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Simon DeCasas
Samba Member
Joined: March 24, 2004
Posts: 61
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 Posted: Fri Jun 13, 2025 1:52 pm Post subject: Engine rod length |
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| 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
Samba Member
Joined: October 30, 2009
Posts: 27645
Location: Colorado Springs
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| Posted: Fri Jun 13, 2025 3:05 pm Post subject: Re: Engine rod length |
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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
Samba Member
Joined: September 30, 2004
Posts: 234
Location: WI
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| Posted: Fri Jun 13, 2025 3:52 pm Post subject: Re: Engine rod length |
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| Use the ones that get your deck height closer to what you want. |
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draksia
Samba Member
Joined: July 07, 2022
Posts: 21
Location: FL
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Rob Combs
Samba Member
Joined: December 30, 2020
Posts: 471
Location: South Bay LA, California
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| Posted: Sat Jun 14, 2025 6:33 am Post subject: Re: Engine rod length |
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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
Samba Member
Joined: November 16, 2003
Posts: 9737
Location: NOVA
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| Posted: Sun Jun 15, 2025 4:44 pm Post subject: Re: Engine rod length |
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The longer one tends to stay in deeper and longer dwell time.  |
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jeffrey8164
Samba Member
Joined: January 06, 2018
Posts: 4140
Location: Georgia
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| Posted: Tue Jun 17, 2025 12:39 pm Post subject: Re: Engine rod length |
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| nsracing wrote: |
| The longer one tends to stay in deeper and longer dwell time. |
That is what she says. 
_________________
Volkswagen!
Turning owners into mechanics since 1938.
“Let he that is without oil throw the first rod”
(Compression 8.7:1) |
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Zed999
Samba Member
Joined: March 04, 2018
Posts: 1386
Location: UK
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| Posted: Tue Jun 17, 2025 1:55 pm Post subject: Re: Engine rod length |
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| 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
Samba Member
Joined: December 30, 2020
Posts: 471
Location: South Bay LA, California
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| Posted: Wed Jun 18, 2025 2:01 am Post subject: Re: Engine rod length |
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| 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
Samba Member
Joined: October 30, 2009
Posts: 27645
Location: Colorado Springs
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| Posted: Wed Jun 18, 2025 9:14 am Post subject: Re: Engine rod length |
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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
Samba Member
Joined: December 30, 2020
Posts: 471
Location: South Bay LA, California
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| Posted: Wed Jun 18, 2025 2:42 pm Post subject: Re: Engine rod length |
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| 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
Samba Member
Joined: December 30, 2020
Posts: 471
Location: South Bay LA, California
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| Posted: Wed Jun 18, 2025 2:58 pm Post subject: Re: Engine rod length |
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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
Samba Member
Joined: October 30, 2009
Posts: 27645
Location: Colorado Springs
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| Posted: Wed Jun 18, 2025 3:08 pm Post subject: Re: Engine rod length |
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My post was not aimed at anyone in particular.
Yeah, AA is pretty cheesy but usable, no change there. |
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VWporscheGT3
Samba Member
Joined: September 01, 2006
Posts: 2170
Location: Gardnerville, NV
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| Posted: Fri Jun 20, 2025 2:08 pm Post subject: Re: Engine rod length |
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| 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.
_________________
If you have any questions about Forged ICON 4032 VW pistons just shoot me a line. |
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