4 bolt main 2 bolt main
Senior Member since 1492
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From: Just because I'm paranoid doesn't mean people aren't out to get me...
St. Jude Donor '09
It refers to the number of bolts on the caps which hold the crank. Here are pictures of a 4 bolts and obviouls a 2 bolt only has 2 bolts per cap.
Instructor
Joined: Jan 2005
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From: OKLAHOMA CITY OKLAHOMA
The # of bolts that hold the crank bearing caps to the block. 4 bolts are desireable for very high horsepower engines. Some two bolt blocks have the place for four bolt caps- thay are just not drilled and tapped.Some people drill two bolts into four bolts (at an angle)-but withought the meat in that area of the engine you can actualy weakin the block. two bolt mains will hold up to alot of hp- more than most streetable rods can turn out.
Thread Starter
Melting Slicks
Joined: Aug 2004
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From: Toronto
thanks jimvette...I'm still new to this and the more I think I know the less I actually do. This has been on my mind since I discovered my engine is a 2 bolt. Obviously not the original motor.
Instructor
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From: OKLAHOMA CITY OKLAHOMA
Originally Posted by kidneyboy
thanks jimvette...I'm still new to this and the more I think I know the less I actually do. This has been on my mind since I discovered my engine is a 2 bolt. Obviously not the original motor.
Race Director
Joined: Jan 2003
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From: State of Confusion
Originally Posted by kidneyboy
What's the difference? 
you'll see 5 (one at the front, one at the rear, and one between each set of cylinders) 'main-bearing caps', which clamp the crankshaft 'up-wards', into the block.
Older 'passenger' cars had 2-bolt-blocks and caps (one on each side of the cap/crank), while later 'performance' blocks had 4-bolt (TWO on each side of the cap/crank) blocks and caps.....
" Thanks for calling, next caller, please.....
"Grumpy", from Malvern, Pa., you're on..... "
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Pro
Joined: Oct 2004
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From: Shoreline WA
Originally Posted by jimvette74
could be original many came with two bolt check the block casting #.
Senior Member since 1492
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From: Just because I'm paranoid doesn't mean people aren't out to get me...
St. Jude Donor '09
Originally Posted by Carl Granquist
On my L-48, where will I find this number? I've had folks tell me both ("probably a 4-bolt main" & "likely a 2-bolt main") and I'd like to check for myself to know for sure. Thanks.
Sometimes though, the casting number may not tell you for certain whether it's a 2 or 4 bolt.
My number listed above is posted as being a 2 or 4 bolt. Go look HERE when you get the number.
I was pleasantly surprised when what everyone told me was a 2 bolt turned out to be a 4 bolt!
Safety Car
Joined: Aug 2001
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From: North Easton Mass
Originally Posted by Carl Granquist
On my L-48, where will I find this number? I've had folks tell me both ("probably a 4-bolt main" & "likely a 2-bolt main") and I'd like to check for myself to know for sure. Thanks.
Rick B.
Thread Starter
Melting Slicks
Joined: Aug 2004
Posts: 2,704
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From: Toronto
My vin# shows mine to be a L 48..so it is supposed to be a 2 bolt main. It's a 78. Thanks for all the helpful answers, tomorrow if its not too cold down in the garage I'll see if I can check the numbers on the block. 
Melting Slicks
Joined: Dec 2004
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From: pr, frederick md.
Originally Posted by Jughead
Casting number is on the drivers side, back of the block, right where the block & tranny meet. Shine a flash light down there & if it's not too cruddy tou should be able to read a number like this--->14015445
Sometimes though, the casting number may not tell you for certain whether it's a 2 or 4 bolt.
My number listed above is posted as being a 2 or 4 bolt. Go look HERE when you get the number.
I was pleasantly surprised when what everyone told me was a 2 bolt turned out to be a 4 bolt!
Sometimes though, the casting number may not tell you for certain whether it's a 2 or 4 bolt.
My number listed above is posted as being a 2 or 4 bolt. Go look HERE when you get the number.
I was pleasantly surprised when what everyone told me was a 2 bolt turned out to be a 4 bolt!
i have several blocks, and for example the #'s are 3970010 on three of them. 2 are 4 bolt, 1 is 2 bolt. the 2 bolt will hold up to alot of hp and tq. more than most will agree to. good luck
Drifting
Joined: Aug 2004
Posts: 1,883
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From: Lubbock TX
Up until the Gen 5 came out in '90, there was no way to tell from a casting number on a big block whether it had 2 or 4 bolt mains with one exception - one '89 - '90 casting is all 4-bolt and it's a Mark IV block.
SBC 400's are a bit different - almost all of the "511" castings are 4-bolt, cast in the first two years of 400 production. 509 and 817 castings are 2-bolt, at least 90% are, I won't say all until I've seen 'em all.
SBC 350 is a crap shoot just like big blocks - all casting numbers after about '70 can either be 2 or 4 bolt mains.
Big blocks have the same number of bolts on all 5 caps, small block 4-bolt blocks only have 4 bolts on the middle 3 caps, front and back are 2-bolt.
Best way to tell easily is to drain the oil and use a borescope through the drain hole. Don't buy the cheapie from Harbor Freight, the camera/light on the end are too big for drain plug and spark plug holes but the more expensive one has a much smaller end and works for both.
SBC 400's are a bit different - almost all of the "511" castings are 4-bolt, cast in the first two years of 400 production. 509 and 817 castings are 2-bolt, at least 90% are, I won't say all until I've seen 'em all.
SBC 350 is a crap shoot just like big blocks - all casting numbers after about '70 can either be 2 or 4 bolt mains.
Big blocks have the same number of bolts on all 5 caps, small block 4-bolt blocks only have 4 bolts on the middle 3 caps, front and back are 2-bolt.
Best way to tell easily is to drain the oil and use a borescope through the drain hole. Don't buy the cheapie from Harbor Freight, the camera/light on the end are too big for drain plug and spark plug holes but the more expensive one has a much smaller end and works for both.
Melting Slicks
Joined: Oct 2013
Posts: 2,569
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From: San Diego California
