Author Topic: Unseizing a piston  (Read 20764 times)

Bob Purton

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Unseizing a piston
« on: April 25, 2012, 10:17:03 PM »
I know we have talked about this before but as I bought an engine from italian ebay recently with a seized piston which I just dont seem to be able to budge I thought I would bring it up again. It was described as seized, condition unknown. I striped it down and fortunately its in really good unworn shape inside, I guess once I do get the piston out it will need reboring to remove the pitting of the bore.

I tried all my usual tricks to unseize it but so far no dice. Did someone last time mention dunking the whole assembly in diesel for a month?

Jonathan Poll

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Re: Unseizing a piston
« Reply #1 on: April 26, 2012, 06:44:10 AM »
I know we have talked about this before but as I bought an engine from italian ebay recently with a seized piston which I just dont seem to be able to budge I thought I would bring it up again. It was described as seized, condition unknown. I striped it down and fortunately its in really good unworn shape inside, I guess once I do get the piston out it will need reboring to remove the pitting of the bore.

I tried all my usual tricks to unseize it but so far no dice. Did someone last time mention dunking the whole assembly in diesel for a month?

Someone on my facebook group was doing a Goggo crank using cider. I know its different, but he said it eats away the corrosion between the ali and steel.

Apart from the usual tricks, I have no idea. Maybe try the tricks here? http://www.hotrodders.com/forum/removing-seized-pistons-24175.html

JP
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Barry

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Re: Unseizing a piston
« Reply #2 on: April 26, 2012, 07:49:43 AM »
In the past I have fitted a suitable diameter of steel on top of the piston then gently tightened the head down on it.  It will move!
Leave it up to you Bob to decide what to put in-between the head and the piston so that no damage is done.


Big Al

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Re: Unseizing a piston
« Reply #3 on: April 26, 2012, 08:06:55 AM »
'Tis I of the diesel dustbin. It will penetrate a seized piston. The chief advantage is it penetrates all the rusted fasteners and so many more of then undo without hassle. Then you have to recall I am a committed dismantler of long unused engines.

It does depend on why the engine is seized. A mechanical seizure is dependant on the breakage. Just having a unit with no use is normally rust binding. That will often go with a soak and perhaps a cycle of heat to break the bonding. The worst is a used but left engine as the rings will often have petrol/carbon crude round them. If the rings rust on the inside they are forced out onto the cylinder with the organic gum making a very effective barrier to any form of treatment. Even heat really makes very little difference.

I just failed to part a Sachs 175 unit. Looked fine but the piston would not clear. I gave it all the usual treatments. I got it moving with a block of wood and a mallet but it then got to BDC and that was enough constriction to stop it. Once again with the mallet and clearly I was hitting it to hard as one fin just shattered from the vibration. The piston itself was no good by now so I gave it to Russell Church as he wanted an exhaust thread to mount on a jig to make Messerschmitt Exhaust systems, available shortly form Russ. Russ reported that even though he put a band saw round the piston thus relieving the pressure some it still refused to budge. I await the mutilated remains on the end of my crank next time we meet. Sometimes the little swine will not come out and that's all there is to it.
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Barry

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Re: Unseizing a piston
« Reply #4 on: April 26, 2012, 08:07:30 AM »
I bought my most recent machine from an old friend who now owns his own engine rebuild shop.  East Dulwich, South London.
I was impressed - Not seen him for 30 years.

Worth a try for engine machining.

http://www.motortechengineering.co.uk/index_files/Page380.htm

http://www.motortechengineering.co.uk/

Bob Purton

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Re: Unseizing a piston
« Reply #5 on: April 26, 2012, 09:39:30 AM »
I have tried tightening the cylinder head down on the piston with a nylon bung in between but no dice. The hot rod forum Jonathan linked me to recommends anything from transmission fluid to coca cola, make you wonder what it does to your insides!  Think I will try diesel followed by my friends hydraulic press. If worst comes to worst I will have to machine the old piston out! Thanks chaps.

