Piston wrist pin issue

Bob Johnson

Bob Johnson

Well-known member
The Model B engine in my pickup had some issue so I took it out and disassembled it. I found the remains of a wrist pin retaining clip in the oil pan. It came from #3 piston. There was a groove in the #3 cylinder wall where the unrestrained wrist pin cut the groove. I also found a problem with one of the retaining clips on the #1 piston. Both clips on #1 were still there but one of them had tore up the retaining groove in the piston and was likely to fail at any time. Below is a picture of the damaged retaining groove in the #1 piston. My question is "What would cause this to happen in to 2 of the pistons?".

Bob
 

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A bent rod will exert a side force on the wristpin that can cause your problem, I also have seen it from the cylinder bore bored at an angle causing the piston to move on the rod --what does the wear on the skirt look like?, is it worn at an angle --like worn on the front on the top, at the rear on the bottom.
 
Kurt in NJ said:
A bent rod will exert a side force on the wristpin that can cause your problem, I also have seen it from the cylinder bore bored at an angle causing the piston to move on the rod --what does the wear on the skirt look like?, is it worn at an angle --like worn on the front on the top, at the rear on the bottom.
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I second that, bent rod.
 
There is some wear on both the front and the back bottom on the piston skirt. Perhaps a little more on one side.The picture below shows both front and back sides. The wear appears to be scratches. There are more scratches in the picture on the right.

That would be 2 bent rods ? How do I check the rods for bends? How do I check the cylinders for an angled bore?

Bob
 

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Bob, within the last 3-5 years I have heard of the same thing in three engines. The rods were all dead-nuts straight but as I recall the keepers were not correct - maybe they were square in section while the groove is round? Unfortunately I do not know the source of the pistons.
 
Rich,

I checked the retaining clip is round and the groove is square. I checked several old piston that I have and they also had round clips and square grooves. I believe that round clips and square grooves are what most pistons have ??

Bob
 
Bob Johnson said:
There is some wear on both the front and the back bottom on the piston skirt. Perhaps a little more on one side.The picture below shows both front and back sides. The wear appears to be scratches. There are more scratches in the picture on the right.

That would be 2 bent rods ? How do I check the rods for bends? How do I check the cylinders for an angled bore?

Bob
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Bob take your rods with the piston on them to a Machine shop that does engines. They have a tool that will check for bent rods or rods that have a twist in them. I have found many that are bent or have a twist in them.
Did it have a light knock at idle?
 
During the time period 5 to 15 years ago I bought rebuilt rods from 3 reputable sources. All were bad because the crank bore and wrist pin bores were not parallel. You can check this in the assembled engine block by moving each piston to the top and looking to see if the tops of the pistons are parallel to the block surface. The problem might have been bent rods with which the wrist pin honing left the 2 bores out of parallel.
 
I put the 2 piston back in the engine with the crank. I tried to measure the piston heights at the front and rear of the pistons but I do not have anything that I can use to get a good measurement. However I did measure the piston to cylinder wall gap at the front and rear of the two pistons while the piston was at the top of it's travel. For cylinder #3 I got a gap of 0.015" at the back and 0.010" at the front. For piston #1 I got 0.010" at the back and 0.015" at the front. I rotated the rods an pistons 180 degrees in the cylinder and the gap measurements switched back to front. I have not yet check the 2 cylinders that did not have damaged wrist pin holes.

I guess that this means that the cylinders and crank are not out of alignment but the pistons are tilted forward or backwards depending on the rod/piston orientation. I can come up with 4 possibilities here. The rods are bend forward or backwards, the rod babbitt (rod bearing) is misaligned, the wrist pin hole in the piston is misaligned, or the piston is worn more on one side than the other.

Did I miss anything? Anyone have any suggestions on what else I can check before take the parts to a machinist?

Bob
 
d.j.

Here is a picture of the other sides (left/right) of the piston. I did not find any abnormal wear there.

Bob
 

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Try this to measure piston heights. Put a straight edge across the top of the cylinder and bring the piston to the top or as close as the straight edge will allow. Now a feeler gauge can be inserted front and back to compare measurements. A twisted rod will be a little harder to measure but if a bar the diameter of the big end is inserted as well as one the size of the piston pin in the little end. The longer the better. Lay the rod on a flat surface and make sure one of the bars is spaced equally above the flat surface. If the other one show a variation the rod is twisted. I think to do this kind of damage you have some serious misalignment issues that you should be able to measure.
 
