engine ran flawless for a season and a half with a few 11.34-11.40 passes and sreet driven. was taken out and torn down to a short block and inspected. cylinder walls showed no indication of any scoring what so ever . engine was put back in service for a few more season with a considerable amount of track time (low 12's - high 11's) and street driven as well . ran well untill a spun rod bearing making a track pass . taken apart and this found in one cylinder. how could a pressed fitted pin move like that ? something like that IMO would of shown up a lot earlier in the life of the engine , like when it was first torn down and inspected . could the pin of gotten bent causeing it to become loose in the rod ??? over heated ??? why just this rod and not all the others ?
What would cause this ?Engine guys step inside .
- Thread starter KWIKR 1
- Start date
UR50SLO
The Reaper
- Joined
- Jun 12, 2001
Which Cyl was this?
And the pin was haning out riding on the side of the block? (as above)
If so the small end of the rod did not have a good grip on the pin and it ended up walking out to the side over time. This is alot of the reason people go with floating pins since they use lock wires to hold the pin in place.
If the cyl didn't get scorred consider yourself very lucky. I would take the time to get full floating pins. The piston cocking in the cyl and pin riding the cyl eventually took it's toll on the bearing.
Hope the crank polishes out and get it all back together!
~Scott
And the pin was haning out riding on the side of the block? (as above)
If so the small end of the rod did not have a good grip on the pin and it ended up walking out to the side over time. This is alot of the reason people go with floating pins since they use lock wires to hold the pin in place.
If the cyl didn't get scorred consider yourself very lucky. I would take the time to get full floating pins. The piston cocking in the cyl and pin riding the cyl eventually took it's toll on the bearing.
Hope the crank polishes out and get it all back together!
~Scott
Lee Thompson
New Member
- Joined
- May 24, 2001
I believe
The pin is pressed into the rod---the pin rotates in the piston bore. For whatever reason it has moved in the piston. I would compare the offending pin to a stock pin. Your answer may be there. Different weight pins hold or tranfer heat @ a different rate.
The pin is pressed into the rod---the pin rotates in the piston bore. For whatever reason it has moved in the piston. I would compare the offending pin to a stock pin. Your answer may be there. Different weight pins hold or tranfer heat @ a different rate.
Lee the pistons were a set of stock replacement .030" FM Hyper Pistons/pins . the owner of the block is waiting to here back from the builder to see if the scoring in the one cylinder is not to bad that it can go .040" .
I've contacted a few good engine builders (non buick) and that they have all seen this problem at one time or another and the census seems to be the rods just could of been over fatigued/stressed from to many heat cylces or from detonation which distorts or weakens the small end .
one also mentioned the possible cause of a lean cylinder resulting in increased heat to the piston causeing the piston pin bore to "bite" down on the pin and literally turn it out of the small end like a screw .
out of the 4 builders I talked to . all 4 agreed on the stock rod most likely being over stressed . One mentioned that the old stock rod Pontiac 400's where well known for doing this when they where really ran hard or stepped on .
I've contacted a few good engine builders (non buick) and that they have all seen this problem at one time or another and the census seems to be the rods just could of been over fatigued/stressed from to many heat cylces or from detonation which distorts or weakens the small end .
one also mentioned the possible cause of a lean cylinder resulting in increased heat to the piston causeing the piston pin bore to "bite" down on the pin and literally turn it out of the small end like a screw .
out of the 4 builders I talked to . all 4 agreed on the stock rod most likely being over stressed . One mentioned that the old stock rod Pontiac 400's where well known for doing this when they where really ran hard or stepped on .
