Thread: Pistons and machining

OK

If one was to machine a automotive or stationary engine piston eg, in order to deepen ring grooves ,where replacement rings are deeper than the old rings , or repair worn ring grooves with a spacer .

What is the prefered method of holding the piston in a chuck ?

Is there a special chuck or fixture available for this job, or is a normal 4 jaw chuck able to do this type of work ?

How is the piston centred ? As they are cam ground normally .

Mike

Absolutely, a 4 jaw is fine. Machined top, second and oil grooves out wider and deeper and top grooves wider for top groove spacers etc and cord rings on dozens of pistons over the years, all in 4 jaw chucks. Round, or cam ground, cast iron pistons or aluminium, no damage to pistons.
Used to use a strip of 1/32 inch thick velamoid( valemoid?) gasket paper around the piston, one layer only. Weet bix packet type cardboard will be good. The strip of cardboard should be as wide as the jaw is long.
On a cam ground piston(even round pistons) just have the gudgeon pin hole lining up between the jaws, leave the pin in if you have it.
Setting up, the top part of the piston will be round, even on a cam ground piston, so set up the o.d. to that. For the axial set up I set up to the least worn side of the ring groove using a thin point on the surface gauge. The unworn or least worn side is usually(almost always) the bottom side of the groove.
Tightening of the chuck is gentle during setting up process then tighten each jaw a bit at a time going around each jaw at a time, you'll get it quite tight (educated feel) without damaging the piston, recheck your set up and your good to go.
Top ring groove spacers, don't know if one can get them easily any more, were not as deep as the ring and a step had to be left in the groove for the spacer to sit down on. The spacer fits in on the top side of the ring groove.

If more info needed just holler.

Cheers.

Good info

I guess spindle speeds are kept low . e.g., For a 4" diameter ?

I've noticed some pistons have a dimple in the crown, where you could locate a centre , for machining the piston .

In the past I used to have these jobs done and paid for it . Now, owning a few lathes , a nice little job I can do .

I'd practice first on a few old pistons to ge the " feel " of things .

Remember pistons are not round either.... Well I suppose some are....

Hi Log,
Just wondering, why do you say to leave the gudgeon pin in? (arent some gudgeon pins held by the conrod and therefore come out when you turn the lathe on?)

Stuart

Speed can be a bit of a variable, try between 250 to 350 or so. Don't have too much top rake on the tool as it can make it dig in. Aluminium will very often build up on the tool so push a bit of steel(side of 6 inch rule) against the build up to get it to break away off the tool. If top surface of tool is polished with a fine oil stone this will help preclude build up problems.

Using a narrow tool( ie tool narrower than the groove) reduces the cutting forces and reduces chance of catastrophic dig in.
Yes if there is a small centre in the top of the piston you can use it for support.

Because narrow tools will try to flex you have to take it easy when doing the sides of the grooves.
Good that you have some old pistons to practice on.

If you need I can take a few pics of my groove cutting tools.
I also found instructions for machining for top groove spacers, doesn't explain the machining process just where to machine etc.,I could scan and post if need be.

If they are split skirt pistons then there is something else that needs to be done before chucking(clamping in the chuck that is) the piston.

Cheers.

Yep .RC. one would have to find an old piston to get one that wasn't cam ground. Morrisman mentioned cam ground pistons in his first post.

My old Ronaldson Tippet stationary diesel has a round cast iron piston though relieved on the sides for some of the length of the piston, basically a bit above and below the gudgeon pin, of course that relief doesn't go all the way around the piston. A lot, maybe most, of old stationary engines had round pistons.

Yep Stuart, press fit gudgeons (press fit in the rod) would come out but when starting to clamp the piston up in the chuck there is enough distortion to lock the gudgeon pin in place. The gudgeon pin if left in the piston helps rigiditise (sp?, is it a word?) the piston for clamping purposes. On chunky thick walled pistons there is no real need for the pin to be in place.
The distortion on the piston caused by the clamping of the chuck is not permanent, the piston resumes it's original shape once released, well assuming a ham fist hasn't squashed it beyond it's recovery abilities.

Also, later pistons are not parallel in the skirt, they are bigger at the bottom.
Also the top section where the rings are is reduced quite a bit in diameter also, this reduction goes down to below the oil control ring. Some times the reduction is staged, being greater on the land above the top ring.

What I've said in these posts is fairly generalised. Morrisman never said what type of piston he is working on or is going to work on so that's why my answers are trying to cover a broad scope.

Just thought, people might not understand what cam ground piston means, simply it means that the piston is made out of round, smallest in line with the gudgeon pin, of course those sides are generally relieved quite a lot as well.

Cheers.

Hi Log,

Thanks, I knew pistons werent round, but thats all I knew.

Stuart

It was a general question

I have had Ford side valve V8 pistons machined for a worn top groove , spacers fitted . I had a 1941 Ford light truck years ago with the 3 and 1/16" bore.

The Chevy 216 motor used cast iron pistons right through until early 50's I think , they are more rugged pistons but the big end bearings cop more load .

I've got a Lister stationary motor, a Indian made copy CS model 6hp single diesel . It has a cast iron piston, but a alloy piston is available for it ..gets it up to 8hp . MIKE

Pics of your ring groove cutters YES ....

Originally Posted by morrisman

It was a general question

I have had Ford side valve V8 pistons machined for a worn top groove , spacers fitted . I had a 1941 Ford light truck years ago with the 3 and 1/16" bore.

The Chevy 216 motor used cast iron pistons right through until early 50's I think , they are more rugged pistons but the big end bearings cop more load .

