Little known fact, Porsche changed the thread pitch on the block side of the crank studs when they made the 3.0 blocks. Same length, same pitch on the nut end but the block went from 1.5 to 1.75……
At the same time they were still making the 2.5 block with the original thread pitch.
I wonder why?
Why? One of the many design questions!!
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chris white
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michaelmount123
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I've seen 3L blocks with both head stud thread pitches. I've got to think it was a matter of which head studs were available at the time. I can't imagine any other reason.
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chris white
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May be 2.7 vs 3.0 blocks?michaelmount123 wrote: Sat May 27, 2023 9:00 am I've seen 3L blocks with both head stud thread pitches. I've got to think it was a matter of which head studs were available at the time. I can't imagine any other reason.
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Tom
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I don't know either. But happy to speculate. 
Very possible they went to coarse threads for logistics reasons (tooling, quicker/easier install, procurement logistics/efficiencies, etc. )
If they did it for engineering reasons, my guess is they wanted to increase the pull-out strength. The threads are deeper on the coarse threads, making them harder to strip out. Coarse threads are often used in aluminum blocks for that reason. The studs for the 104mm blocks are both shorter and are responsible for more combustion area per stud, compared to the 100mm blocks, and both of those things would seem to subject the threads to higher pull-out forces. Porsche also had more experience than they wanted with pulled 911 head studs, so it may have been top of mind for them....
If you look at the torque specs, for example, the 2.5 heads get torqued to 15 ft. lbs., plus 180 degrees (in stages). The 3 liter head studs are shorter, so they get 15 ft. lbs., plus 150 degrees. And the 89 2.7 head studs are shorter still, so they get 15 ft. lbs. plus 120 degrees. I'm not a trained engineer, so just spit-ball'n here, but it stands to reason the shorter the stud, the less distance it needs to stretch to achieve the same clamp force. A one inch stud stretched to 5 thousandths of an inch would seem to have a lot more clamp force than a 10 inch stud stretched 5 thousandths.... I assume this is why the shorter studs are torqued to spec with fewer degrees. So when the engine is running, the shorter 3 liter studs can't stretch as much as the longer 2.5 liter studs. As such, it stands to reason that the shorter studs inherently transfer more of the combustion forces to the threads (whether or not the studs ultimately get overstretched and fail). Combine that with just having a bigger combustion area to deal with, and I wouldn't doubt if Porsche made the switch intentionally to avoid 911-like pulled head stud issues.
Very possible they went to coarse threads for logistics reasons (tooling, quicker/easier install, procurement logistics/efficiencies, etc. )
If they did it for engineering reasons, my guess is they wanted to increase the pull-out strength. The threads are deeper on the coarse threads, making them harder to strip out. Coarse threads are often used in aluminum blocks for that reason. The studs for the 104mm blocks are both shorter and are responsible for more combustion area per stud, compared to the 100mm blocks, and both of those things would seem to subject the threads to higher pull-out forces. Porsche also had more experience than they wanted with pulled 911 head studs, so it may have been top of mind for them....
If you look at the torque specs, for example, the 2.5 heads get torqued to 15 ft. lbs., plus 180 degrees (in stages). The 3 liter head studs are shorter, so they get 15 ft. lbs., plus 150 degrees. And the 89 2.7 head studs are shorter still, so they get 15 ft. lbs. plus 120 degrees. I'm not a trained engineer, so just spit-ball'n here, but it stands to reason the shorter the stud, the less distance it needs to stretch to achieve the same clamp force. A one inch stud stretched to 5 thousandths of an inch would seem to have a lot more clamp force than a 10 inch stud stretched 5 thousandths.... I assume this is why the shorter studs are torqued to spec with fewer degrees. So when the engine is running, the shorter 3 liter studs can't stretch as much as the longer 2.5 liter studs. As such, it stands to reason that the shorter studs inherently transfer more of the combustion forces to the threads (whether or not the studs ultimately get overstretched and fail). Combine that with just having a bigger combustion area to deal with, and I wouldn't doubt if Porsche made the switch intentionally to avoid 911-like pulled head stud issues.
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chris white
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Some good try’s…..but these are the main girdle studs not the head studs!!!
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michaelmount123
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Oops. My second grade reading teacher would be ashamed.chris white wrote: Mon May 29, 2023 12:26 pm Some good try’s…..but these are the main girdle studs not the head studs!!!