SS/GN
Well-Known Member
- Joined
- Jun 30, 2005
I agree with the engine builders,however if the rods were not properly reconditioned this may have reduced the life span.I have seen many HP motors with stock rods live long and safe as long as they were done properly.I have seen on a 400 sb where the rod was heated too much on the install process and caused the wrist pin to get tight in the bore.Oil also plays in to the equation like on new GM trucks with piston slap.Good luck,JMHO
Kevin
Kevin
Sure that's possible and can typically be said about any engine related machining procedure . Kevin not directing this at you
. But people sometimes always tend to right away jump on the "could be" or "must be" the builders or machine shops fault band wagon . Lets look at the real basic facts . We are talking about a set of GM 23 year old stock production rods that where never designed or intended to be used in the way these rods have been used . they more than likely could have been hammered on , heat cycled and detonated till the cows come home for the last 20 years before they were ever installed in this engine . maybe they have been abused 8 x more than a set that have been living in a 10 sec buick for the the last 5 years . who knows ? bottom line is no matter what particular part we are taking about no matter how good the machining is . can only take so much undue stress and can or will eventually break down over a period of time . it's just a question of when . IMO that's just fact with anything . the engine is being pulled because of a spun rod bearing it's just very unfortunate that this problem has reared it's ugly head and made his basicly simple engine rebuild a little more difficult and a PIA . 
Warp6 I guess they are having a sale on them this month .
what a drag . I would think a bend in a rod that much to push a pin sideways like that should be pretty easy to see in the rod . Very high cylinder pressures can cause the piston to "grab" the pin and spin it in the connecting rod...especially if the high pressures distort the small end of the rod. Often seen in nitrous engines. That's why people run full floating pins in high horsepower applications.
Billy T.
gnxtc2@aol.com
Billy T.
gnxtc2@aol.com
- Joined
- May 26, 2001
Lee the pistons were a set of stock replacement .030" FM Hyper Pistons/pins . the owner of the block is waiting to here back from the builder to see if the scoring in the one cylinder is not to bad that it can go .040" .
I've contacted a few good engine builders (non buick) and that they have all seen this problem at one time or another and the census seems to be the rods just could of been over fatigued/stressed from to many heat cylces or from detonation which distorts or weakens the small end . .........
out of the 4 builders I talked to . all 4 agreed on the stock rod most likely being over stressed . One mentioned that the old stock rod Pontiac 400's where well known for doing this when they where really ran hard or stepped on .
My first race car was a Pontiac, and that is exactally what happened on the first rebuild.
That is another reason performance builds use full-floating rods/pistons.
Does not happen often in Buicks, however seen a few. After 22+ years of hammering these engines, the original parts are old and starting to fatigue, especially cast cranks and rods designed for use in economy 6 cylinder engines.
how is the other rods and pistons in the engine?
Those pistons are not really a turbo app piston. those are actually meant for the older buick 350 and happens to have the same C/R and pin height.
The pistons don't disapate the heat very well in a turbo motor and the pin starts to bind in the piston. The wrist pin don't get the oiling the stock piston/pin gets further adding to the issue with those pistons.
This is also a problem with the sealed power forged units too.
The fix for that is to drill a oil hole from the inside of the pin bore to the outside of the oil ring groove so that it will force oil down on the pin to keep it lubed and cooled.
The Ls engine suffers from this aswell and those guys drill the pistons too!
Richard Clark has built a fixture for this.
I thin what has happen here is the piston binded on the pin and then the pin started to walk out of the rod.
Those pistons are not really a turbo app piston. those are actually meant for the older buick 350 and happens to have the same C/R and pin height.
The pistons don't disapate the heat very well in a turbo motor and the pin starts to bind in the piston. The wrist pin don't get the oiling the stock piston/pin gets further adding to the issue with those pistons.
This is also a problem with the sealed power forged units too.
The fix for that is to drill a oil hole from the inside of the pin bore to the outside of the oil ring groove so that it will force oil down on the pin to keep it lubed and cooled.
The Ls engine suffers from this aswell and those guys drill the pistons too!
Richard Clark has built a fixture for this.
I thin what has happen here is the piston binded on the pin and then the pin started to walk out of the rod.
dr_frankenstein
Mad Scientist
- Joined
- Mar 17, 2008
this is also why "On Center" blocks were made, to helpo correct this issue because of the offset causing the pin to load in the direction it came out.
A.j.
A.j.
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