Thread: 2 bolt or 4 BOLT

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I am looking to buy a 350 from a guy i know, but its only a 2 bolt main? How much safe power can i put it two it and how much stronger is the 4 bolt? the engine is going in a 82 chevy 4x4

I have the same question and hope it gets answered here. So far the history page on Arkus Duntov notes that up to the 327 SBC there were only two-bolt mains in the block and it says that the need for a four-bolt on the street is exaggerated EXCEPT FOR TOWING!. Also in the book by David Vizard "How to Build Max Performance Chevy Small Blocks on a Budget" the No. 3 buildup he shows reaches about 380 H.P. and he comments that a two-bolt block would have been adequate. Then there are the Milodon angle-bolt main caps which convert a two-bolt to a four-bolt with added bolts on an angle. That means an added $150 for the after market caps and added line boring charge. From What I have "read" it looks like a two-bolt is OK up to 350 H.P. unless the engine will be involved in towing. Who can tell about a two-bolt failure and the conditions of that failure?

Don Shillady
Retired Scientist/teen rodder

Ill be running about 400 hp with 10:1 compression

I wouldnt worry about it, unless you plan to be in situations were you might tend to cause spark knock for any period of time. Ive built many 2 bolts & buzzed the piss out of em & never broke one, actually now that I think about it, its been an awful long time since Ive had a 4 bolt myself.

Steveo , either block will work well for you for the street.
The 010 casting 2 bolt blocks have a higher nickel alloy and will allow for more twist than the 020 castings.
With the cams available for the street out there now you won't be twisting ( with any power ) over 6500 rpm anyway. But if you are going to build a solid lifter strip motor thats going to turn up towards 10k than it may be an issue, and I would then go with the 010 block & have it splayed to a 4 bolt so that you can feel safe with that Blower, or 250 NoS shot as far as the lower end goes anyway....
Good Luck

hambiskit
I was also told the 2 bolt was the better of the two and would take the higher RPM's but never got a good explanation of why. By the way your user name makes me hungry

This question is of great interest to me since I also have a '76 Corvette two-bolt, so I did some "look-see" research at nearby "Joe's Machine Shop" in Ashland, VA. The proprieter is Mr. Joe Butler who showed me a shelf full of track trophies which he said is only half of his trophies for circle track racing with the rest at home and he says he would not run a four-bolt block on the 1/2 mile tracks he races on (running a 5.34 rear gear!). Then he showed me two blocks in his shop where it was clear that the outer bolts of the four-bolt tap through the block webbing at the bottom of the cylinders while the two-bolt blocks are solid there and I was able to look at both examples there in the shop, the two-bolt was being bored/honed at the time and I could see down into the cylinders. I wish I had a picture to show, perhaps someone else has a good picture of this? Of course it is clear from comments on this Forum, magazines and conversations with racers that there are variations on this belief and others and I do not know what class Joe runs in, but he said he is running a two-bolt 377 with a 5.34 rear gear for the 1/2 mile tracks. That's good enough for me to just rebuild my two-bolt 350 and that is what I plan to do. If anyone else has a picture of the bottom of the SBC cylinders that would show this point clearly. I was surprised due to my dinosaur experience limited to flatheads where the cylinders go straight down with no webbing at all, but I can see how the webbing at the bottom of the cylinders in the SBC add strength and the added holes could weaken this, but this means to me that it is OK for my intended motor at no more than 300-320 H.P. As Streets said, for the street there is probably little difference, but to me this means that the two-bolt is OK and according to Joe Butler, the two-bolt is actually better, so I will go ahead with a mild two-bolt 350. If you plan to use a blower or push 400 H.P. or more that is your call to use/not use a two-bolt, but surely if 1/2 mile racing is OK with a two-bolt that is a comfortable cushion for the two-bolt on the street? It would be interesting to hear from a drag racer who has used say up to 250 H.P. from nitrous, but maybe all of this is relative to the rods as a weaker part. Maybe the main caps are much less of a problem than the stock rods and Joe Butler encouraged me to purchase new rods rather than recondition the old ones. As usual I talk too much and it amazes me how some of you can say so much in a few words, John Wayne/Gary Cooper genes?

Don Shillady
Retired Scientist/teen rodder

You cant really use a HP number as a scale, it depends alot on how that # is achieved. a 400 HP motor for example useing typical TRW or Speed Pro pistons is going to be a heavier rotating mass than say Arias, Ross, SRP type pistons,that extra mass will be more critical(not to say TRW or Speedpro pistons are bad, just there not very light). same goes for the rods.
it also depends on the RPM range. If your building a motor with decent compression ratios & nice flowing heads, you may not have to buzz it nearly as high, same goes for maybe using a roller cam verus a flat tappet cam.
It also depends somewhat on how hard it has to work, if its a light vehicle ,with less load,it wont have to work as hard as say in an Impala...or a truck that tows alot of weight.

