[Half-Inch Stud]

Originally Posted by Chipped Tooth

.....
Reason is lots of performance connecting rods have this feature..... I think rods are a higher stressed item than a main cap.

But I'm not saying I'm right.

Driver side Rear Axle Cap/Bolt had the toughest duty. See how thry run.

[JD311]

I called pro-gram about my caps being drilled for 1/2 in. Apparently they used to make the outer bolt holes 7/16, but the input they've gotten in the last few years was to make them 1/2 in. They can custom make 7/16, but they all come 1/2 standard now. Had a lengthy conversation on the phone. He also noted Pontiacs had two different dowel sizes, .309, and .3125 (5/16). My blocks all measure 5/16, their caps are 5/16 as well. He said I could still use them for 7/16 by using a insert like a t bolt washer. (like something you use on a 302 ford with 7/16 bolt holes and a 351w head with 1/2 in bolt holes. They also do main spacers for Pontiacs using 351c bearing.

[LMSRACER]

Originally Posted by JD311

I called pro-gram about my caps being drilled for 1/2 in. Apparently they used to make the outer bolt holes 7/16, but the input they've gotten in the last few years was to make them 1/2 in. They can custom make 7/16, but they all come 1/2 standard now. Had a lengthy conversation on the phone. He also noted Pontiacs had two different dowel sizes, .309, and .3125 (5/16). My blocks all measure 5/16, their caps are 5/16 as well. He said I could still use them for 7/16 by using a insert like a t bolt washer. (like something you use on a 302 ford with 7/16 bolt holes and a 351w head with 1/2 in bolt holes. They also do main spacers for Pontiacs using 351c bearing.

The Main Caps that I sent back were drilled 1/2" and the Main Caps that they're sending back are to be drilled 7/16". Like I said earlier, it's a preference thing. I listed why I prefer the smaller holes already.

Question:
What Year did Pontiac switch to 0.3125" Dowels?
I've checked many over the years and most of the time, I get 0.3090" to 0.3095".....

Another "Repeat" Question:
Let Me ask again, Has anyone actually measured the Main Webbing Thickness at the Base of the Cylinder Bores of an IA-2 or MR-1 that has been Bored 4.350" or Larger?
If so, then what was this Dimension?

Have a Good One,
Larry S.

[JD311]

We have an IAII in a car right now, I never measured it. Its bored to 4.35, it looks thick lol.

[LMSRACER]

O.K., I just checked Three Different Blocks for Dowel Pin Sizing.
1967 400 Block = 0.309"
1971 455 Block = 0.309"
1974 350 Block = 0.309"

The 1974 350 Block is the Newest Block that I currently have.
So, the Question is:
If Pontiac Changed over to a 0.3125" Dowel Pin, When did it Occur?

Regarding the Question on the IA-2 and MR-1 Blocks:
On an IA-2 or Mr-1 Block the Thickness Between the Bores at the Deck Surface is approximately 0.270" @ a 4.350" Bore. Therefore, I know that it is going to be Less than that at the Base of the Cylinder Bore. I'll bet that it is "Considerably" Less....

Larry S.

[gene simmons]

Larry,
I had a 1974 400 #481988 that the main web was cast with a notch,to clear the dipstick tube. It appeared to be one year only. None of the 67-78 blocks that i have seen had this.
Does your 74 350 have it, and if it does,do you have a photo?

If you were to drill for 4 bolt,the bolt hole would be thin on the dipstick tube side.

[Half-Inch Stud]

OHC6 has registered caps in a deep-skirt block. See how they run.

I'm annoying myself. sorry all.

[Half-Inch Stud]

5/16 dowel has been an excellent Shop upgrade for the 1st 4 caps. maybe just the middle 3 caps. can do all 5 with no harm though.

[JD311]

Sorry for the incorrent info, just checked my blocks again, my helper used a pos mic. I knew I should have checked them myself lol. My 59 389, 74 455, 67 400 are all .309, and thats 3 different gens. I have 4 sbc's early and later that have the exact same size dowels for the heads as well. Guess thats what GM was using. So yeah, i'd like to see one with .3125, I didn't check every block I own though......lol

[Chipped Tooth]

Half inch, getting into grandpa cough medicine again?! Lol

I'm not sold on the solid Dowel pin set-up but not much a guy can change since the factory already has the holes drilled.

[taff2]

Bill 'Grumpy' Jenkins used to put dowels in some of his high rpm SBC race engines to stop the caps moving around, and we all know how successful he was.

[Chipped Tooth]

The Dowel wer only used in aluminum BBC blocks he used...... I'm not positive but I think aluminum moves around a little more than iron.

Maybe things have changed since then......

[taff2]

Jenkins tried them in his small blocks according to his book 'The Chevrolet Racing Engine'.

[Chipped Tooth]

I believe that was before decent fasteners where available.

Bill Jenkins had the register moving apart thing figured out back in the 70s he would take the dowell pins from pontiac engines and use them in the 2 bolt 400 engines that he was playing around with . He called them tire test engines and match race motors. Bill would install a dowell pin half way from the register in the block to the main bolt hole to keep the cap in the register, and he would use factory fasteners where ever he could he climed the stretch was the best on the gm bolts

Before ARP came along SPS was the rod bolt to buy, and GM made some great rod bolts as well called Boron bolts. The SBC was a lighter crank compaired to the 90 pound BBC. Yes we spun 283,s and 327,s 350,s to the moon, dowel pins work to a point but not well in BBC with the heavy cranks. Along with GM Ford made some great fastners as well.

[Chipped Tooth]

And only a practice he used on 2 bolt 400 sbc blocks.

[gavin]

These are the sonics for a .040 68-69 no.9790071 400 block and a .060 73 no.485428 455 block.The 400 block could easily go to 4.210 .The 455 is so bad I almost don't believe the chart.If you add .030 to take it back to stock thickness its still not good.Im hopping my own no. 485428 block is better than this when I get it checked.I will do my 62 389 block when time and money allow.