Barry

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Re: Unseizing a piston
« Reply #6 on: April 26, 2012, 09:46:26 AM »
Any chance of a good soak in the oven or is the engine all in one piece?

Could try a blow torch on the barrel whilst pressure is applied using the head tightening technique?

Bob Purton

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Re: Unseizing a piston
« Reply #7 on: April 26, 2012, 10:22:03 AM »
Any chance of a good soak in the oven or is the engine all in one piece?

Could try a blow torch on the barrel whilst pressure is applied using the head tightening technique?

Its now just a crank shaft dangling from a barrel.
As someone who takes two hundred year old instruments apart for a living I have tied all the obvious. The blowlamp always works when its brass in brass or steel in brass, thats assuming you are not having to retain an original patina.  steel bonded to alloy appears to be another ball game. I may use you oven suggestion just before it goes under the press. Thanks.
« Last Edit: April 26, 2012, 10:23:40 AM by Bob Purton »

Big Al

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Re: Unseizing a piston
« Reply #8 on: April 27, 2012, 07:57:33 AM »
I am assuming it is a two stroke. If not then you can pump oil into the cylinder to push it out. Clearly that does not work if there are ports etc. unless you go to a lot of trouble blanking everything off.

Heat breaks the bonding a bit. I think for clearance of a piston it wants to go in a fridge. Normally the engine has correct clearance at running temperature and will seize when hot as the piston is to big. Therefore put it on the deep freeze and the piston should proportionally shrink more. The problem is that the rings are of similar material to the cylinder so they will not dramatically alter with temperature relative to the barrel. A couple of cycles of this treatment of hot to cold might release some of the binding though.
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Barry

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Re: Unseizing a piston
« Reply #9 on: April 27, 2012, 08:02:49 AM »
Heat the whole thing up then pour some dry ice in the piston from underneath and hey presto the piston will fall out!  (probably in small pieces).

steven mandell

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Re: Unseizing a piston
« Reply #10 on: April 27, 2012, 09:37:26 AM »
I used a thick hard wood disk and several carefully placed sledge hammer blows after soaking in penetrating oil for a week on a Lotus engine.
I finally gave up when I felt that the danger of a miss ruining the block was too high.
However the machine shop that bored it 30 thousandths over had no problem removing it when they got it under their god knows how many ton press.
So maybe just having the right fixture and strong enough press will do it for you too.

Big Al

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Re: Unseizing a piston
« Reply #11 on: April 27, 2012, 10:12:14 AM »
A press applies even pressure not available by mallet. The mallet applies shock not available by the press. As with pullers the ideal is to wind up the pressure and apply a shock to break the bond. A press, like a puller will probably eventually win out but it can be at the expense of deformed parts or tool. If your unlucky the part can deform locking into place ever more firmly! In tool terms the Isetta/Goggo dynostart puller, or more likely home gleaned bits to replace said item is a classic case of an own goal. Banging the crap out stuff is normally a course to success or destruction, see 175 barrel story. I persisted as it was not a valuable part otherwise I would have stopped and made a few calls.
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Jonathan Poll

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Re: Unseizing a piston
« Reply #12 on: April 27, 2012, 09:45:11 PM »
Have you tried putting WD40 in it, leaving it overnight? That usually frees the rust, but I've never tried it on a cylinder (I'm still young, only unseuzed a couple of cylinders. I still need practice ;) )
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Barry

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Re: Unseizing a piston
« Reply #13 on: April 27, 2012, 09:51:33 PM »
Instead of trying to push it up or down, have you tried rotating it Bob?

Jonathan Poll

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Re: Unseizing a piston
« Reply #14 on: April 27, 2012, 09:54:25 PM »
Instead of trying to push it up or down, have you tried rotating it Bob?

Depends if the crank is also seized, if so hyou cant lift the cylinder to rotate ;)
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