Well I suppose the main feature of this Forum is communication and I appreciate comments that I can learn from. In fact it is cheaper to recondition my old rods than to buy new ones so that is a good tip if they are better besides. Are all 400 blocks two-bolts? If so that explains Joe Butler's preference for two-bolt blocks, but as far as I know I was looking at two 350 blocks (I will check), one with two-bolt mains and another with four-bolt mains. The facts of the matter in my case are that I do not have a spotless drivers record in Va and I am 67, so I have no intention of competitive racing (sorry!). While I can REMEMBER excitement from earlier days I just want to go cruising in roughly a 300-500 mile radius to car events of various types. I note that most of the feature cars in Street Rodder have stiff rears (for mileage) and a mild 350 SBC, while on the street it seems that everyone has a 383 or a Mopar 440! I guess I aspire to the cruisin' crowd. As far as jollies, my best memories have to do with solving certain equations and presenting the results at international conferences where the egos are enormous, so my reverting back to teen activities after 40 years are out of date (you want to buy a 4" Merc stroker crank?) and partly the fact that I may not be able to solve new equations anymore, so I just want to have some fun and one good memory left to me is the surge of power from a 350 SBC in low gear and maybe a good 0-60 time, so let an old man have some fun. Come to think of it low gear in a '47 Ford flathead was pretty good too, but I kept breaking the fragile trans (three in all), still the torque from a flathead up to 1500 rpm was/is almost as good as the SBC, but by 2000 rpm the SBC is a new game entirely. Well tomorrow I have to teach the Carnot cycle to a class I agreed to teach to finance my engine rebuild and maybe they will like the joke about the Corvair with a "Car No Engine" under the front hood! (Got to pay for the engine somehow!) I will admit that the picture of the bunny with pancakes on it's head by Treekiller was the best laugh I have had in a long time so I guess there is some good fun on this Forum.

Best Wishes,
Don Shillady
Retired Scientist/teen rodder

Sure there is less webbing in a four bolt block, go splayed in a two bolt block, but make sure the cylinder stays in round when they hone it. In other words, make sure the mains are torqued down when they bore/hone to make sure the cylinder isn't deformed after torquing the splayed bolts. And on another note, the big issue here is that you don't want any side play or 'slide' of the cap. The four bolt block will just 'hold' it with more bolts. The new LS1s have 6 bolt mains from stock just for info sake. For you, I would stay with the 2 bolt unless the 4 was nearly the same price. The crank and mains I would leave....the rods however....

A two bolt block is good to 500 hp as long as you use a good set of main studs, not sure of max HP without the main studs.
If I remember right this info came from Grumpy Jenkins. He also said that 4 bolt 400 blocks are weak and will tend to crack in the main webs due to the lack of material there because of the outer bolts for the 4 bolt. I know a few folks running 2 bolt 400 blocks putting out over 500 hp with good main studs and haven't had a problem with it yet.
I personally have a 400 block with 4 bolt mains because I couldn't find a 2 bolt block that was borable .030 over. These blocks will wear a bunch on the bottom of the cylinder areas due to those short 5.56 inch rods and that long stroke. I would have prefered a 2 bolt and bought a new set of splayed 4 bolt caps and had them installed, but it didn't work out that way.
And just for the record, someone mentioned that a 377 is a 400 with 350 rods, actually it's a 400 block with a 350 crank.

Streets has shown there are/were four-bolt 400 blocks as well as two-bolt 400s. Joe Butler says he only uses two-bolt blocks (whether they are 400s or 350s, likely 400 block for a 377, the issue is two-bolt versus four-bolt) for circle track racing with a 5.34 rear. To me that is enough to believe that the two-bolt 350 block is adequate for street performance at least up to the 380 H.P. I found in the book by David Vizard. The fact that Joe Butler can wind up a two-bolt block with a 5.34 rear in a competitive situation convinces me that at the very least a two-bolt block is strong enough to handle performance upgrades, but the points have been made that the crank+piston mass load and side motion are other issues. Since 350 H.P. looks like a dream to me and I will probably end up with only 250 H.P. with a torque cam and hopefully 17-18 mpg, it would seem to me that the two-bolt is fine for my needs and let others show one way or the other which is best. Still it seems pretty amazing to me that a circle-track racer would prefer and only use two-bolt blocks and be successful, so it looks like the two-bolt is pretty good after all.

Don Shillady
Retired Scientist/teen rodder

well Don i run a 327 2 bolt main, and to say the least i have put that engine throu hell and back, so i know 2 bolt mains are strong enough for 380 hp, b.c im running 410 to the weels at the fly weel its about 418hp, and that engine has turned 7000rpms with out a problume so 2 bolt mains are good i think. oh ya i have about 20,000 miles on it, most people will say its a fresh engine if its turning 7000rpms, so its fresh but not brand new.

Cd