Ok to use press to put pistons on rods? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 08-03-2010, 11:05 AM
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Ok to use press to put pistons on rods?

My buddy is going to build a motor and he started by pressing the new new pistons on the rods. He used a press and put them on cold rather than heat up the rod and then put the pin in. They went on fine and look fine but is this ok or should he buy new rods and start over? I think he should but he doesn't really have the money for new rods again.

Last edited by enduro54; 08-03-2010 at 11:17 AM.
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Old 08-03-2010, 11:21 AM
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I doubt he hurt the rods, I'd be more concerned with the pistons.
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Old 08-03-2010, 03:17 PM
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The pistons are forged and the pistons float on the pins. There is nothing wrong with the pistons.
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Old 08-03-2010, 03:34 PM
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The problem is that the pressure put on the pistons by pressing the wrist pins through the rods may have damaged the pistons.
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Old 08-03-2010, 04:34 PM
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Quote:
Originally Posted by enduro54
My buddy is going to build a motor and he started by pressing the new new pistons on the rods. He used a press and put them on cold rather than heat up the rod and then put the pin in. They went on fine and look fine but is this ok or should he buy new rods and start over? I think he should but he doesn't really have the money for new rods again.
The rod is heated not the pin nor piston. While called a press fit (pin to rod) it's really an interference fit achieved by heating the rod in an oven to expand it and if wanted cooling the pins in a freezer or on dry-ice to shrink them. They should just slide together with minimal effort not require a press.
Then when they both return to the same temperature there will be a slight dimensional difference providing the grip to hold the pin in the rod.

The problem that could develop with a simple press is that there is quite a bit of load put on the piston which can deform it or cause internal stress fractures to it. Pistons that are separated from rods are not considered usable regardless of other condition, such is the force to remove the pin. One should assume the same situation applies in pressing the pin into the rod while being supported by the piston when done with both at the same temperature.

Can you get away with what you guys have done is a maybe. If it doesn't result in a failure you're money ahead, if it does, the engine is probably 100% junk by the time you get it shut off so all the money and time that went into it goes to naught.

Having the pistons dye penetrant inspected for cracks would probably ease your mind, while costly to do, a lot cheaper to find out now than after parts blast their way out of the block. This process, also, would be less expensive than new pistons as well.

Frankly; I wouldn't trust it further than I could throw it, so if it were me, I'd be ordering new pistons.

Bogie
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Old 08-03-2010, 05:02 PM
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machinest with a brain

Pistons were pressed on in the "old" days but was a very poor way to do it.
The answer about heating the rod is correct. The heating of piston has nothing to do with it. Pins that are tight should be fit before hanging then on rods. A good machinest, ya thats not so easy to find, can fit pins.
Few machine shops check the small end of OE rod for press fit tollerances. It is something like .008 tenths to 1 and a half thousands.
A light hone with the correct stone and mandrel will keep pins from stopping part way in when heated in a rod heater. Do not use a torch!!!!!!

If you are lucky and do not distort the pistons with a press they will float on the pin easy, if the piston will not move with it's own wieght it is to tight and may be damaged, or may sieze when started.
Ring lands may also get damaged when pressed.
A press is to remove pistons only and do not exspect to re use pistons pressed off.
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Old 08-03-2010, 06:37 PM
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I always heat the rods in an electric Sunnen pin oven. On the other hand. My partner has been cold pressing pins for forty years and never misses.. The secret to cold press is the piston fixture that holds the piston by the internal pin flats. Doesn't impart undue stress to the piston.. If you lay the piston on a flat surface and try a cold press. You just junked a piston..

Our pin press jig looks like an inverted J with inserts that support inner surface of piston.. With the correct arbor and depth stop. A little spray of Sunnen press fit lube and presto. A cold correctly inserted rod/piston assembly.. Incedently I'm told that with the Sunnen lube you can press a telephone pole up a squirrels azzhole.. Strange feeling stuff...
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Old 08-04-2010, 12:16 PM
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press

Very few shops have that type of pin press fixture.
Rod heater is the no brainer way to go.
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Old 08-04-2010, 02:02 PM
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Regarding the OPs original question, I agree that for anyone to do this, at the very least, would demand some serious tooling and technique. Most use a rod heater. The induction heaters seem to work well, and quickly. But OlBogie makes a great point (I do usually agree with him :-)). In fact, it was gonna be my comment. A NDT using dye penetrant will indicate any damage to visible surrounding surfaces. Note the word visible. If the rod bore itself were damaged, I think the piston would likely not float well. Seems like a 100% visible surface dye penetrant examination along with the simple "float freely" test should give pretty good assurance that all is well. Not guaranteeable, perhaps, but pretty good assurance none-the-less.
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Old 08-04-2010, 04:09 PM
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piston press

Ok, I'll chime in on this one.
Anyone else using a raytec next to the rod oven?

We recently built our own oven using a 2" X pipe fitting laying flat. Small propane tip comes in from each side... Rod lays on a horizontal shelf welded to the front side of 2"X pipe fitting, and small end inserts to center of x. Back of x extends about 3 inches with close nipple to a 90 pointing up; another close nipple to a cap adapter which reduces to 1/2 pipe. Little 1/2 pipe nipple exhaust stack to draw heat one way. Small hole drilled on both sides of x, 1 inch past small end, or one inch past center of x. Twin Ratec Rangers mounted aimed through holes. When they register 675F you have 15 seconds to drop rod into install fixture and push lever which already has pin loaded and partly installed in piston of course.
Lever doubles install power but was designed only for control, smooth entry. More than that it keeps your hands a bit further from the heat. Not that the heat is critical but it lowers the pressure on the operator..... I don't know.. Kind of the way a running engine on a lawn mower or pressure washer has a tendancy to push the operator?? Just me?
Anyway, you will have about 20 seconds to install at 700f.. 25 seconds at 750, but anything over that and the the temp elevates too fast and the rod is subject to become blued. Reason we built it was that the standard electric rod oven often leaves half the set of rods blued.
We experimented on pistons until we discovered that 660 to 675f worked every time with no risk. We do have a full curtain around our unit to prevent moving air from causing us to have to over heat the rods. And we found that if we preheat the oven for ten minutes it works really nice, no pressure.

So anyone have an opinion on this deal?
Duntov
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Old 08-04-2010, 04:10 PM
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All excellent replies . In the old days .(1985) I used to heat the rods on a hot plate and tap the pins in with a plastic hammer and a special wrist pin drift while holding the pistons in my lap while seated . Poor man's method but never had any problems . Built several engines this way . Worked at a small shop where we rebuit stock engines .
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Old 08-04-2010, 04:44 PM
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Duntov . . . Sounds pretty resourceful to me. Not to mention it seems technically sound.
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