Knurling pistons

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Knurling pistons

Post by k-star » Sun Jun 23, 2013 3:16 pm

This is a question I though I would never be asking. I have a customer doing a high dollar restoration. The engine is 100% correct down to the factory original pistons. He dropped off the block last week. It had the machine work done to it maybe 20 years ago, but never assembled. The bores are about .0030" over stock size now.There is some slight surface rust in the bores. I am sure I can clean it without going bigger then .0035". It is a 350 Chevrolet with factory forged pistons.

I am about 95% sure the thing is going to have piston knock.

When the restoration is complete I am sure it will be a 100 mile a year driven car.

Would knurling the pistons take care of the piston slap.

Don't everyone pile on and say how stupid it is not to bore it, that's not the question. I know what the correct thing is to do, but the restoration world see's things different some times. I am just trying to get all the options to give him.

Do the pistons need to be removed from the rods to knurl them?

Can anyone here do them?
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Re: Knurling pistons

Post by bigjoe1 » Sun Jun 23, 2013 3:28 pm

I have had piston skirt coated to reduce the clearance ,and that worked out very well. but you would still have to remove the pistons from the rods.


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Re: Knurling pistons

Post by CREngines » Sun Jun 23, 2013 3:37 pm

I have a knurler i picked up for a rainy day. Never used it though but i know the process works to a degree. The trick is to knurl them up big and put them in tight so they clearance themselves. The coating may be a little more kosher these days. If you decide to pursue it i could probably loan it to you.
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Re: Knurling pistons

Post by Truckedup » Sun Jun 23, 2013 4:01 pm

What's the clearance? I think GM forged pistons in performance engines were around .005 clearance?
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Re: Knurling pistons

Post by ap72 » Sun Jun 23, 2013 4:44 pm

I'm guessing sleeving or plating the block is out? The slap at .010" over is enough to break pistons, .030 over just sounds retarded. I guess noise and smoke and running like shit don't matter on a restoration?
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Re: Knurling pistons

Post by lorax » Sun Jun 23, 2013 4:56 pm

ap72 wrote:I'm guessing sleeving or plating the block is out? The slap at .010" over is enough to break pistons, .030 over just sounds retarded. I guess noise and smoke and running like shit don't matter on a restoration?
I think the number was .003

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Re: Knurling pistons

Post by robert1 » Sun Jun 23, 2013 4:58 pm

Knurling is not an option I would use. Each one of those knurls is going to be a high spot on the piston and could lead to wear in the cylinders. If they don't want new pistons then they get what they get. You can't wave a magic wand over them and make them new. I've done some pretty high end restorations, from a numbers matching 65 convertible Vette to a 348 57 Pontiac, and they got new pistons.

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Re: Knurling pistons

Post by ap72 » Sun Jun 23, 2013 5:01 pm

Sorry for my mistake! .003" isn't nearly as bad. I've ran that before and it'll slap a hair cold but otherwise it was fine.
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Re: Knurling pistons

Post by lorax » Sun Jun 23, 2013 5:09 pm

I think I would have them coated as Jue said. They can lay the stuff on up to 20 microns thick,
In the old days ny grandfather would have them tin plated, but I don't know if anybody does that any more. Dry film coating is your best bet.

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Re: Knurling pistons

Post by DaveMcLain » Sun Jun 23, 2013 5:16 pm

I'd knurl the skirts to close it up a little it'll work fine and it won't cause any problems. I've used this fix in exactly this situation a lot of times without trouble.

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Re: Knurling pistons

Post by PackardV8 » Sun Jun 23, 2013 5:18 pm

Each one of those knurls is going to be a high spot on the piston and could lead to wear in the cylinders.
FWIW, the aluminum high spots on the pistons aren't going to wear the cast iron bores.
There is some slight surface rust in the bores. I am sure I can clean it without going bigger then .0035".
Knurling is still good old school science. I've seen everyone from Rolls-Royce restorations to a John Dianna Jr Stock record holder running knurled pistons. If the pistons are .003" and you have to hone to .0035", knurl the skirts to zero so they are an oiled push fit into the bores. They'll be fine, run quiet and won't cause any problems.

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Re: Knurling pistons

Post by Dave Koehler » Sun Jun 23, 2013 6:20 pm

Mr. Vines is correct. From your measurements you can get by with this. Might coat them also. Just be sure the customer understands the future possibilities and you don't want to hear a phone call about a tick-tick. I would still try to get him to bore it so you can sleep. Never seen a show judge asks for the heads to be removed.

IF someone has a Perfect Circle Machine in the area those can do the job with the rod on. There is an upright bench model that also can do it with the rod on but the name escapes me.
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Re: Knurling pistons

Post by wyrmrider » Sun Jun 23, 2013 6:30 pm

I'd want to pull the pistons off the rods and touchup the pin bores
most lube would be long gone
nothing wrong with knurlong
remember when we had to pay extra to have groves cut in our new fordgtrues?
knurling would actually hold some oil on the skirts
you can always just knurl the non thrust side
those factory TRW style pistons - any noise would go away shortly in any case
find out what the oem specs were on those
are they bushed or press fit?
any rust spots on the locks and they break

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Re: Knurling pistons

Post by IDT-572 » Sun Jun 23, 2013 6:55 pm

Back when being poor would have been a step up for me, knurling was something that I had to do because of needing the engine running and not having the coin to do it right.

I never saw any more wear on the cylinder because of it though. The knurl holes a lot of oil till the piston and cylinder wall get to know each other.

I have a friend, I let have used parts I traded for and scraped together an A headed 514, the piston to cylinder wall clearance is between .009 - and .011 tightest to the most clearance (4.440 bore). Is it the way it should be if he had the coin to do it correctly? NO, but it is running and he is enjoying it. sealed up good. The file fit rigs were put in and all had over .020 ring end gap (no filing required LOL) I just told him to get it up to temp before extracting vengeance on it..........

Sometimes one can't do what would be the best to do, but what you end up with is good enough for who it's for.

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Re: Knurling pistons

Post by robert1 » Sun Jun 23, 2013 8:05 pm

PackardV8 wrote:
Each one of those knurls is going to be a high spot on the piston and could lead to wear in the cylinders.
FWIW, the aluminum high spots on the pistons aren't going to wear the cast iron bores.

I guess yall have never seen cylinders scuffed at the bottom of the cylinders below the ring travel. Will it work maybe, is it junk absolutely.

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