most pipe/tuning will expand when cut.
Once in the lathe it shoud be able to stay in place an you just swap pistons with less chance of scoring and a full diameter grab on them.

 

I use a pin vise to hold the piston squarely by the pin bore. My buddy made a CNC program to profile mill this 352 FE piston with a remainimg dome to fit the combustion chamber. But if you merely want to cut it down a flycutter in the mill could be used.
This 352 piston had plenty of ring land left after milling .050" off the top (to be used in an inline six race motor).

 

Recently had a friend tell me that on his Lincoln Y block. He had too much compression and was going to swap to a blower. So he took the heads off. And covered everything up and hit them with a grinder with a flap disk and then finish them with the file while still in the motor. It ran great!

 

I would check weight of each afterwards . Pretty important to be close to the same weight , in the rotating mass

 

Have done it both ways. Simple flat top OEM slugs, a one off deal, I would use the lathe if they fit.
Some pistons, the bottom of the skirts is not flat enough to locate off of.
I remember someone used to sell a piston fixture that located off the wrist pin.
That would have been helpful, back when we were buying semi finished pistons from Mahle and Cosworth.
The shape of some domes, are not suited to being modified on a lathe. Depends on what mods you want to do.
Frenchtown showed a good example above.

 

I would still mill them since the set up is easy. Bore some soft jaws to fit the piston O.D. I have made up many sets of aluminum vise jaws.

Drop the piston in place, make up some shims to fit the top ring groove so they all sit the same. Then flycut the pistons. I agree of weighing them, even before just to get some perspective.

 

Set 2 up at a time using a wrist pin to tie them together on the mill. You're welcome.

 

Damn that's a good one. I should have thought of that.

 

I would measure the pistons to make sure they are round. I learned a long time ago some are somewhat oval for expansion. Clamping an oval piston could distort it.

 

If one has the correct special finger chuck jaws which grab the piston in the ring lands, then it's cake in the lathe. We regularly cut down the tops of Ford and Chevy pistons to use in our Packard V8s.

No, pistons aren't round. Yes, clamping them can/will distort the skirt. Trying to clamp pistons in a standard 3-jaw or 4-jaw chuck is not best science machining. Many slipper skirt pistons can't be safely done that way.

jack vines

 

As @Burt Kollar stated above, the piston isn't round so clamping it in a lathe may deform the built in oval skirt. Also, the chuck jaws may mar the piston or the coating if its a coated piston. If using a lathe, the right way would be to get a set of aluminum jaws and bore them to the pistons diameter....to spread the grip further. Some machinists put thin copper or plastic on the jaws and tape it in place to avoid marring, but you could still ruin a piston.

Its probably better to use a milling machine vise and grip a piston on each end of the wrist pin hole. You can still use something soft to insert between the vice jaws and the piston. The difficulty here is getting a vice big enough to hold the piston.

Here is a picture of a homemade pin remover. If your pistons have this type of relief on the sides, you can get one of those inexpensive milling machine vises off Amazon thats bigger than this cheapo. Get two "parallels" or some bar stock to use for parallels.
Set the parallels on the vise jaws and set the piston on top of them at the inset locations. This should raise the piston high enough for the piston skirt to clear the vise and allow the vise to be tightened on the piston.

Here is what I'm trying to say...........


Pin Remover






This way you are gripping the strongest part of the piston and not possibly deforming the skirt.

 

Just curious, but how would you bore the jaws for a radius?
Thanks

 

A couple of set ups that come to mind are. Bore the soft jaws, insitu on the vise with a boring head in the mill. Fixture them to a rotary table on the mill using an end mill. Fixture them to a face plate on the lathe.

 

Thanks. ^^^.

 

If one has the correct special finger chuck jaws which grab the piston in the ring lands.

^^^^^^ Absolute best way.

 

Lathe,,when I would build my 348 c-in,,SBC 400 block with 327 big-J crank n spacer mains,I'd use long rods,but the 400 stock pistons{ That come with a bowel top/dish} I would cut away the outer lip of bowel=now a flat top,then add my small valve pits/eyes.],I loved my odd rig. I out ran a lot of super high$ 355's. Had to use a big oil cooler,too keep the spacer mains happy.

 

I get what you are saying, but that does require the use of two machines. OP may have or have access to both.

 

It's two different fixture/tooling options, for using the mill. To bore/cut an arc in the vise soft jaws. And one for accomplishing the same thing in a lathe. If you don't have a boring head or rotary table for your mill.

 

Many thanks for all the advice, I am retired and no longer have access to equipment

Does anyone make a table top mill perhaps ??

Have always done my own machine work except for bore/hone/millling block, heads

I will most likely take my stuff to Jeff at HEB in Pasadena, he knows FE's


Ricky.

 

I bought a SB mill maybe 20/25 years ago.
I never used it.
Sold it after 7/8 years of collecting dust.
Could have used it many times since.
I too keep an eye out for a decent well tooled bench top mill, maybe someday!

For me it would be like my lathes, just the convenience of doing something when I want to. Not driving to a place and driving back to pick things up.

 

Not understanding the purpose of the rotary table, in this case.
Can you elaborate ?

 

I would just bore the diameter of the piston, leave a gap between the jaws before the boring. I would grab the piston way up high on the ring lands where it is the roundest and strongest.

Holding it by the piston pin location, something to explore for sure. It would be a simple fixture, you would need to keep the piston as square as possible.

 

Just a second option to mill a circular relief. In a pair of soft jaws, if you don't have a boring head.

 

Yes, you can find plenty of them on Facebook Marketplace if you watch for a while. Expect to spend ablout $1000/$1500 depending on how nice they are. For about the same money ($1500/$2000) you can pick up a decent full size mill. I can provide you with free instructions on how to make a phase converter cheaply. I have one I made 30+ years ago and its still working just fine. The small table top units often suffer from a lack of ridigity. If you have room for a small lathe and a mill, it will make retirement a lot more interesting.

 

One other idea came to mind about setting a piston in a vice to mill it. If a lathe is also available, I'd get a piece of pipe and bore it so the piston slips in it. Then face each end of the pipe so the ends are parallel.
Then a used top ring could be placed in the piston and the piston inserted into the pipe. The pipe would sit on the vice with the bottom of the piston protruding out. Then it sits on the vice and would be easy to just slide each piston in and out. The pipe would be the "parallel" but it would sit atop the vise more easily.

 

What actually secures the piston or fixture to the mill or vise ?