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2-bolt or 4-bolt

2942 Views 17 Replies 16 Participants Last post by  NAIRB
I am building a 355 chevy motor, and I am wondering if it will be really important for me to have a 4-bolt engine block, I have a 2-bolt block I sent out to be machined already, but it has been bothering me, wondering if it will stand long. But that doesn't worry me, because I'll easily be able to sell it again. But what I am wondering, is will the 2-block stand the abuse I will give it? I am building a primarily street motor, with a little drag strip use. Also the parts I am putting in it should end me out with ~400hp or so.

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There are a great deal of 2 bolt chevys running on the strip without issues. I think with limited dragstrip use this setup will be more that adaquate. Under certain conditions a 4 bolt block is the best setup, you'll be fine for a long time.

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I agree with PC Hotrodder. For limitted strip use, a 2 bolt should hold up just fine. If you are seriously worried about it though, I would go ahead and get a 4 bolt. Built right, a 4 bolt would be ABSOLUTELY bulletproof!
Thanks for the input, I have also read places about having the block splayed to a 4-bolt, but have also read about this causing odd stress on the block.

Any opinions on having this work done?

Thanks again for the replies, and all future replies.
The splayed 4 bolt is superior to the regular 4 bolt main.

The 2 bolt main should be safe to 6500 RPM. If you are worried you might tax the 2 bolt main setup, then I suggest main studs, and not worrying about it any more. Noone I know has ever seen a main cap fail in a street type engine and you probably never will either.

4 bolt mains are highy oveerrated and probably 60% of the engines passed off as a "350 4 bolt" are really 2 bolt. There is no external way to tell you see, and many just add it on because it "sounds better". DOn't worry about your 2 bolt block!
I have a 1981 camaro with a 350 2-bolt main. It has a forged crank, 9.7:1 forged pistons, and the edelbrock performer rpm package. I would estimate its pushing a little over 400 hp and I beat the crap out of this motor. I red line it at 7200 when I race someone or do burn outs. I drive it every day and and I race it on weekends. Ive been driving it for over a year now like that every day and I haven't had any problem yet. So from my personal experience with a 2 bolt block, im sure it will hold up easyly with your application.
in every article i have seen it said that the 2 bolt block was good to 500 hp i have built several 2 bolts that were 350 to 450 hp with no problems
My engine builder says the Chevy 2 bolts are actually stronger! If you go with the splayed bolts you'll be very happy. Chevy 4 bolts don't do much other than weaken the block itself with te straight bolts.
Well, I think your engine builder is a little over zealous. The traditional 4 bolt block is not "weaker" than the 2 bolt arrangement, though the second bolts do come through a thin part of the web. The deal is that the 4 bolt arrangement allows greater clamping force with less torque on the bolts (check the specs!). Bolts can fail, main caps and webs almost never do. The 4 bolt arrangement also improves bearing saddle distortion.

However, the splayed arrangement is better because it does away with the thinning of the webs, and I think it may improve distortion of the bearing saddles over the traditional 4 bolt arrangement as well.
Hi there;
About your 355 sbc.
If I was building the same and didn't expect to
rev over 6000rpm. I would use the two bolt block.
If the engine makes peak horsepower above that
and you will be revving above 600 by a considerable amount. then use a four bolt block.
I think that there are some who will dissagree
but that is what I would do.
Well that is my opinion. Good luck what ever
way you decide.

kenneth leger
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I have built a stroker 383 from a 4 bolt main block... why did I use the four bolt?...cause I didn't want to go through what you're going through.... worrying if I did it right!!!! Just get a four bolt and feel great when you hammer it!!!!
I think that the 2 bolt main should be fine considering you don't over-rev. It should be fine considering that you are only shooting for 400 hp. Just make sure that the chassis mounting area is secure and not rusty or corroded. I've seen plenty of 500+ hp cars run 2 bolt mains all day long. I think that 4 bolt is way overkill.

pce out-
nitro guy
As many racecars as i have seen and built, the 4-bolt blocks do better on roundtrack applications unless you are building an all out 500hp drag motor. Short answer....2-bolt is my choice when it is still streetable.
depending on your final compression, your 2 bolt block will work. if you're gonna run this engine on the street and run pump fuel then you dont want to run more than 10:1 at which 2 bolts are more than sufficient. i say this because i have personally built race engines for Nascar Late Models at Rockford Speedway in Rockford,Ill. the engines that i built were 2 bolt blocks running 9.5:1 (limited by rules) turning 7500rpm and held up for several seasons without teardown or freshening.however the debate bout 2 vs 4 continues
don't sweat it , unless you are going to run a blower ,or turbo ,or nitrous ,your 2 bolt will be just fine ..modest camshaft ,single 4 barrel750 cfm ,just drive it and enjoy it .......................don't forget to pressure-prelube the engine before you fire it ,make sure you have the good assy lube on lifters ,lobes ,rocker arms etc ..make sure you break cam in as the supplier reccommends ,usually 30 /45 minutes at 1800 rpm..should have a dandy engine ..set lifters/rockers engine off ,per MOTORS manual ,much less messy ..good luck ,let me know how it works out ..jim p...
it should be fine. but if it is worrying you then buy a 4 bolt and say the extra money was for peace of mind. cheers!
my 2 cents
i have heard and read that 4 bolt mains were simply used to make up for a low nickel content in the blocks. if i recall a quote properly: "four bolts in mush are better than two".
Since you already have a two bolt main block , just use it and don't worry, a stock 2 bolt main block will take more abuse than you would think. If you happen to have a four bolt main block, then use it, if you want. I wouldn't go out of my way to have a four bolt block unless, it was an extreme duty application, and in that case, I would have a two bolt block four bolted with splayed caps or use a bow-tie block anyway.
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