problem with pistons

6

67rcks

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Feb 20, 2008

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Aug 1, 2008

#1

• Aug 1, 2008

• #1

I got my pistons back from a motor shop today. They put new pistons (with new wrist pins) to old connecting rods. About half of them seems pretty tight (wristpin to piston) while the other half seems quite loose (e.g. piston won't stay level if holding it up). One or two connecting rods are furthermore little offset, about .05" ie they are not perfectly in the middle of the wrist pin.

Do connecting rods have to be in the middle of the wrist pin? Is it normal that new pistons would have such big tolerance? These are speed-pro brand I got from summit not the best quality or cheapest ones. Can it be somehow fixed or it's a trash?

 

D

D.Hearne

New Member

Sep 29, 2000

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south louisiana

Aug 2, 2008

#2

• Aug 2, 2008

• #2

Sounds like you need to find a better machineshop to do you work. Pins should be centered in the rods, pin to piston clearance should be the same for all. I'd also bet some of the pistons are installed backwards too.

 

B

bnickel

Founding Member

Aug 21, 2002

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lubbock, texas

Aug 2, 2008

#3

• Aug 2, 2008

• #3

i agree with D, take them to another machine shop and see what they think. i hope they didn't heat the pin end of the rod with a torch like a lot of people seem to do....that always seems to me like they are screwing with the metalurgy of the rod. i've always used a hydraulic press to press them in myself. i've seen a lot of rods come apart at the pin end and you could easily tell that someone used a torch to install the pins because of the buing that's always present from hetaing the rods with a torch. whether or not that was the actual cause of the rod breaking or not i can't really say but it does make me nervous to see that.

 

D

D.Hearne

New Member

Sep 29, 2000

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south louisiana

Aug 2, 2008

#4

• Aug 2, 2008

• #4

The older machine shops sometimes used a heater to heat the pin end, that would be preferable to a torch, at least the heating would be evenly done. But I'd prefer a pressed fit without the heat too.

 

6

67rcks

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Feb 20, 2008

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Aug 2, 2008

#5

• Aug 2, 2008

• #5

He did not do anything with pistons, just heated rods + pressed new wrist pins (he had some laser thermometer to control heat). And today it turned out that a few of them have pin about 0.030" shorter than the distance between piston clips so pin+conn rod slides inside the piston. well are these pistons trash?

 

D

D.Hearne

New Member

Sep 29, 2000

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south louisiana

Aug 2, 2008

#6

• Aug 2, 2008

• #6

As long as the pins are centered, you're fine. The rod & pin need to have lateral movement in the piston to function. Just as long as the pin doesn't slide in the rod.

 

T

TOM B

20+ Year Stangneter

Nov 15, 2002

872

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16

Harrisburg Pa

Aug 2, 2008

#7

• Aug 2, 2008

• #7

I agree with "D" but I like full floating pins. I would have had the bushings honed for a full float and not worry about pin centering, less stress on rod to.
On a 302- 351 the rod to piston relationship is,
With the piston notch or arrow facing toward the front of motor the numbers on the rodsand caps should face to the outside of the block. 4 one way and 4 the other way.

 

B

bnickel

Founding Member

Aug 21, 2002

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lubbock, texas

Aug 3, 2008

#8

• Aug 3, 2008

• #8

TOM B said:

I agree with "D" but I like full floating pins. I would have had the bushings honed for a full float and not worry about pin centering, less stress on rod to.
On a 302- 351 the rod to piston relationship is,
With the piston notch or arrow facing toward the front of motor the numbers on the rodsand caps should face to the outside of the block. 4 one way and 4 the other way.

Click to expand...

the rods also have a slightly larger chamfer on side of the big end for the fillet on the crank, this side goes to the outside edge of the crank and the smaller chamfer on the rods face each other.

 

6

67rcks

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Feb 20, 2008

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Aug 3, 2008

#9

• Aug 3, 2008

• #9

D.Hearne said:

Sounds like you need to find a better machineshop to do you work. Pins should be centered in the rods, pin to piston clearance should be the same for all. I'd also bet some of the pistons are installed backwards too.

Click to expand...

Your bet is right. 1-4 are installed backwards. If I had a shop press I would do it by myself. Now I can only hope my crank is machined right... have no plastigage to check it.