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Engine rod length
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Simon DeCasas
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PostPosted: Fri Jun 13, 2025 1:52 pm    Post subject: Engine rod length Reply with quote

Building an engine 78.8mm X 90.5 want low to mid torque I have 2 rod choices 5.5" and stock 5.39 with a torque cam, what's the best rod length for longevity and power?
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modok
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PostPosted: Fri Jun 13, 2025 3:05 pm    Post subject: Re: Engine rod length Reply with quote

The difference is trivial.
Use 5.5 if you are using 34mm pin height pistons and forgot to deck the case 1mm to match.
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bedlamite
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PostPosted: Fri Jun 13, 2025 3:52 pm    Post subject: Re: Engine rod length Reply with quote

Use the ones that get your deck height closer to what you want.
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draksia
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PostPosted: Fri Jun 13, 2025 4:02 pm    Post subject: Re: Engine rod length Reply with quote

This post helped me out quite a bit

https://www.thesamba.com/vw/forum/viewtopic.php?p=9938600#9938600
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Rob Combs
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PostPosted: Sat Jun 14, 2025 6:33 am    Post subject: Re: Engine rod length Reply with quote

I’m building up a 78.8 crank/90.5 piston engine myself.

My case was decked .6mm/.024” IIRC. Scat crank. Scat 5.394” H-beams. AA “B” compression height pistons. I needed a set of .020” shims to put me at .0413 deck on the tight one and .0429 on the roomy one.

The shims miked out at about .018” after deburring the stamped edges.

The parts don’t work out to exactly the “theoretical” specs, but the theoretical value should get you close on what to order.
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nsracing
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PostPosted: Sun Jun 15, 2025 4:44 pm    Post subject: Re: Engine rod length Reply with quote

The longer one tends to stay in deeper and longer dwell time. Laughing
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jeffrey8164
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PostPosted: Tue Jun 17, 2025 12:39 pm    Post subject: Re: Engine rod length Reply with quote

nsracing wrote:
The longer one tends to stay in deeper and longer dwell time. Laughing

That is what she says. Rolling Eyes
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Zed999
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PostPosted: Tue Jun 17, 2025 1:55 pm    Post subject: Re: Engine rod length Reply with quote

I'm probably dreaming, but isn't the whole point of a 78.8 crank to work out with stock rods? Otherwise why the strange size? My 76 crank needed 5.5" rods to push the pistons 2.5mm further up the cylinders to get a good deck.
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Rob Combs
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PostPosted: Wed Jun 18, 2025 2:01 am    Post subject: Re: Engine rod length Reply with quote

Zed999 wrote:
I'm probably dreaming, but isn't the whole point of a 78.8 crank to work out with stock rods? Otherwise why the strange size? My 76 crank needed 5.5" rods to push the pistons 2.5mm further up the cylinders to get a good deck.


Correct.
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modok
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PostPosted: Wed Jun 18, 2025 9:14 am    Post subject: Re: Engine rod length Reply with quote

People make the assumption that all "b" pistons are the same.
THey aren't, they never were.
They were whatever pin height was ordered, or whatever it said in the piston catalog

But since people so universally assume they are, I vote we make all B pistons 35mm pin height moving forward.
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Rob Combs
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PostPosted: Wed Jun 18, 2025 2:42 pm    Post subject: Re: Engine rod length Reply with quote

modok wrote:
People make the assumption that all "b" pistons are the same.
THey aren't, they never were.
They were whatever pin height was ordered, or whatever it said in the piston catalog

But since people so universally assume they are, I vote we make all B pistons 35mm pin height moving forward.


Agree with you 100%. My above response was perhaps overly simplistic. The way I worked around this issue was to make a spreadsheet based on some of your earlier posts elsewhere on this site to get the nominal, expected dimensions for pistons, cylinders , block decks, etc, even if they weren’t perfectly reflective of the parts out there. Your expected dimensions posts are accurate enough to accommodate accurate parts ordering, and I for one thank you. It made my build thus far way easier. Paying proper attention to advertised pin heights is extremely important to the calculations.

For now, my spreadsheet works for me, but probably only me. It’s a mess, but it’s my mess and I understand my own madness, even when no one else does.

Maybe I can get the spreadsheet closer to ready for prime time/general public use, making it look & work something like CB’s engine calculator. If that would be helpful…
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Rob Combs
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PostPosted: Wed Jun 18, 2025 2:58 pm    Post subject: Re: Engine rod length Reply with quote

Adding to above, it’s not your posts that contain inaccuracies. It’s the natural variation in mass-manufactured parts.

For example, my AA cylinders measured out to .1mm longer than the “expected” or nominal 112.5mm length. This alone contributed to .004” additional deck height compared to the “projected” deck height in my spreadsheet.

Another example of why it’s so important to mock up and make adjustments accordingly, whether that means getting different shims, trimming off cylinders, or whatever.

Just wanted to make sure that came across clearly…

Thanks again!
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modok
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PostPosted: Wed Jun 18, 2025 3:08 pm    Post subject: Re: Engine rod length Reply with quote

My post was not aimed at anyone in particular.
Yeah, AA is pretty cheesy but usable, no change there.
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VWporscheGT3
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PostPosted: Fri Jun 20, 2025 2:08 pm    Post subject: Re: Engine rod length Reply with quote

modok wrote:
My post was not aimed at anyone in particular.
Yeah, AA is pretty cheesy but usable, no change there.


shameless plug here.. the ICON 90.5 B will be 35MM compression height.
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