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Old 08-25-2008, 10:11 PM
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Adding metal to create an oversized piston?

Looks like I've got to bore out my cylinders and try and find some oversized pistons for an antique engine I am working on. Manufacturer stopped making them years ago.

Is there any way to add metal to an aluminum piston? Can a small defect in a piston be TIG welded?

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Old 08-25-2008, 10:24 PM
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I don't know about older pistons but most modern pistons are not perfectly round. There are several piston companies that will custom make any piston you want. maybe this would be a better option.
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Old 08-26-2008, 12:21 AM
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Back in the old days they used to ...what they called "expand the pistons", which basically meant the knurled the skirt to make them a little bigger. A band aide approach at best which only lasted a very short time. You didn't say what engine you have but you might give www.egge.com a call, they make engine parts and rebuild kits for all kinds of older engines....good luck
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Old 08-26-2008, 12:39 AM
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You can knurl the skirt or plasma spray the exterior to increase the diameter, I have had the best results with knurling. You can add 0.010" to the OD of a piston with knurling quite easily, even did one 0.030" once.

Make sure you knurl oversize and then polish to fit the bore.

Be careful you support the skirt while you do this, there are many ways but a couple come to mind that work well.

1.) Fill the skirt with Wood's Metal and then center drill for support in a lathe.

2.) Machine an aluminum plate with grooves that snaps onto the skirt to prevent the skirt from spreading or collapsing under the pressure of the knurling tool. If necessary machine the inside of the skirt slightly so it fits in the groove snugly.

Usually I use a combination of the two techniques by using cerrobend to fill the skirt and then machine the AL plate to support the skirt "ears", then chuck the piston head above the top ring land.

I recommend using a single 45 degree knurl wheel to do the work as a double wheel gives a crosshatch knurl that won't last very long. Do the work in the lathe by rotating the piston by hand with no feed and do one skirt ear at a time. Too much pressure will crack a cast piston easily so everything you can do to support the piston assy will help.

A 0.001" cleanup cut on both sides of the top ring groove is advisable while the piston is in the fixture since the chuck jaws will tend to collapse the top ring land slightly. Remember piston skirts are egg shaped and manipulation of the infeed is necessary to get a good knurl on the entire face of the skirt, usually 3-4 passes 3-4 knurls wide is needed to get a good running surface on the skirt depending on the width of the skirt.

I've done many old pistons this way and all of those engines are still running to this day. They don't burn oil and have good compression, the key is stabilizing the piston so it doesn't rock in the cylinder once up to temp. Lots of clearance above the skirt (around the ring lands) doesn't hurt much unless your looking for high output.

Plasma spraying the skirt is a lot more work and I have seen it flake off after a few hundred heat cycles so I don't recommend it unless its mostly for show and you don't mind grinding it with a cam grinder to get the barrel shape back onto the piston.
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Old 08-26-2008, 02:48 AM
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I too wondered the very same thing.
I suppose knurling isn't too bad of a fix if the knurls kicked up ALOT of surface area.
Spray on coatings (if bonded properly) work as well.
A better choice is to use an allready manufacturered piston & alter if necessary to fit the application.

I have a .060 over 400 chevy that will go another .010-.015 & still have suitable wall thickness.
Only problem is that pistons aren't made any larger than .065 over.
454 chevy & 401 AMC are likely candidates to use but I'm just going to wait until they start making larger pistons for the 400.
Like .080 over (350 pistons)
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Old 08-26-2008, 05:17 AM
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JB don't hold your breath waiting for an .080" over 400 piston. Ain't gonna happen unless it's custom.
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Old 08-26-2008, 07:44 AM
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Bore the block and sleeve it back to fit the piston.. I've dove that before when can't go any further than .060 over bore and the block has matching number to the car.
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Old 08-26-2008, 08:29 AM
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Cast your own pistons!!
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Old 08-26-2008, 08:37 AM
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Quote:
Originally Posted by Double_v23
Cast your own pistons!!
LOL
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Old 08-26-2008, 10:55 AM
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Reminds me of that movie "The Worlds Fastest Indian", the guy from New Zealand was casting his own bike pistons, quenching them in a rain barrel. Same barrel the little kid got water to brew the tea.
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Old 08-26-2008, 10:56 AM
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Quote:
Originally Posted by SSedan64
JB don't hold your breath waiting for an .080" over 400 piston. Ain't gonna happen unless it's custom.
Yeah sure, that's what was said about .080 350 pistons. Now they're fairly common.
I was on the phone with one of the Keith Black techs about the block I have & I told him it would probably clean up fine @ .065.
Low and behold, 3 months later a .065 over piston was added to KB's lineup.
http://store.summitracing.com/egnsea...115+4294782135

The only thing I can make of it (because I seriously doubt it was just me that caused the change), is that they've recieved many other calls wondering if anyone stocks larger than .060...

There were many early 400 blocks punched out to the .060 oversize over a decade ago (mine was built in 1992, probably on it's 2nd overbore)
Now that they're worn out many people are picking up the cores very cheap
& trying to see if they are in fact "unbuildable"
I aquired my block for free with a good std. block for $300.

The seller gave it to me with the bottom end & heads, calling it worthless since it was .060 over allready.
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Old 08-26-2008, 01:55 PM
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Quote:
Originally Posted by SSedan64
Reminds me of that movie "The Worlds Fastest Indian", the guy from New Zealand was casting his own bike pistons, quenching them in a rain barrel. Same barrel the little kid got water to brew the tea.

Yeah I was thinking of the same thing when I wrote it.
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Old 08-26-2008, 06:59 PM
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You can always have the cylinders slightly bored and then have them nickel/chrome plated back to std. bore.

Seriously, it's not that uncommon in aviation engines, i don't see why it would be a problem for an auto engine.

Also, that plating wears out alot slower than regular iron cylinders would, just make sure that if you do that to only use cast rings. Chrome plated rings on a chrome bore is a no-no.
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