Loose Main bearing cap bolt
Main bearing bolt sitting on windage tray So I tore my 346 down and found a loose main bolt. It is the inner bolt closer to the crank. Spinning 7200 rpm. Never acted like it spun a bearing.. quiet, good oil pressure, etc. I looked at the windage tray and found that the head of the bolt was rubbing on it so it's been on it for awhile. I only put ~1200 miles on the engine. I never touched the main bolts. They were factory torqued by GM. I replaced the rod bolts and built up from there. Should I be concerned about this OR just order a new bolt, torque it and go? Edit: They are TTY bolts right?
Last edited by frito1; Dec 15, 2008 at 04:30 PM. Reason: added another question
10 Second Club
Joined: Nov 2002
Posts: 1,471
Likes: 3
From: Berkeley, California
My take is that you had bad luck (loose main bolt is rare) and good luck (discovery before disaster must also be rare). I am especially surprised that your motor held together spinning to 7200, because the inner bolts carry a bigger share of the load than the outer bolts. I would try to reinstall the original bolt. You may find stripped threads in the block. The factory main bolts can be reused. They are installed with the torque-angle method but they are not TTY.
OK I saw the torque angle in my book and assumed they were TTY. Thanks. Should I even take a peek at the bearing or just assume all is good? I kind of hate disturbing bearings if I'm not going to replace them. I know my rod bearings look like new still. No noises so I got lucky... be happy, torque it and go?
Bad thing with the angle meter method on those bolts..... there is no real great way of checking the rest other than making sure they are at least snug tight.
Bad thing with the angle meter method on those bolts..... there is no real great way of checking the rest other than making sure they are at least snug tight.
Teching In
Joined: Aug 2008
Posts: 44
Likes: 0
Well you could trq it to the spec it says, get an electronic tq wrench to make it easy, and keep cranking up the setting on the wrench till you can get the bolt to go a tiny it further, subtract 5 ft lbs from that and go through all the bolts making sure they are atleast that. Make since?
^^ Yep, I got ya.
However, I just felt the tightness of the rest and very few of them were near tight enough. So I loosened everything back up and started the torque sequence back over. No big deal.... except, why did this happen?? My ARP rod bolts were still on the money when I hit them with the torque wrench. Remember, I never touched the main bolts.
I'm wondering if the ARP's are heavy enough (read: crank slightly out of balance) to be working the main bolts loose? Is that possible? If so, the Katechs are the easy solution, but I'm not by any means sold on this idea. I didn't feel an abnormal vibration
I am hoping for a pretty informed answer here. Thanks
However, I just felt the tightness of the rest and very few of them were near tight enough. So I loosened everything back up and started the torque sequence back over. No big deal.... except, why did this happen?? My ARP rod bolts were still on the money when I hit them with the torque wrench. Remember, I never touched the main bolts.
I'm wondering if the ARP's are heavy enough (read: crank slightly out of balance) to be working the main bolts loose? Is that possible? If so, the Katechs are the easy solution, but I'm not by any means sold on this idea. I didn't feel an abnormal vibration
I am hoping for a pretty informed answer here. Thanks
Last edited by frito1; Dec 28, 2008 at 05:51 PM.
OK I just talked to a very reputable engine builder when I picked my AFR head up from him. He said it was very unlikely 3 grams heavier ARP rod bolt caused the main bolts to back off. I'll just hope that they initially stretched causing them to loosen and will hold the torque now
