[Shiny]

Just curious - what is the purpose of that "window"?

Trapping blast shot may not have been anticipated when that design decision was made.

Are the windows machined? If so, the extra cost must have had good justification.

[steve25]

Those where necessary to accommodate the big diameter/ shoulder of the rod bolts in that parting line area of the rod bore.

This helps to keep the rod big end and cap end in alignment under heavy G force loads when the rods tend to pinch in there.

According to the engineer who designed the rods back then he wanted them to not be stressed to not much more then 60% of there ultimate strength at 6000 rpm .

[Shiny]

Originally Posted by steve25

Those where necessary to accommodate the big diameter/ shoulder of the rod bolts in that parting line area of the rod bore.

This helps to keep the rod big end and cap end in alignment under heavy G force loads when the rods tend to pinch in there.

According to the engineer who designed the rods back then he wanted them to not be stressed to not much more then 60% of there ultimate strength at 6000 rpm .

Thanks

That explanation now begs the question of why have the large-diameter bulge in the middle of the bolt? That decision led to the window apparently.

My only guess is to limit the amount of stretch, which by design would be primarily in the smaller diameter sections of the bolts. Again, I expect that bulge added cost, so why was it needed? Why wouldn't the small diameter be made continuous?

Those necked-down sections look like tensile test specimens. Easy to calculate*the tensile stress in a controlled region of the bolt. In this case, though, TWO regions. Why?

The 60% of UTS in my experience is a decision to mitigate fatigue risk. The generous radii in and out of the necked-down sections would suggest the same.

Interesting to see how complex those bolts are and how their design spilled over into the rods.

Cool beans!

Mike

[mgarblik]

Thanks for the additional information on the rod bolts. My rods are A series rods. The bolts do not fit tightly in the rods. Actually, loose enough to concern me. I am ordering a set of ARP rod bolts. They are specific to this application of course. Hope they fit a little more tightly in the rods. I will stretch them to the minimum and check the housing bores for size and shape.

[Elarson]

The large diameter section SHOULD exist at the parting line and SHOULD be a close enough fit to act like a dowel pin at that joint to properly locate the cap. The necked down sections are to remove the stress concentration at the base of the thread. Another indication the Pontiac engineers were pretty savvy.

Eric

[Shiny]

Originally Posted by Elarson

The large diameter section SHOULD exist at the parting line and SHOULD be a close enough fit to act like a dowel pin at that joint to properly locate the cap. The necked down sections are to remove the stress concentration at the base of the thread. Another indication the Pontiac engineers were pretty savvy.

Eric

Doh!

Thanks for explaining. Makes sense now, except I'm back to wondering the purpose of the window in the rods again.. ???

[sdbob]

Back in 1974 factory sent out notice all new SD rods to be dissembled and throughly cleaned of the shot used in production.

[mgarblik]

I finally had a chance this week to do a little more investigation into the SD connecting rods and get some other gaskets, seals and hardware ordered. People really familiar with the SD-455 engine may already know this, but I found it interesting. I ordered new connecting rod bolts and nuts from ARP. They came in yesterday. The interesting thing is the factory rod bolts. They are a very unique part with an unusual thread pitch. Pontiac utilized a 7/16-24 thread on these bolts. This is a non-standard coarse or fine thread. It is called a 7/16-24 UNS (Unified Special Thread) . I searched all over the internet and the only other place I found that size and pitch used was on banjo bolts for brake calipers and some fuel systems. Very unusual. Not sure why Pontiac decided to use this non-standard thread. Possibly to gain a little more thread engagement and a little more cross-section in the bolt. Or possibly to make the bolts and nuts tamper-proof by not allowing standard hardware nuts to be used which would certainly be a disaster. Just another interesting tid bit about the OE connecting rods. The replacement ARP bolts are standard 7/16-20 fine thread. The ARP replacements are slightly lighter than the OE nuts and bolts. OE Bolt= 52.2 G ARP Bolt= 49.8 G OE NUT=8.2 G ARP Nut = 7.9 G

[sdbob]

Thanks for the info. Back in 75 I ordered a complete set of SD rod bolts. Why? I guess if I ever didnt want to reuse rod bolts, I had new ones.

[mgarblik]

Here are a few pics of the OE and ARP SD rod bolts/nuts. Square head is OE, football shape is ARP. Also the UNS thread pitch chart showing the unusual 7/16-24 thread.

[steve25]

I don't like that ARP replacement bolt!

That reduced head size concerns me .

That extended top pad on the factory bolt was engineered there to better releve the bending forces trying to widen that upper part of the rod out to each side because the the rod will tend to go Egg shaped.