Bob, from your photos of the piston wear I doubt there was any major cylinder misalignment or rod bend. I see four possible causes. 1) If the retainer groove depth was not at least 1/2 the clip wire diameter the floating wrist pin could easily knock them out. 2) The two clip ends that you squeeze together should be bent out of the flat plane of the ring, and point outward. If installed backwards and if they point inward, the wrist pin could knock the ring out of the groove by touching the ends rather than the ring in the groove. 3) The clip rings may have had a helical twist, unable to sit in a flat plane without spring stress. 4) The wrist pins may not have enough, or the wrong chamfer. The chamfer should contact the ring, pushing it deeper in the groove. If the pins butt square against a round wire ring it can pop out of it's groove.

If you use pistons with square grooves for tru-arc internal rings the wrist pins should NOT contact the ring with a chamfer.

Personally, I HATE floating wrist pins which require retaining rings. If you look at my avatar, those are 283 Chevy pistons now in one of my A's. The wrist pins are press fit into modern rods, no retaining rings required on the piston. You can get those rods and pistons from Rich Falluca, Antique Engine Rebuilding in Skokie, IL. Modern narrow rings with a Moly top ring, too.
 
Additional information

I was able to check the "tilt" of the pistons front to back. I used s straight edge and feeler gauges. I took the measurements twice. The first time I put all the pistons and rods in the same orientation that they were originally. Then I rotated the pistons and rods 180 degrees in the cylinders so they were now flipped front to back. Here is what I found.

Original orientation
#1 front 0.011" back 0.013" difference 0.002"
#2 front 0.004" back 0.014" difference 0.010"
#3 front 0.021" back 0.014" difference 0.007"
#4 front 0.018" back 0.028" difference 0.010"

180 degree rotation (front to back)
#1 front 0.022" back 0.019" difference 0.003"
#2 front 0.021" back 0.011" difference 0.010"
#3 front 0.018" back 0.025" difference 0.007"
#4 front 0.022" back 0.011" difference 0.011"

As you can see the tilt reversed when I rotated the pistons and rod. So that means that the tilt is due to the pistons or the rods and not the crank or bore. To see if it was the piston or the rod I rotated just the piston on one cylinder and I found that the tilt did not change. So that tells me that the tilt is due to the rod and not the cylinder. The 3 possibilities are that the rods are bent, the upper bushing was reamed incorrectly or the lower bearing was reamed incorrectly

Please note that the piston that had the catastrophic failure was #3. #1 was the one with the retaining clip that had started to tear up the king pin hole but had not broken loose yet. Pistons #2 and #4 showed no signs of wear at the retaining clip grooves. That means that the pistons with the least amount of tilt were the ones that had problems. None of the piston pins showed any up-down or side-side play in the pistons.

I am now not sure what happened. I do not want to repair the engine until I can figure out what happened, since it might happen again if I do not address the unknown cause.

Bob
 
Retaining clip details

MikeK

I took some measurements. The retaining clip has a diameter of 0.088". When it is in the groove in the piston it sticks up 0.030" to 0.038" above the pin bore. So well over 50% of the width of the ring is in the groove. The rod pin is hollow so the end of the clip have nothing to hit on the pin. The chamfer on the ends of the pins is very slight. I would estimate that the chamfer is about 0.010". Since the ring sticks out of the groove 0.030" to 0.038" the chamfer can not force the ring into the groove.

Bob
 
To make the piston tilt measurements more meaningful, the cap must be tight on the crankshaft. If you have a really good fit on the crank, it should be O.K. If loose at the crank, take out a shim or put in a piece of smooth aluminum foil to get the best readings at the top of the piston.
 
azmodela said:
Now would be a good time to put it back together with teflon wrist pin buttons!
Click to expand...

Have you used them? There have been reports that grit embeds in the teflon buttons and wears grooves in the cylinder walls. Some VW and Harley guys use them in rebuilds, the reports are mixed. Some say never without filtered oil.
 
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