Which Block ?
 

Howdy all, been a while.
Gathering parts for a 400 stroker. I have a 73 400 block and an Eagle 4.25 forged crank. Going with the forged long rods and good pistons. I had originally planned to use the block with factory 2 bolt caps and studs. However, I recently found a local guy with a 70 block drilled for four bolt mains. He wants $500 for the block. So, if I'm all in on rotating assembly should I drill the 73 block for 4 bolt mains and buy caps or buy the 70 buy caps not have to drill just align bore/hone? Engine going in 77 TA and 70 400 has the 5 lug bosses for motor mounts. Thanks all !!!!

your 2 bolts stronger than his drilled 4 bolt. id keep the 988, i just had 2 of mine honed with 2 bolt studs. if you were gonna put 4 bolt caps and line bore, your 988 blocks better. you can use splayed caps. i have a 988 block and a set of splayed caps i plan to use at some point. for now i have the 2 studded 400s, i also have a 4.250 2 bolt block that im gonna just run with bolts

Undrilled 2 bolts is the best. You can run splayed caps with them, better than straight bolt.

Everything hinges on how much power and or how high a rpm the motor will need to spin depending on your Cam pick.

Your Cam pick will be a result of the average type of driving that will be done with the car and the overall gearing and compression the motor will need for that Cam to make the needed cylinder pressure to go along with the fuel octane you will / can run.

Block wise if your looking for the ultimate strength and have a big enough wallet then go with the 73 block and put on splayed main caps.

In terms of the 70 block if your only looking to make up to 550 hp then the stock 2 bolt main caps fitted with studs is fine, especially if you take the added time to prep the main caps by sanding roll over all the sharp edges but for the bearing parting line.

The 2 boltsmain caps of the short stroke motors are stronger then the 2 bolt caps from the 3.250 inch main motors because they are thicker since they are only bored to 3”, not 3.250”

Some things to note here are these.

1)
On a few 1970 400 blocks I have had that where drilled for 4 bolt mains I have found cracks in the main web parallel to the the outer wall of the outer main bolt holes, so when you look at that 400 block look very carefully at the walls of the bolt holes.

2)

If you build the 400 with just its stock main caps put Allen head 1/2” course bolts in the 6 unused outer holes and red Loctite them in .

This will strengthen the main web area, but also make sure that the Allen bolts you use do not bottom out.

After doing this the line bore needs to be checked just as it would from just switching over to studs from the stock bolts.

3)

The 73 block has a tad thicker cylinder walls then the 70, that’s one of the reasons for the changed part number as they knew catalytic converters where coming and the motors would run hotter.

Personally from the way it sounds like you will use the motor I think either block will serve you just fine with two caps when fitted with studs for just that added measure of safety.

Without doing a sonic check, you will never know if one block has thicker walls than another, where it counts.

I checked a 71 and 73 block I had, both very different castings, and the 73 had core shift and the entire passenger bank was the thinnest, on the thrust side. From a visual inspection, all other items looked good (lifter bores, frost plug holes) and I would not have suspected thin bores.

It really doesn't matter. We can all sit here and over think every measurable aspect of both blocks, but I'd make my decision solely based upon the condition of the block, and since you already have a good block, I'd just use that.

I'd use what you have and buy the other for a back up. Even a good 400 block is getting harder to find!

i agree with that if he were comparing 2 988 blocks but that 70 is a different casting and not as good....but if he has tge extra coin its still a nice block to have as back up like skip mentioned....if on a budget though id put the 500 into studs n line hone.


i had 2 of the 70 400s...i sold them both an kept 3 988 blocks just because what steve 25 mentioned and alot of others in old threads.

One thing I found is that the dowls in the blocks aren't worth a crap
and let the mains walk all over the place, So I started making my own.
Went to the hardware store and bought some 1/4 inch round stock and
cut them 1 inch long and replace all factory dowls. I was getting cap walk
on all my performance motors, since I've done that I don't get any cap
walk at all, no more fretting. The fit is tight and makes for a little more
work to get them off but it seams to of stregnten the lower end up
trementliessly. I use studs and factory bolts.
I don't line hone for this app either, Been doing this for a long time
and no problems at all, bearings seam to look better between
teardowns.these are all two bolt main motors.
Sorry about some of the bad spelling LOL

GT