Info on main caps

[brian89gp]

I just found out that the Chevy 400 large journal motor uses almost the same size main jounal, 2.649" compared to the 2.647" of the 60v6. Just figured I'd point it out for anyone that wanted to research it further. Maybe a .002" shim for the upper, or perhaps boring out the block side to 2.649" and using oversized 400 bearings. No idea if the bearing/journal widths are the same though.

1970-72 was 4 bolt main
1973-80 was 2 bolt main
I have seen splayed main caps to convert a 2 bolt main block to a 4 bolt cap, so there is a difference in the block.

[v6h.o.]

hmmmm..

[RacerX11]

The splayed main cap conversion kits I have seen have a journal diamter that is much smaller than the finish dimension. This is to allow maching on the cap to fit it to the block. Once fitted, the cap is line-bored out to the final dimension. With this extra material, you probably wouldn't need to worry about oversize bearings.

My neighbor has a 400 2-bolt block sitting is his garage. I will try to take a look at it and get some dimensions of the caps.

Does anybody know if the bowtie block 4-bolt main caps are still available? I checked the part numbers on GMPartsDirect, but they didn't come up.

Marty

[614streets]

Are you guys making 450 hp? Why 4 bolt mains on low hp engines?

[SappySE107]

Could be....think high RPM/DOHC. Its not a question of needing it, but rather wanting it if its as simple as slightly modifying the mains and machining the block.

Brian posted it in pushrod for some reason...but he has a turbo DOHC and a few of us are looking at higher RPM.

[RacerX11]

I'm not pushing 450 HP, but I will be running somewhere around 300 at the flywheel with the supercharger. The 2-bolt mains become an issue at around 450 HP on a small block, but probably lower with the V6. 450 HP across 5 mains ratios out to 360 HP for 4 mains if you figure the force per main cap. 360 isn't too hard to hit.

I plan on a turbo DOHC sometime, with forged rods and pistons. 400-450 HP would be doable. I would hate to put the money into a built bottom end, and have the main caps be the weak link.

I have also thought about a stud girdle, or possibly using the 3X00 cross-bolt main caps to make the bottom end stiffer.

Marty

[brian89gp]

More pushrod people, plus most the DOHC guys that would be interested in this stuff read this section where most pushrod guys do not read the DOHC section.

Anyway, there are splayed billet steel 4 bolt mains (meant as an upgrade on the 2 bolt block) on ebay and such for $80. Pretty cheap for some reassurance and bottom end strength.

[SappySE107]

The DOHC splash shield may help strengthen the bottom end up as well. Its still available from GMPD.

[Cotton]

why do you guys type 3X00, when referencing a 3800?


with "juice", and a turbo, 500hp should be attainable, right. So if you build the bottom end as close to bullit-proof as you can, later add-ons arent as worrysome

[SappySE107]

we dont reference a 3800. 3x00 is 3100 and 3400.

[phantom505]

500 HP is very realistic. I'm having the tranny built to handle that power. I might be building a high winder too. This would be great info to pass on the the builder and see what he can do. I'm not 100% sure that I'll do this project, but it's looking pretty good.

[94Camaro]

umm...interestingly enough..tiago ran on a dyno with 16psi and hit 330 rwhp / 360 rwtq and he has the stock 3.4 iron block (gen 1?) with 2-bolt mains..i dunno if 4 bolt would be necessary other than for high RPM applications.

[phantom505]

i dunno if 4 bolt would be necessary other than for high RPM applications

Bingo!

[Corpse]

I need to get a 400 cap but, when compared to a two bolt 350 cap here is what I found:

The 350 cap is slighlty wider on the main, I mean thickness-wise. The bearing tang notch is also out of place.

The 350 cap is also a little shorter, height-wise. The 660 Main is more reinforced in this are. But, as for the sides, the 350 cap is a little thicker. Which is good becuase, I would figure the side thickness to help more than the end.

So, permitting that the bolt holes are hopefully the same, I think it will just be a situation like using pre-68 SBC connecting rods.

The bearing width on the 660 is about .69 or .7 inches whereas the 350 is about .75-.76 Inches.

The 350's main cap also seems to have a better grain to the metal, ie: seems like a higher density metal construction, I dunno though.

the width of the 660 caps is about .83" as opposed to about .89".

The thickness at the bottom or top of the cap, whatever you call it, the end, is about .95 on 660 and about .78 on the 350 two bolt.

The sides, at the point I measure were 660: about .875" and about .93" on the 350.

This of course, is pure speculation as I do not have a 400 cap off an engine right now to measure. But, i am going to pull one and measure due to the closeness in size.

I've always wondered this for my self, due to the fact that the main sizes are nearly identical. Anybody have a 400 main cap to take bolt hole measurements off of? It will take me a few days to pull one and check.

-Dave

P.S. I wanted to add this. I hope that you guys are not pondering the thoguht of converting to a 4 bolt. I though people just wanted to use 400 2 bolt caps. Unfortunately, as i know with the Gen II blocks, adding more main bolts would be rather out of the question. If you look at the block, there is webbing on the sides of the main bolts that goes to the block for added main support, then outside that area, the block is weaker. If you were to drill more holes you would either hit the support web or tap into the weak spot. I would just not advise this.

[Nester]

Main Caps

Well i'm new to posting in this forum, but am a long time viewer and poster of other forums regarding the 660 such as s10forum.com. Anyway i have taken particular intrest in this topic, i love parts swaping and this sounds just like our abilty to use small journal SBC rods in our engines as they have the same length, and main journal diameter. One thing i wanted to point out was that, the Bowtie 660 aluminum racing block (our holy grail) utilizes 4 bolt main caps, so theres another route on the path to retrofitting 4 bolts on an iron block. I'm going to try and see what i can find concerning the Bowtie caps, maybe they can be had seperately from the motor, but i doubts it. Anyway i look forward to hearing more on the subject and posting to the great forum.

Peace

EDIT

Well it did not take much searching...

Bowtie 4 bolt main bearing caps with splayed bolts

10051177 #2 main bearing
10051178 #3 main bearing

I was not able to get prices on these part numbers but there they are. maybe someone else could. Depending on the price it might make more sense to fab your own from a 400 block or just purchase these and get your iron block machined to accept them. (drill/tap two holes at the appropriate angle). One thing i wonder is if the main journal bulk heads have enough meat to have the additional holes drilled into them. Reading the description of the bowtie block it notes

This block has wider main bearing bulkheads than a production cylinder case; four-bolt caps are installed on the two intermediate main bearings.

I guess someone with a bare block and 400 4 bolt caps could tell us.

EDIT