4 bolt or 2 bolt main

[Kman]

I have a 350 that I thought was a 4 bolt untill I pulled the pan its a 2 bolt . what is the advantage of using a 4 bolt over a 2 bolt other than its being heavy duty? I was going to rebuild this for my K20 should I use the 2 bolt or look for a 4 bolt?
Kman

[flyin99]

2 bolt mains are fine, unless you are just building a racing machine.....500HP or better.........if you want to add some insurance to the bottom end, you can use ARP studs or bolts, to strengthen it up and you will be fine in a truck.

[tom hand]

IMO 4 bolts are total overkill for any streetable motor. In 20 years of being a mechanic I have never seen a motor that died because a maincap let go. The rods will always ventilate the block first. I have even seen motors break the crank and the mains still be fine. Chevy didn't even start making 4 bolts in small blocks till after they quit making the high horsepower ones, go figure. Save your money and build the 2 bolt.




http://www.hotrodsandclassics.net/us...ocknumbers.htm

[Kman]

I was kinda leaning in that direction of rebuilding the 2 bolt thanks for the info
Kman

[mikep]

Its not that the mains let go. The 4 bolts prevent the main caps from walking at hi rpm and help prevent spun main bearings. Ive seen a few of those. Chevrolet certainly did put the 4 bolt main blocks in Hi HP small blocks most notably the LT-1 in the corvette and Z28 and also in the 302's in late 68 and 69. The 4 bolt was built in response to main cap failures they were having in the higher HP small blocks such as the 350 and 375 HP 327's. The problem was especially noted in 4 speed cars and trucks which tend to shock load the mains a lot harder than automatics .

For my dollar its a good upgrade if you are driving a manual tranny truck. For an auto its probably overkill.

[Fast68Chevy]

http://www.nastyz28.com/sbchevy/sblock.html

halfway down page: http://www.nastyz28.com/sbchevy/sblock.html

[Kman]

Thanks guys for all the information I'm going to continue to tear this 2 bolt apart and take it to the machine shop next week for block and head work. this is going to be a stock build
Kman

[mikep]

I wouldnt worry about the 2 bolt thing either. The 4 bolt main concept was part of a big engine redesign in 67-68 to adress some shortcomings they had noticed during the previous decade of small block production. Kepp in mind that this was during "Boss" Ketterings tutorship of the engineering dept at GM and he was notorious for common sense engineering decisions that increased reliability and saved money at the same time. The 4 bolt main thing was one of those. They had previously adressed the problem in the 283's and 327's of spun main bearings by installing forged steel crankshafts to keep the crank from flexing under load or at hi rpm. After the redesign they went to a larger balancer to absorb more torsional flex, 4 bolt mains to stiffen the bottom end , and larger journals on the crank to stiffen up the casting.This allowed them to eliminate the steel crank in all but the 1 ton truck and a few very hi performance 350's as a across the board cost savings since the larger balancer and 4 bolt mains took care of a lot of the flex problem the steel crank used to band aid.
The whole reason for the redesign , other than some emissions concerns and the addition of a canister oil filter , was to cut back on warranty engine changes which are very expensive and to make the engine cheaper to produce.

I remember I once bought an MG . My dad was looking it over and we both noticed it had a pretty good sized oil cooler mounted ahead of the radiator. I thought " what a good bonus " my dad commented that the early ones probably didnt have the oil cooler but they were having problems with the motors over heating and blowing up so they added the oil cooler to band aid a design fault. I think he was right.