Broken Rear Main Cap!
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Pro
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From: Clio Mi
Broken Rear Main Cap!
Hey, guys i was investigating a oil leak on my 496 and i found it was comming from the rear main area. i was using the blue felpro gasket.. so i pulled the steering linkage down pulled the pan.. removed the oil pump and then pulled the cap.. to my surprise it looks like the cap was cracked in 2 spots.. on closer investigation it looks like the fly wheel bolts hit it in 3 spots... it was my fault.. i kinda remember torquing the fly wheel bolts in ,and them being to long .so i replaced them with something grade 8 and shorter from arp.. my question too you guys is what do i do now? aftermarket cap, or find a gm replacement... where do you them from.. I'm writing this at 8:00pm and my machine shop is closed.. any suggestions?? help a young guy out Thanks. Rich (neptune75)
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From: Sierra Foothills CA
Several years back I became good buddies with my builder/machinist. He always had good spares or knew where to get them and knew the difference between good usable parts and junk. I trusted his judgment completely. Do you have a connection with anybody like that? If so start by calling him in the morning- he may have just what you need. Good luck. 
Drifting
Joined: Apr 2007
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From: Cameron Park CA
To do the job right, don't you need to completely disassemble the engine and then line bore/hone the main journals so everything lines up? I guess you could slap a new main cap in and check the bearing clearances with plasti-gage, but that seems like a bubba fix to me. Just my 2¢.
Cheers,
Pete
Cheers,
Pete
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I hope you get lucky and find one, I sold a 400 block to someone and the machinist said the rear main was not the original ( I didn't know I never had it checked out ) anyway it ended up being a pain in the **** to fix although I don't remember the details 
I hate to say this but I would definitely say it would have to come apart to do it right
I hate to say this but I would definitely say it would have to come apart to do it right
Last edited by MotorHead; Apr 21, 2010 at 09:05 PM.
Team Owner
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From: Crossville TN
If you still have the pieces to the broken cap, a good machinist should be able to get enough measurements from it to fabricate another one with the right dimensions. Then you can install it with some Plasti-gage and find the right size of bearing needed to fit in there. Unless someone has a really good reason not to do so, I think you can just leave the rest of the assembly alone.
Le Mans Master
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From: Graceland in a Not Correctly Restored Stingray
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From: Clio Mi
i have no problem tearing it down.. i have a little over 13k into the engine. on the other hand it is just a stock 4 bolt main end cap... i have access to a cnc 3 axis machine that could take computer controlled measurments to the strictest of measurments, and make me one out of what ever metal i want.. what do yu guys think about this option? it is only craked where the rear main gasket is too. i will post a pic of the brakes tomorrow
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Thread Starter
Pro
Joined: May 2009
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From: Clio Mi
Several years back I became good buddies with my builder/machinist. He always had good spares or knew where to get them and knew the difference between good usable parts and junk. I trusted his judgment completely. Do you have a connection with anybody like that? If so start by calling him in the morning- he may have just what you need. Good luck.
Race Director
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
If it is recommended to have your block line honed just because you are using ARP studs then I would not be doing anything but taking it apart.
You could try the cheap and easy fix you mention but you could damage your crank or do some other damage that might make cost more in the long run
You could try the cheap and easy fix you mention but you could damage your crank or do some other damage that might make cost more in the long run
Le Mans Master
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From: Butler Pa
Team Owner
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From: Crossville TN
Well, taking the block down and boring a new cap on the rear main...and trying to maintain alignment to the other crank bearing bores would take a bit of luck, too. With a good machinist, fabricatings a replacement cap isn't 'rocket science', if you can get the necessary measurements off the original cap [as I stated above].
Le Mans Master
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From: Butler Pa
They do it every day. It's called line boring and line honing! Do you have a clue as to what you're talking about?
With a good machinist, fabricatings a replacement cap isn't 'rocket science', if you can get the necessary measurements off the original cap [as I stated above].
Last edited by bashcraft; Apr 22, 2010 at 08:49 AM. Reason: missing quote
Le Mans Master
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From: League City Tx
Damn this sux Neptune....sorry to hear this for you...
Is the cap completely broken? Just a single crack at the seal area? Post up some pics..... If it's just a crack on the outer edge and not that long, I might would try to braze/cast weld it and run the damn thing.....
Ofcourse the "right" way to fix it is a tear down, new cap, line bore and new blueprinted main bearings but.....depending on the cracks size/location.....I may either just leave it or attempt to weld it....if it's only in the seal area etc....although that sucker is just bound to leak...(the seal)...
I hope you get it fixed....if it makes you feel better, I frag'd my fresh built TH400 last weekend and have to pull the tranny out for a full teardown.....rebuild etc...$1500 down the tubes.
Is the cap completely broken? Just a single crack at the seal area? Post up some pics..... If it's just a crack on the outer edge and not that long, I might would try to braze/cast weld it and run the damn thing.....
Ofcourse the "right" way to fix it is a tear down, new cap, line bore and new blueprinted main bearings but.....depending on the cracks size/location.....I may either just leave it or attempt to weld it....if it's only in the seal area etc....although that sucker is just bound to leak...(the seal)...
I hope you get it fixed....if it makes you feel better, I frag'd my fresh built TH400 last weekend and have to pull the tranny out for a full teardown.....rebuild etc...$1500 down the tubes.
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From: Reno Nevada
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i have access to a cnc 3 axis machine that could take computer controlled measurments to the strictest of measurments, and make me one out of what ever metal i want.. what do yu guys think about this option? it is only craked where the rear main gasket is too. i will post a pic of the brakes tomorrow
Guys, line bore and hone is to correct the block core shifts of our old used blocks or new out of spec blocks. It's not the main caps that are bad, because they are blue printed. The block is what shifts or outright manufacturer defects.
Le Mans Master
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From: Butler Pa
The machine shop I deal with always align hones when caps are replaced. Regardless if they're stock or splayed billet caps.
Melting Slicks
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From: Elkhorn WI