63dartman
Well-Known Member
If the connecting rod journals of the crank are taken down 10 thousandths do you use a 10 thousandths oversized bearing or undersized bearing? I think I may have screwed up
connecting rod bearing size?
- Thread starter 63dartman
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moper
Well-Known Member
The journals are undersize. the bearing shells need to be oversized to fit them. you can get .001 or .002 larger to get clearances perfect too.
its easy to get confused. "size" is original factory measurement. Undersized journal is outside diameter. Undersize bearing is inside diameter. Sure the bearing wall thickness is greater but that not the refernced dimension.
63dartman
Well-Known Member
Thanks everyone. I'm using a cb481p clevite77 it has .010 stamped on it so I'm guessing they are for a crank that has .010 removed to make it right. Well, that is exactly what my bearing journals are measuring at. I still can't figure out what caused my rod bearing failure???
moper
Well-Known Member
You're right. They are correct for a "ten under" crank. As for why it failed... Need lots more info including clearances for main and rod, and pics of the wear surface and backside of the bearings that failed.
"Exactly". Does that mean the clearance is .000"?
63dartman
Well-Known Member
Exactly is a "relative" term. I need to get a set of micrometers to get accurate measurements. I have been using a pair of calipers and it is tough to get consistent measurements. Hard to tell what the original clearances where since the bearings are all scored up and missing metal Iam going to try and get some pics of the bearings and measurements on Sunday. I have been working all week and it's hard to find time when your working 12hr shifts.
Iam going to try and get some pics of the bearings and measurements on Sunday. I have been working all week and it's hard to find time when your working 12hr shifts.
Gotcha. Yeah, calipers aren't accurate for that kind of work.
63dartman
Well-Known Member
First....My camera sucks and I need to get a new one. Here are some pics of the connecting rod bearings. When I took them apart some of the bearings were tight in the caps and some just fell out. The ones that fell out looked like they where squeezed tighter than the cap. They didn't even conform to the caps. There where gaps between the bearing and the cap. Also a pic of the mopar chain tensioner. Looks like it held up and did it's job. About 5k on this engine.
63dartman
Well-Known Member
here is a different pic showing the bearings and how they have different profiles. I think the caps were mixed up too. The caps are stamped 1 thru 8 and they were not on the corresponding cylinder number.
green1
Well-Known Member
Chain loose; wrong bearings. Crank looks like what?
63dartman
Well-Known Member
crank looks fine....loose chain? I posted the tensioner just in case someone was thinking about getting one. No issues with the timing chain.
green1
Well-Known Member
Ah. I'm old school; no tensioner. But those bearings have spun, and mis-matched.
green1
Well-Known Member
I have a crappy camera too, but if we are locking at the back of the bearings;Rods now to be re-sized. Been there.
63dartman
Well-Known Member
What would be the best fix for this issue? I'm thinking of starting over with a new crank and connecting rods. What else should be checked?
63dartman
Well-Known Member
Also the last pic is not matched bearings. I just pulled them all out and put them in a pile. I just matched them up to show the differences.
dstdevl
bruce t johnson
dartman, I'd take the crank and rods to a competent machine shop, have the rod cap and beams matched up and rebuilt if needed. have the cranked miked and make sure its good, polish or regrind if you have to. Get set up with the correct brgs and reassemble the engine. Bruce
dstdevl
bruce t johnson
dartman, I'd take the crank and rods to a competent machine shop, have the rod cap and beams matched up and rebuilt if needed. have the cranked miked and make sure its good, polish or regrind if you have to. Get set up with the correct brgs and reassemble the engine. Bruce
daredevil
Well-Known Member
and when you assemble it be sure to use lube on all the rod bolts and use a good torque wrench. tighter than spec is not good. Loose is worse. I,m wondering if your rod bolts have fatigued and arent clamping correctly.
