109 deck/stud thread count

[Mike T]

I had a mishap that pulled the top 4 threads out of a head bolt hole on my motor. Not thrilled about it but after a closer inspection it seems that I got lucky and it will be ok.

I counted between 11-12 threads on the ARP studs so I tried looking down into the block see how many were left to engage. I really couldn't tell so I made a tool by cutting a bolt down until it was only 3 threads tall and TIG welded a small 6 inch long rod on it.

This tool allowed me to engage the threads in each hole and count turns until it went beyond the threads and into the water passages. After doing this I subtracted the 3 threads on the tool and found that the outboard bolts have much deeper threads (16-18 on average) than the intake side (11-12) .

The damaged bolt hole was on the outside of the block and still has 14 threads. I think this will be fine since it still has more threads than the intake side from the factory.

The difference in thread count may be old news but I though the info was worth posting.

 

[bison]

I would call arp and order studs with more thread for those locations. You can get away with at least .250" more thread. It won't matter if the threads on the studs are not engaged into the water jacket but I wouldn't want more than a thread or two in the jacket because it will begin to oxidize and removal would be more difficult in the future


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[Mike T]

I would call arp and order studs with more thread for those locations. You can get away with at least .250" more thread. It won't matter if the threads on the studs are not engaged into the water jacket but I wouldn't want more than a thread or two in the jacket because it will begin to oxidize and removal would be more difficult in the future


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As it stands I have enough thread depth to engage all 12 threads on my current ARP studs. Do you think I should get studs with .250 more thread for the outboard bolt holes only since there is ample threads in the block?

I'm still surprised at the substantial difference in thread depth between the inner and outer head bolt holes....all 109 blocks must be cast very similar.

I would think the ideal situation for strength would be to have all holes 18-20 threads deep and the shoulder of the stud (slightly larger diameter) to fit snugly into the recessed portion of the deck.

 

[S10xGN]

The golden rule of thread engagement is your good as long as the threaded length is greater than the pitch diameter. So for 7/16-14 studs, a minimum of 7/16" depth would be at least 7 threads. I'd be even less concerned with studs than with bolts since it's a straight pull with no twisting...

 

[Mike T]

The threads were pulled out at the machine shop but it was totally my fault. I supplied the hardware that I wanted in place during the hone job and missed the fact that the head bolts were too short for the 109 block.

I was watching as he was going through the torque sequence and the first bolt that received 50#s pulled the 4 engaged threads. So with that theory each thread must be good for about 12.5#s

A mistake like this gets me wondering about the strength of different areas of the block. I would like to see a main stud attached to a device that could pull it straight out of the block to see where it would fail (block crack or threads fail) and how many pounds of pressure it would take to fail.

 

[Mike T]

I would call arp and order studs with more thread for those locations. You can get away with at least .250" more thread. It won't matter if the threads on the studs are not engaged into the water jacket but I wouldn't want more than a thread or two in the jacket because it will begin to oxidize and removal would be more difficult in the future


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I now see what you were trying to tell me. The outside studs are barely long enough as they were and now with the lower thread depth I will need longer studs.

I like things symmetrical and it was killing me to have one oddball bolt hole. Since the outside threads are so much deeper the only solution that satisfied me was to drill the outside holes to the same depth. It worked out well and it looks like the ARP main stud kit for a small journal 327 will come with 8 4 inch studs that will be perfect for what I need.

The drill worked perfect with a stop that I made for it and I even considered countersinking the end studs that sometimes require extra washers but those holes only have minimal threads to work with so I left them alone.