I mean for a street rebuild I would use the ARP bolt but other wise I would chance a 3rd or even 4th torque up of the stock bolt.

[mgarblik]

Originally Posted by steve25

I don't like that ARP replacement bolt!

That reduced head size concerns me .

That extended top pad on the factory bolt was engineered there to better releve the bending forces trying to widen that upper part of the rod out to each side because the the rod will tend to go Egg shaped.

I mean for a street rebuild I would use the ARP bolt but other wise I would chance a 3rd or even 4th torque up of the stock bolt.

I do like the flat, square head of the OE bolts better. It does provide a larger landing area on the rod. Unfortunately, I do not know the full history of the OE bolts. I believe this may be the third time this engine has been apart. Not sure how many stretch cycles it went through on the last rebuild or before that. Typically, three stretch cycles would be the minimum for a single rebuild. 1. to assemble the rods to hone them. 2 to assemble the rods with the bearings for measurement of oil clearance. 3 for final assembly. My guess on the head design by ARP is they looked at their existing dies for 7/16" rod bolts and chose a die that fit the rods with adequate clearance not creating a stress area near the beam. Then made a couple thousand bolts on a short run of bolts. They are so oddly shaped and so specific, they didn't want to start from scratch with new dies for just a few thousand fasteners. Probably the same reason they went with regular 7/16-20 fine thread. I have little doubt the ARP bolt is made of premium material. They also fit nice and snug in the rods. About half the OE bolts just fell out of the rods. Another possible indication they have been overstretched at some point. If I have a chance to dyno this engine, I will certainly take it to the RPM range where the power drops off. I assume around 6000 RPM. No plans to race it or anything like that. But if I want to run it hard on the street some, don't want to be worrying about connecting rods coming loose.

[mgarblik]

Finally had some time to work on the connecting rods for this SD engine. I installed the ARP bolts and stretched them to .0075" stretch as directed by ARP. Then measured the big end housing bores. Each housing bore closed up .0015"-.0018" smaller than the spec. This is NOT unexpected. The ARP bolts are much stronger than even the stock SD bolts and have higher clamping load. The torque to reach that stretch was around 90 ft. lbs. So as a general rule, on any connecting rod, when changing the bolts, plan on honing the rod big ends back to specs. to get nice straight and round bores for the bearings. Should be a fairly easy fix. Will make that happen today and move on to a little block hone touch-up. Then a thorough cleaning.

[hurryinhoosier62]

Mike, if memory serves me the OE SD bolts were produced by SPS. I wonder if they are still available through them?

[mgarblik]

A short update on some engine progress. The connecting rods are finished as mentioned on an earlier post. I have included a few pics. I also verified the oil clearance on the 8 rods. All are .0031"-.0034". This is with Clevite 77 bearings that were manufactured in 1978. Going to reuse them as they were never run. Block will be honed today. It will only take .0005"-.001" to get all 8 cylinders where I want them. Going for .0065" piston to wall clearance. Factory clearance for SD-455 is .006"-.0068". With the .030" overbore, .0065" sounded like a good number. As most know, these pistons ran very loose cold in the SD engines giving them a unique diesel like clack cold when everything is correct.

Note: As most machinists know, it is not standard procedure to hone rod big ends with the pistons attached. But these pistons and rods are a press pin design. With new pistons and pins already installed, much greater chance of damaging the pistons, pins or both removing them by pressing out. Rods could be touched up without removal.

[sdbob]

Thanks for the update. Interesting as I've never actually witnessed this work done.

[mgarblik]

Happy to be back on here with an update. I have been down with a nasty case of COVID for nearly a month. Just getting back into the swing of things. Way back in early October, Paul K asked me to sonic test the block. Since I took so long, I decided to map the entire block which I did yesterday. The results will be posted below. As Paul stated then, the cylinder walls are not as thick as ideal in some places. Certainly nothing like an IA, IA II or KRE aftermarket block. I don't have a basis for comparison to a regular 455 stock block. It's safe to say boring the block for a set of 8 , 1/8" sleeves would not gain you anything. I wouldn't do it unless to save a cracked OE bore. The bottom of the bores appear to be very well supported and thick. Next on tap is to install new cam bearings and begin the assembly process. I plan to weigh this block in the next week and a regular 73, 455 block I have sitting around here for a weight difference, if any. Stay tuned.

I see the sonic test will not post directly. Will have to post as a jpeg.

[mgarblik]

Sonic test results. Feel free to modify to make it easier to read.

[mgarblik]

Some additional pics of block and sonic test. You can see the reinforced cam bores. About 1/8" of additional material.

[steve25]

Thanks for the up date and posting the sonic test results.