63dartman
Well-Known Member
Daredevil and dstdevl thanks for the help. I'm really thinking about getting rid of the connecting rods they just seem to be the culprit here. I will get the crank redone and new connecting rods to go with it. If I get the Scat I beams or something similar should I have the rotating assembly balanced again?
alsdemon
Well-Known Member
can,t you use placstic gage to check ?the new set up .looks like there was somthing up on the reassmebly. resized rods if done right should bee fine. but the crank needs checked.and the oil feeds hole are free of dirt. did it have oil presserbefore they spun? als
moper
Well-Known Member
A few thoughts.... I also like tensioners... but that chain needs to be replaced witha performance roller version. Summit (brand) has some decent stuff that isnt too expensive.
On the bearings: Non e have spun. All are trasahed and look to be from oil starvation. That can be misleading tho. The reason they fell out of the rods are the crush is gone. Crush holds them in place and allows the bearing to achive the correct final shape when torqued. Crush is beaten out of a bearing. Detonation and/or heat and/or breakdown of the oil wedge for some reason will do it. You need a good set of micrometers to really measure the crank. It looks to me like it's not ok. Oil goes from the main bearings to the rods. And the rods dont look good at all. So either there isnt enough oil moving thru the system, or the main bearings are to wide in terms of clearances, or the rods are too wide in terms of clearances. In any event, the lower end needs work. As far as re-torquing bolts they can be re-done a number of times with no problem. But you can't over-tighten them or they become junk at that instant. The numbers on caps and rods SHOULD match. But if in the past the rods were re-sized the shop might not have re-numbered as they should have. If this was in my garage, I would be getting the crank to a grinder to check for taper and surface quality and the rods would be re-sized properly with ARP bolts. Plastigage can verify good clearances, but you can't see taper or roundness with it.
On the bearings: Non e have spun. All are trasahed and look to be from oil starvation. That can be misleading tho. The reason they fell out of the rods are the crush is gone. Crush holds them in place and allows the bearing to achive the correct final shape when torqued. Crush is beaten out of a bearing. Detonation and/or heat and/or breakdown of the oil wedge for some reason will do it. You need a good set of micrometers to really measure the crank. It looks to me like it's not ok. Oil goes from the main bearings to the rods. And the rods dont look good at all. So either there isnt enough oil moving thru the system, or the main bearings are to wide in terms of clearances, or the rods are too wide in terms of clearances. In any event, the lower end needs work. As far as re-torquing bolts they can be re-done a number of times with no problem. But you can't over-tighten them or they become junk at that instant. The numbers on caps and rods SHOULD match. But if in the past the rods were re-sized the shop might not have re-numbered as they should have. If this was in my garage, I would be getting the crank to a grinder to check for taper and surface quality and the rods would be re-sized properly with ARP bolts. Plastigage can verify good clearances, but you can't see taper or roundness with it.
ramcharger
Dismember
Too much side clearance can allow the oil wedge to dissipate as well. Get the scat I-beams and have the assembly re-balanced and mic out the crank and rod journals. Be sure to mic out each journal in 3 equidistant spots and from the center out to edges but not on the fillet. Generally, light scoring that runs the circumference of the journal won't cause a problem but if the journal is barrel shaped (larger in the middle and smaller toward the edges), that's a bad deal and will need to be reground. Obvious burn marks or metal transfer is bad too.
Like moper said, crush is gone from heat and pressure. A double roller is also cheap insurance and I've used the Summit brand rollers with no ill effects. It's cheaper (or just a few bucks more) to get new rods with the machining prices here in Denver. That will be decision only you can make. Used rods kinda freak me out as they have a finite life measured in cycles and Lord knows how many times these rods have been cycled. Just food for thought.
BTW, how much oil pressure were you running? If crank and rod clearances exceed .002 you really should be running an HV pump.
Like moper said, crush is gone from heat and pressure. A double roller is also cheap insurance and I've used the Summit brand rollers with no ill effects. It's cheaper (or just a few bucks more) to get new rods with the machining prices here in Denver. That will be decision only you can make. Used rods kinda freak me out as they have a finite life measured in cycles and Lord knows how many times these rods have been cycled. Just food for thought.
BTW, how much oil pressure were you running? If crank and rod clearances exceed .002 you really should be running an HV pump.
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