I've got a Lister stationary motor, a Indian made copy CS model 6hp single diesel . It has a cast iron piston, but a alloy piston is available for it ..gets it up to 8hp . MIKE

Pics of your ring groove cutters YES ....

Yep, sorta thought it was a general question, hope the eplanations I've given have been helpful.

We used to even have a manual piston grooving tool, it was only for doing top groove and top groove spacer.

You left the pistons on the rod, cleaned the carbon out of the top and second groove then clamped the rod in a bench vice with the bottom of the skirt down against the jaws, this made it so the piston wouldn't rock.

The tool was a round hollow frame gadget that sat over the piston. It had 3 round rods with thin flat ends, these rods were pushed in to the 2nd groove, twisted so the flats contacted each side of the groove, then the "fingers" were locked. The flats of course were actually curved a bit with no sharp edges now oil the fingers and second groove and you can now proceed. Now the whole thing could be rotated around the piston with the handle which stuck up vertically. It wasn't a big thing and it was made of aluminium so quite lite, can't remember but could do up to about 5 inch or so piston.

Also on the tool was a small slideway arrangement that could be wound it by a knurled knob. In the slideway block there was a cutter which could be changed to suit the groove width. The slideway block could also be adjusted up and down so it could be brought into alignment with the top groove.

Now once set to height you just wound the cutter in to touch then rotate the whole gadget using the handle, holding the handle and turning the gadget caused the tool to advance inward. As you were getting close to depth you had to slow down so as not to dig into the bottom of the groove.

When the cutter just started to touch the bottom of the groove you would then let go of the handle and just turn the gadget by hand so the tool skimmed the bottom all way around. Can't remember exactly but if then you wanted to machine the top of the groove for a spacer you then raised the tool up by the width of the spacer, actually using the spacer as a gauge in the adjusting mechanism, now you wound the gadget using the handle and it would feed in, you wound it in for so many turns and that would be to the depth for the spacer.

Can't remember exactly but pretty sure that somewhere around the gadget there was enough room that you could shove the ring into the groove, with the spacer sitting on top of the ring(if fitting a spacer) to see if right fit and depth had been achieved.

Thinking about it it may have been able to be used to widen and or deepen 2nd groove also if the fingers were set to run in 3rd groove and likewise 3rd groove if fingers were set in 4th groove.

I've very occasionally seen these for sale usually "use unknown" type of thing.

I'll dig the cutters out and get pics up as soon as I can.

Cheers.

Instead of using ring spacers you could buy the cast iron and make your own rings to suit, I haven't done this myself but my brother has, I don't know where he got the cast iron bar from but I could find out. I have cut ring grooves a few times to fit non standard rings to something you can't get parts for, thinking about that the last time was probably 15 yrs ago I opened up the grooves on a compressor piston and fitted a set of Briggs & Stratton rings. A thing to consider is that these days many newer engines have a chromed top ring.

Evening Mike, thought this might go with your 1941 Ford light truck! Your earlier question was about mounting, and a particular jig is illustrated - but that was 1925 - may be of interest in passing.

Mark

I have 5 groove tools, here are pics of two tools I've used many times over the years, not for some years now though. These tools are for aluminium pistons of automotive size.









You can use wider tools(if fits in groove of course) if the piston is being held rigid enough. Tungsten carbide tools can also be used of course but I've mostly always used toolsteel.

Also, in my earlier explanations the pistons were automotive size and were not that long and thus would fit fairly deeply into a largish 4 jaw chuck so the grooves being machined were fairly close to the chuck jaws which was good.

On longer pistons such as stationary engine pistons, and if using a smallish chuck the piston will be sticking out quite a bit so hopefully there is a small centre in the end of the piston. Of course some of the diesel pistons will have a deepish chamber in the top of the piston which would be a bugger. There are ways to get around these problems though.

I was just Googling for any info on the manual piston grooving tool that I explained in my earlier post, didn't find anything.
I did though find a site with some pics of piston groove machining (in a lathe) so there is a bit of info there. Here it is Shop Work page 18

Cheers.

Look at the bottom of the piston, most have a machined area inside the skirt. The base of the piston usually has a machined face as well.

Chuck up a short bit of round and machine a step on it then turn it around and rechuck it with the minor diameter in the chuck and the step hard against the chuck jaws to prevent it slipping back into the chuck. Machine a step in the front face with a minor diameter .02 larger than the machined face inside the piston. Tap the piston on to this mandrel and use a live centre in the tailstock to keep it there. Many pistons have centres because they were machined the same way The piston should run true, grind up a groove tool and machine the grooves. I have used this technique for deepening and widening ring grooves. For example using a holden 179 oil ring in a 4 hp MacDonald diesel. Last week I machined a squish area on some kawasaki pistons which had no centre so I used some aluminium scrap between the tailstock and the piston crown.

Many pistons can be machined this way as long as the mandrel does not move.

If full access is needed to the crown then use a jig which pulls on the gudgeon pin or chuck on the ring belt (this is not usually cam ground)

It is also possible to grind down thick rings on a surface grinder.

Originally Posted by markpest

Evening Mike, thought this might go with your 1941 Ford light truck! Your earlier question was about mounting, and a particular jig is illustrated - but that was 1925 - may be of interest in passing.

Mark

Good link there, I'm pretty sure I have that South Bend Shop booklet download, I have a number of them downloaded and saved.

Not that long ago a piston cam grinder was up on Ebay(Aussie) it went around a couple of times, don't know if he sold it or not, something like that wouldn't be much use to anyone now days; though the cam action may be able to be locked out in which case it could be used as a cylindrical grinder of sorts.

Cheers