arnoldjr

So I was looking for info yesterday to see if the #7 heads that i have were any good and I came across a page, don't remember what it was called. The guy mentioned having his main bearing cap machined for use of a "Strap and stud kit" What is a strap and stud kit?

TripDeuces

Here's a pick of the strap kit. They are 1/2 inch plate added to the top of the main for strength. A picture is worth a thousand words.

arnoldjr

Oh ok. Thanks. Is this really needed if I'm planning on building around 300 horses?

TripDeuces

Not at all. Some factory engines were rated at 400hp and had no problem. If you were shooting near the 500hp range I'd start getting concerned.

arnoldjr

Ok cool. Thanks for the info. Don't need 500hp lol. Would just get me into trouble!

joe_padavano

I have to say that as a structural engineer, I remain amazed that people think these straps actually do something. First, these straps have negligible stiffness as compared to the cap itself. The load will ALWAYS be carried by the stiffer load path. Second, unless the holes in the straps are match honed to the bolts and are a light press fit, the strap will carry NO appreciable load. These straps are about as effective at increasing bottom end stiffness as the infamous oil restrictors are at keeping oil out of the rockers. Should I be skeptical that they are originally from the same source? This is another case of eyeball engineering.

To effectively add stiffness to the bottom end, you need a total girdle that is tightly fitted. Some simple pieces of bar stock will do nothing. Billet main caps would be a MUCH more effective choice.

TripDeuces

I think the original purpose of the strap was to add a rigid support to help stop the main cap from egging in shape at rpm. It's stronger than the iron cap and should not flex downward as much. I have to agree with the girdle. Anything that ties all the caps together has to ad rigidity to the block plus you get the added benefit of the material across the bottom of the cap similar to the strap by itself.

The one thing I never understood about them is some want you to remove 1/2 inch of material from the cap to install the steel strap. So you're giving up 1/2 inch or iron for a 1/2 inch of steel? Although stronger overall I can't see that as a huge benefit. Why not leave the cap alone and install it right on top just like a girdle would be?

joe_padavano

Unfortunately, you are confusing strength with stiffness. Stiffness is what prevents the cap from egging and you increase stiffness two ways. First is to use a stiffer material (and the modulus of steel is not that much more than the modulus of iron). The second is to change the section properties. The deep cap is MUCH stiffer than a piddling 1/2" steel strap. Second, since the strap is not attached to the cap, it simply flexes between the two bolts.

I'll say it again, the strap does virtually NOTHING to increase stiffness of the cap, especially if you machine 1/2" off the cap first. That actually DECREASES the stiffness.

Now, the next failure mode IS strength related, it's fracturing of the cap. Again, the strap does nothing to change this.

cutlassefi

I agree and disagree. First of all I don't mill .500 off the cap then use a .500 thick strap, I think that's stupid too.

The last 3 I've done we milled about .200-.225 off the cap and used a .500 thick strap, that should add to stiffness.
However cap walk is another issue and I believe with more mass and better fasteners (I use ARP studs with the straps) this is kept to a minimum.

Jmo.

captjim

I'm not sure how, especially with the holes having a little clearance in them.

Two questions. 1) are the stresses on a 350 not a LOT less than a 455 with the lighter piston and shorter stroke?

2) thoughts on a halo girdle? I get creamed for saying it does nothing but give you a handle to pick up the pieces. Obviously, a well machined full girdle tied into the rail adds support.

Run to Rund

Sam Murray ran a best of 10.82/122 mph with his NHRA D/S 1967 W30 (400). He must have been making over 500 hp, yet used only studs with stock caps. It appears that the only people having a lot of trouble are those adding a boatload of nitrous, which gives a vertical torque curve, lol, or maybe a 403 with windowed main webs.

This thread is about a 300 HP 350, and I see no reason to stray away from stock caps and bolts. And, re-used stock bolts will be fine if they come up to torque spec smoothly and quickly.

cutlassefi

Not sure what you mean here but I just mill off enough to show a couple of threads, no sense in having a half inch of unused stud sticking up.

captjim

What I mean is that there is a little room for the strap to move, the only thing really holding it is clamping force. They are not a precision fit, right?

cutlassefi

Correct.