Here are my thoughts on 4 vs 2 bolt main...Following my recent post on a new 400 small block project.

http://forums.corvetteforum.com/showthread.php?t=937801

While you could start out with a two bolt main setup and use the best insurance, of running ARP bolts on everything in the motor. Why not double your main bearing cap strength by using four ARP fasteners per cap. And given the starting strength of approx. 170,000 psi of each fastener, I would think that some other portion of the motor would let go before bolt stretch would occur, thus opening up the main bearing clearances under load.

Many people argue that 2 bolt mains are stronger than 4 bolt mains but, I beleive this argument was formed before modern fasteners were developed. I also believe that this argument can be circumvented by having your block properly magnifluxed to identify any weak areas of the block before begining an engine build.

Something to note. I have seen, one of the main reasons for the recipricating assembly breaking loose from the fasteners is due to overly heavy parts, used becuse the owner is in search of the heavy duty reliabiliy implied by the factory some 30 years ago. Another factor of course is to overspeed the motor using factory fastners. I.E. Using the old tech. of the factory forged crank, factory pink rods, and forged pistons, all of which contribute to an overall weight of at least 1/3 more than what you need to run along with the factory balance of approx. 30-40 grams on the piston and rod assembly. That window of margin is downright sloppy vs. a good balance of 1-2 grams total(piston/rod small and big ends together). You could run a lightweight cast crank from Scat which has the strength of the old forged crank with a 1/3 less weight, the KB Hyperutectic pistons which is a cast pistons that contains 14-16% silicone to stiffen the aluminum and provide very good piston expantion under pressure while remaining light weight. For connecting rods running the newer factory PM rods might be a good low cost approch however, if you can afford to take down your motor once a season and do not run the car at the 1320 only, I have used forged Manley Aluminum rods that weight just over 500 grams, and they served in my daily driver as well as the once in a while blast down the track all day on a saturday, for over two years without fail.

Sorry if I carried on too much. I just want to share the experience of running a cheap to build and reliable as the sun rising tomorrow motor. The small block chevy motor is by far the cheapest motor to build from the 265 to a 400. Your experence and imagination is the only limit. Hope I can intregue a few of you so stay tuned monthly "400 Small Block Build-Up" .

 

Sounds like you've done a lot of the thinkin' already.

Here's what I did, I have some 7000 HARD miles on this setup.
SBC 400 '509' casting 2 bolt main. Line honed with ARP studs.
Scat 9000 series lightweight internally balanced crank, Scat 6" H beam rods: 645g, SRP Forged pistons: 424g. HV blueprinted Moroso pump, and all the block modifications to help oil drainback, ect. (One thing I would suggest you do is install deck plugs. These decrease the amount of upper cylinder wall distortion present on these motors, its only $15 from moroso and any qualified machinist, or hobbist can do it) Do not install oil restrictors, let the cam and valvetrain get the oil it needs for street duty.
I have made 46 passes at the drag strip shifting from 6800-7300rpm without issue.
I am happy how this combo runs, its begging for more gear, but I still drive it back and fourth to town and take cruises when the weather permits! Compression is at 10.7:1 and runs just fine on 91 octane swill with 36-38 degrees total, and inital timing (locked out).

Good Luck, i'm sure you'll be happy with the SBC 400.

 

Congratulations! You've reinvented the wheel!

Yes, 400 SBC 2-bolt blocks in the stock configuration are stronger than 400 SBC 4-bolt blocks in the stock configuration. However, an original 2-bolt block with aftermarket splayed main caps and good-quality fasteners is about as strong a bottom end as you're going to find.

 

Cool set-up, wish more folks would embrace the big inch small block idea using more current technology and attention to details. I really like that color red and the meaty AR wheels. Bet you love sneaking up on the high dollar snob cars and handing them there a$$ on a platter. Those 6" rods really help to keep the pistons hanging around a bit longer and it shows in your incredible 10.7:1 comp and still run on 91 octane gas with the full advance, what a fantastic combo, what type of heads do you use on this set up? Thanks for the great info also. Happy building and crusing.
RJ

 

2 bolt main blocks are stronger than 4 bolt blocks but it has nothing to do with the fasteners. I've torn down several hundred small block engines and have yet to see even a factory fastener fail. 4 bolt blocks fail under high rpms since the outer bolts weaken the webbing between the pan rails and main saddles because they go into one of the thinner parts of this structural area. They don't break fasteners, they crack the webbing and if not caught, they'll pull the main saddles entirely out of the block. I've personally seen it twice and it's not a pretty sight. As stated in one of the other posts, a 2 bolt block with aftermarket splayed caps is about as strong as you can get short of aftermarket. Short of that, a good 2 bolt block with main studs will handle just about all most of us care to throw at it. Keep in mind that just like the valvetrain and rods, the biggest and most destructive enemy of any block's bottom end is high rpms.

 

That is what I have, a 2 bolt main with ARP main studs. Crank letting go is the least of my worries

 

If the 2 bolt mains are stronger than 4 bolts, why did GM use 4 bolt mains in their higher performance engines. I could believe the idea that it was an engineering mistake; intially thinking it was stronger. GM used 2 bolt and 4 bolts mains for many, many years. If the 4 bolt was not stronger, why didn't GM stop using them after a few years when there was sufficient data/information showing the 4 bolts mains were less reliable. Is it just because GM didnt want to admit the problem and any potential law suits that may have followed?
Just a side question, what is the purpose of spending over $200 for the Callies Billet Main Caps?

 

This is only true in a 400 block because there is less "meat" material in the crank case, a 350 4 bolt main is stronger then a 2 bolt main.
The Callies billet main caps convert a 2 bolt block to a 4 bolt block with the outer 2 bolts on each cap on an angle which is directed into the part of the block with more material. Most aftermarket and performance blocks have these styles of main caps, the are called splayed caps.

 

The 400 SBC is a completely different animal. Similar, but different enough so that they got a bad rep for being, "weak" or being, "prone to overheating" after being built up by folks who weren't aware of the slight--but critical--differences between them and the other SBCs.

 

I agree with most of what has been stated in this thread 2 bolt vs. 4 bolt blocks, but I have torn down 2 differant 350's, one 2 piece rear seal and 1 one piece seal out of probably 100 motors, and found a rear main cap bolt broken. The head was popped of of both of them and they unscrewed by hand when the cap was removed. Now were they over torqued or did the fastener fail, I can't say, all of my motors now get ARP main cap bolts or studs whichever the customer perfers. No used factory fasteners here, rod bolt, main bolts and headbolts are all changed. I've never seen a head bolt fail but have seen rod bolts break and thats not a pretty site either. One other thing I've notice over the years is main caps being broken, its always the center or #3 cap and on a 4 bolt block, never seen a 2 bolt main cap break, and the ones I witnessed didn't hurt the motor, it was noticed during rebuild. When this happens the block gets the billet caps and a align bore.

 

I think it needs to be mentioned that the 2 bolt main cap on a 400 has a lot more meat than the 2 bolt on your typical 350. It has a much wider sadle and resembles the 2 bolt main cap in the big blocks. I will try to dig up a picture.

 

Here's what I was talking about.

SBC 350:


SBC 400: Sorry the picture isn't better, this is all I could find.


Pretty big difference between the two.

 

The reason the 2 bolt mains are stronger than the 4 bolt blocks is because the main webs are thicker! Use a 2 bolt block with ARP studs for strength. Use 2 bolt block drilled for splayed 4 bolt caps for more strenght. For a few more bucks forget the OEM block and buy an after market block with 4 bolt mains.