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Easy way to hold piston/rod assy for dome machining

9K views 16 replies 7 participants last post by  4 Jaw Chuck 
#1 · (Edited)
Working on a set of 289 domed old school L2249N TRW pistons, the dome is solid so lots of meat for mods. Research has told me leaving about 0.040-0.060" of the dome will give me about 10.5:1 comp with the 55cc heads we have ported.

The engine has less than 5K on it so it doesn't need rings or bearings but the pistons mean it won't run on pump gas, block is zero decked already so the quench is in the gasket. We have the piston/rod assy out of the block, they are so clean the carbon wiped off with a paper towel and some solvent. This is an old build that sat for close to 30 years, if it even ran for a week I would be surprised, internals are mint...even the cam is hardly broken in (280 duration, 0.510" lift)

Needless to say I don't want to take the pressed in pins out or even change rings but I need to hold the pistons on either a mill or lathe to machine off the dome.

My first idea was to bore out a tube to bore diameter, slit it and then clamp the piston inside so I can hold it and then just machine off the dome as required. Another similar idea was do the same thing on the mill with a welded plate on the bottom for clamping the jig to the table...the lathe method is easier and I wouldn't even need to take rings off to do it. The pistons have a slipper style skirt so nothing to hold onto there but the whole assy would be close to 8 inches tall so cuts would have to shallow meaning taking longer to do.

Anyone else done something similar? Better way of doing it? Ever use PVC pipe to do something similar, definetely cheaper jig that way.

This is what they look like on top, not my photo sorry.



Open to ideas outside the box, thanks.
 
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#2 ·
Problem with holding the assembly in a tubular fixture in the lathe is that the piston is largest diameter at the bottom of the skirts and smallest above the top ring land.

I'd build a 4-1/2" OD x 1/2" wall split ring fixture to hold it hanging off the side of the mill table, gripping the piston around the area just below the oil rings, then swing the head of the mill around to get it aligned above the fixture.
 
#3 ·
The way I see it you need to have the rods pressed off the pistons. Taking material off the piston dome will require the motor to be rebalanced. I would never reuse piston rings. If I was doing it I'd have the rods pressed off the pistons, clamp pistons down on mill table by the flats under the piston pin boss and fly cut the piston tops. Then take the rotating assembly to get rebalanced. When it comes back from balancing assemble with new rings, bearings, and gaskets.
 
#4 ·
All of these concerns are mostly a non-issue.

Low mileage rings left on the piston and put back in the same cylinder bore seal up just like they were never removed, just as long as there was no ring ridge they were forced past on removal.

From Chuck's description of leaving some of the dome intact, it would appear he is looking to remove about 6-7cc. Weight of aluminum is roughly 2.7 grams per cubic centimeter, so it will be less than a 20 gram change. Since it will make the piston lighter, you are working in the correct direction to have a slight overbalance from the crank counterweights....and a slight 2% overbalance to about 50-53%is the current high perf balance target, compared to past years standard of equal 50% balance.

So balancing really ends up a non-issue....stick it back together and no one including the engine will ever notice a difference. If most guys realized what goes into balancing and what actually goes on inside a running engine they would realize how not exact a science balancing is.....none of the factory high performance engines of the 1960's was ever precision balanced from the factory, not even 7000+ rpm screamers like the Ford 289 Hi-Po and Boss 302, Chevy 302, the Fuelie 327, the LT-1 350, the 340 Six Pack Mopar, etc were simply target balanced, with parts fitting a typical 50 gram range.

For some reason, probably lack of understanding of how it is done, balancing is considered almost "black magic" by a lot of car guys.

EDIT: Sorry HCompton, you posted pretty much the same thing while I was typing my long version.
 
#6 ·
Unfortunately the mill I have access to is a heavy duty Bridgeport style and although the head tilts it doesnt swing.

I think building a fixture tall enough to support under the pin bosses is the best way to do it...at least that way I can put enough clamping force on the assy and be able to machine off the dome in two passes instead of tickling it with multiple cuts. I think I have some thick wall square tubing I can notch out to build it with instead of a stack of blocks, then I can clamp using the pin hole with a simple pin with tapped holes in the end and some tabs welded on the tubing to bolt too.

Not worried about reusing the rings, done that dozens of times before with no issues...just have to make sure the rings are taped up so the chips don't get in there.

Thanks guys!
 
#7 · (Edited)
Yeah, not being able to swing the head eliminates hanging it over the side of the table.

I think you've got a good idea now, just make sure to side brace your tubing uprights, as the taller you get off the table the easier chatter will show up.

Best of luck.....I'm betting you'll be just fine though :thumbup:
 
#8 ·
Thanks guys, the fragile ring lands and unavailability of replacements make this a dangerous proposition if messed up...gonna err on the side of beefy to be on the safe side.:thumbup:
 
#13 ·
I am thinking of a piece of heavy wall tubing bored to fit the piston. Split it in two halves one to be welded to a fixture clamped to the Bridgeport and the other to be clamped on the piston so the piston is clamped between the halves.. hope you can see this in minds eye. Would not be the first time I have built a custom fixture for a one time job..

Sam
 
#14 ·
Basically what I was thinking of doing except just one split, my spindle isn't large enough to take the big rod end so it would have to hang out which isn't ideal...would need a center rest which this lathe isn't equipped with. I would have counter bored the tube so the wider skirt had room and leave the ring area a thou over ring land diameter so it would be a snug fit.

The pin boss is always the strongest area of a piston so clamping down to the table using that as support is the safest and able to take the most stress, also is machined flat to the piston top so squaring the piston isn't an issue. I can just drill a hole through the notched square tubing jig and use the rod bolts to torque it down to the tube.

Hard to describe but easy to make, just a piece of square tubing with welded tabs on the side to bolt to the mill table...machined flat across the top/bottom so its square to the table and notched on two sides to allow the skirt to hang outside the tubing. The top of the square tubing supports under the pins bosses.One bolt through the tubing for the big end to pull on and use the rod bolts to pull the assy tight together...yes the jig has to come off the table for each piston but thats minor.

If I had only one to do or the piece wasn't that valuable I would just stack blocks under the pin bosses and use table clamps on a pin through the piston pin but thats a little rickety meaning light cuts...probably 0.010" at a time which would take an hour...same thing with a flycutter...I need to take a 3/8" off the top of these things and a 6 inch carbide face mill will take that off in one pass not tickle it with a single point flycutter.

I'll take a pic of the setup when I'm doing it, much easier to show than describe.
 
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#15 ·
Did the pistons yesterday, three hours start to finish including fixturing and clean up. Just in case your getting a quote for doing such a thing.

Not the sturdiest fixture I've rigged but enough to take a 0.350" depth as a roughing cut and then two 0.050" finishing cuts to get a 0.045" dome. Lots of extra meat on these old forgings, took a lot of weight off them, the domes are solid.
 

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#17 ·
Wasn't sure if the step blocks were square and equal height but they were which made it pretty simple, also the head does swivel on this mill if I needed it.

If I was going to do a lot of pistons I would have turned the nose of one of the side clamps round in the lathe and inserted it directly into the pin instead of using the bar, limited rigidity since the pinhole was only 0.600" ID.

Certainly happy with the result and money saved.
 
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