bottom end keeps going out
- Thread starter hondaracin339
- Start date
yeah i used the puller to pull it in most of the way. then used the bolts for the case in sequence to slowly push it in the rest of the way, then backed the bolts out a tad. had my buddy rotate the crank as i torqued down the case bolts, and spun beautifly the whole time.
only thing that comes to mind for me is that one side or the other was not seated in the bearings the whole way. the crank was not centered in the cases. that is a very odd scenario though. especially with it passing a leakdown.. whats the cyilnder wall and piston look like after this happens?
the cylinder and piston are good, and have survived two crank failures. still have crosshatching and all is good on top.
i cut the current failed cranks conrod off to get a better look at the needle bearings, and wow. its almost like theres no oil getting in there and they just grind away..but with a lack of oil comes heat, and no heat marks AT ALL.
i cut the current failed cranks conrod off to get a better look at the needle bearings, and wow. its almost like theres no oil getting in there and they just grind away..but with a lack of oil comes heat, and no heat marks AT ALL.
i would def. use the puller to seat the crank into the bearing races.. other than that i couldnt tell you. if it was out of center that bad the piston/cylinder would show it.
This IS strange... You said the top end still looks good, but was it recently rebuilt, too? I'll assume it was. Who did the machine work, and what brand of piston are you running?
There is somehow an excessive load being placed on the Big End bearing of the rod. It won't take long, even under otherwise-ideal operating conditions, for this bearing to fail. It is HIGHLY UNLIKELY for you to get two bad new cranks, so I'm going to point the top end.
Why? It is possible that the machinist who did the work accidentally bored the cylinder off-center when he went to oversize it. More than a couple thousandths of an inch off center, and the rod, in theory anyways, can slap side-to-side and damage the Big End bearing.
I asked about the piston...what shape is the Small End needle cage bearing in? Is it damaged, too, or is all the damage on the Big End bearing only?
There is somehow an excessive load being placed on the Big End bearing of the rod. It won't take long, even under otherwise-ideal operating conditions, for this bearing to fail. It is HIGHLY UNLIKELY for you to get two bad new cranks, so I'm going to point the top end.
Why? It is possible that the machinist who did the work accidentally bored the cylinder off-center when he went to oversize it. More than a couple thousandths of an inch off center, and the rod, in theory anyways, can slap side-to-side and damage the Big End bearing.
I asked about the piston...what shape is the Small End needle cage bearing in? Is it damaged, too, or is all the damage on the Big End bearing only?
This could be a few things. The crank should measure 2.280' from web to web. If it's too tight, it will pinch the lower bearing when it gets hot.
Another scenario is if the bore is not perpendicular to the crank. I see this from time to time. Most stealerships have a boring bar that's mounted to a plate. They mount the cylinder by clamping the top of the cylinder to the plate. Bad move!!!!!! If the top of the cylinder is not parallel to the bottom, your bore will not be perpendicular to the crank. No way to check this without a lath or Bridgeport. You may be able to take a machinist square and set it on the bottom of the cylinder and use a set of feeler gages at the top to see if it's straight.
The rods could be bent but very unlikely if you bought new cranks. You can count the cases out if they are a matched set.
Make sure the holes that lube the bearings are clear.
Finally, if you're assembling the bottom end by sucking the cases together with the bolts ( not the right way to do this) the main bearings are going to have a load on them. I assemble cases with the flywheel side up. If you did it like this, take a soft brass or copper punch and smack the clutch side of the crank till it turns free. Again, this is NOT the correct way to assemble a bottom end but it may get you out of trouble. Good luck and call if you have questions.
Ken 860-230-5003
Another scenario is if the bore is not perpendicular to the crank. I see this from time to time. Most stealerships have a boring bar that's mounted to a plate. They mount the cylinder by clamping the top of the cylinder to the plate. Bad move!!!!!! If the top of the cylinder is not parallel to the bottom, your bore will not be perpendicular to the crank. No way to check this without a lath or Bridgeport. You may be able to take a machinist square and set it on the bottom of the cylinder and use a set of feeler gages at the top to see if it's straight.
The rods could be bent but very unlikely if you bought new cranks. You can count the cases out if they are a matched set.
Make sure the holes that lube the bearings are clear.
Finally, if you're assembling the bottom end by sucking the cases together with the bolts ( not the right way to do this) the main bearings are going to have a load on them. I assemble cases with the flywheel side up. If you did it like this, take a soft brass or copper punch and smack the clutch side of the crank till it turns free. Again, this is NOT the correct way to assemble a bottom end but it may get you out of trouble. Good luck and call if you have questions.
Ken 860-230-5003
wow i never thought about the bore being not true. hmm..well all the damage is in the big conrod bearing. no bluing, no discoloration at all. is there a way to check to see if the bore is straight?
Yes go to Ken. He knows what he is doing. The good news is that Blaster cylinders can be bored out several times, up to .080" over stock, so even a bad bore job may be fixable in the right hands, with the proper tools. Give O'connor Racing a call.
ive come up with why ken may be right about the bore... conrod bearings on flywheel side are worse than clutch side. there looks to be more stress on cylinder on flywheel side as well...so i guess time for a bb kit haha. thanks for all you guys help!!!
Blaner
Your Friendly South African Ambassador
wait my bad first two were oem. the one i have now to put in is a hotrods..im an idiot haha. heres a couple pics of current progress.
just case half
you can see the bad crank (conrod can touch the main body)
idk if u can make out the flatness of the needles
what do the thrust washers on either side of the rod look like? the rod should never be touching the crank on the side like this in the first pic? i think i see one in the second pic. i guess though if the bearing is that messed up the rod will no longer be perpendicular to the crank..very interesting post!
just thinking out loud
if you're using the bolts to bring the cases together, you might be closing up the clearance on the big end bearing. when installing cranks i always use the tool to pull everything up tight, never had a motor/crank related problem doing it this way
if you're using the bolts to bring the cases together, you might be closing up the clearance on the big end bearing. when installing cranks i always use the tool to pull everything up tight, never had a motor/crank related problem doing it this way
