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bigworm0860
Samba Member
Joined: January 04, 2009
Posts: 33
Location: Midland Texas
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 Posted: Tue May 24, 2022 8:17 am Post subject: Longer Rods???? |
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Need some advise from the experts. I've read all six pages in the deck height section and I think I have a solution but would like some confirmation if possible.
My 2020 build:
New Case
76mm crank
92 TW cylinders with A pistons
5.325 I Beam rods (don't know why I bought shorter than stock rods)
42X35.5 heads cc'd at 57
C35 cam
My deck height is .228" which gives me about 6:3:1 CR (not the 8:5:1 I was hoping for)
As I understand it, I can either have my cylinders machined down or get longer rods. I don't want to jack with my cylinders so if my calculations are correct I can go with a 5.5" rod and would gain .175" and bring my deck height to .053" which would be in the sweet spot of .04-.06."
I researched going with a "B" piston but the difference in size won't make up the difference I need.
Another concern is will the crank need to be re-balanced if I go with a longer rod?
Am I on the right track here? |
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Dan Ruddock
Samba Member
Joined: October 25, 2012
Posts: 3594
Location: Sarasota, in my adopted state of Florida
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| Posted: Tue May 24, 2022 9:23 am Post subject: Re: Longer Rods???? |
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| bigworm0860 wrote: |
Need some advise from the experts. I've read all six pages in the deck height section and I think I have a solution but would like some confirmation if possible.
My 2020 build:
New Case
76mm crank
92 TW cylinders with A pistons
5.325 I Beam rods (don't know why I bought shorter than stock rods)
42X35.5 heads cc'd at 57
C35 cam
My deck height is .228" which gives me about 6:3:1 CR (not the 8:5:1 I was hoping for)
As I understand it, I can either have my cylinders machined down or get longer rods. I don't want to jack with my cylinders so if my calculations are correct I can go with a 5.5" rod and would gain .175" and bring my deck height to .053" which would be in the sweet spot of .04-.06."
I researched going with a "B" piston but the difference in size won't make up the difference I need.
Another concern is will the crank need to be re-balanced if I go with a longer rod?
Am I on the right track here? |
I find it hard to believe you have that much deck with A pistons. Something does not sound right. Most likely your not at tdc or you have B pistons. |
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RickS
Samba Member
Joined: May 05, 2005
Posts: 430
Location: Speonk, NY
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| Posted: Tue May 24, 2022 9:31 am Post subject: Re: Longer Rods???? |
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| Dan Ruddock wrote: |
| bigworm0860 wrote: |
Need some advise from the experts. I've read all six pages in the deck height section and I think I have a solution but would like some confirmation if possible.
My 2020 build:
New Case
76mm crank
92 TW cylinders with A pistons
5.325 I Beam rods (don't know why I bought shorter than stock rods)
42X35.5 heads cc'd at 57
C35 cam
My deck height is .228" which gives me about 6:3:1 CR (not the 8:5:1 I was hoping for)
As I understand it, I can either have my cylinders machined down or get longer rods. I don't want to jack with my cylinders so if my calculations are correct I can go with a 5.5" rod and would gain .175" and bring my deck height to .053" which would be in the sweet spot of .04-.06."
I researched going with a "B" piston but the difference in size won't make up the difference I need.
Another concern is will the crank need to be re-balanced if I go with a longer rod?
Am I on the right track here? |
I find it hard to believe you have that much deck with A pistons. Something does not sound right. Most likely your not at tdc or you have B pistons. |
Unless he has a raised deck case.
What case are you using? |
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Pruneman99
Samba Member
Joined: February 22, 2012
Posts: 5013
Location: Oceanside
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| Posted: Tue May 24, 2022 11:05 am Post subject: Re: Longer Rods???? |
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| If you do have B pistons, 5.5" rods and 76mm crank should put you very close to where you want to be. |
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bigworm0860
Samba Member
Joined: January 04, 2009
Posts: 33
Location: Midland Texas
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| Posted: Tue May 24, 2022 12:29 pm Post subject: Re: Longer Rods???? |
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I think I figure out where I screwed up. I think I ordered the wrong pistons and cylinders.
What I got were 92mm pistons to fit 94mm heads for a 78mm and larger crank. What I needed was 92mm pistons to fit 94mm heads for up to a 76mm crank.
Would that make the difference in my wacked out deck height? |
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modok
Samba Member
Joined: October 30, 2009
Posts: 26785
Location: Colorado Springs
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| Posted: Tue May 24, 2022 1:03 pm Post subject: Re: Longer Rods???? |
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| Yes, that's it. |
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bigworm0860
Samba Member
Joined: January 04, 2009
Posts: 33
Location: Midland Texas
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| Posted: Tue May 24, 2022 1:27 pm Post subject: Re: Longer Rods???? |
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| Thanks for the input gentlemen. Much appreciated! |
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Fender38
Samba Member
Joined: April 11, 2015
Posts: 75
Location: Las Vegas, NV
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| Posted: Tue May 24, 2022 2:01 pm Post subject: Re: Longer Rods???? |
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| bigworm0860 wrote: |
I think I figure out where I screwed up. I think I ordered the wrong pistons and cylinders.
What I got were 92mm pistons to fit 94mm heads for a 78mm and larger crank. What I needed was 92mm pistons to fit 94mm heads for up to a 76mm crank.
Would that make the difference in my wacked out deck height? |
It may have been by mistake, but actually you ordered the correct P&C's, and the wrong connecting rods. Get some 5.5" rods and you'll be better off. 76mm crank, "A" pistons and 5.325" rods will work. But 76mm crank, "B" pistons and 5.5" rods are a better option. Should go together nearly perfect. |
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jpaull
Samba Member
Joined: February 22, 2005
Posts: 3462
Location: Paradise, Ca
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| Posted: Tue May 24, 2022 10:02 pm Post subject: Re: Longer Rods???? |
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Ordering those pistons "wrong" was a blessing in disguise in my book also. Ditto what Fender said.
_________________
[email protected] MPH 1/4 Mile & 8.1 @ 83.7MPH in 1/8 Mile with Mild Type 1 VW Mag Case 2234cc commuter engine in stock weight bug w/only .491 total lift(CB2292 Cam), 42x37 heads, 48idf's, Street tires, Belt on, Mufflers, Pump gas, video of the run here: https://youtu.be/M3SPqMOKAOg
Transmission by MCMScott:
Rhino case, Klinkenberg 4.12, Superdiff, 002 mainshaft with 091 first idler. Weddle 1.48 Third & 1.14 Fourth. |
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bigworm0860
Samba Member
Joined: January 04, 2009
Posts: 33
Location: Midland Texas
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| Posted: Wed May 25, 2022 6:33 am Post subject: Re: Longer Rods???? |
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| What is the difference between "A" and "B" pistons? And how do I know which ones I have? I bought them from LJ Aircooled and the description on the website nor the label on the box specifies A or B. |
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bigworm0860
Samba Member
Joined: January 04, 2009
Posts: 33
Location: Midland Texas
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| Posted: Wed May 25, 2022 6:39 am Post subject: Re: Longer Rods???? |
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| Also, will the crank be effected in any way going from 5.325" rods to 5.5" as far as balancing goes? It's a brank new crank from CB Performance. |
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jimmyhoffa
Samba Member
Joined: February 16, 2014
Posts: 1057
Location: St. Louis
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| Posted: Wed May 25, 2022 6:56 am Post subject: Re: Longer Rods???? |
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If the rods are weight matched and balanced end-to-end you're good, but be forewarned I just balanced a new CB 78.4 crank and it was 7 grams out on the gear end, which is pretty severe. Gotta check 'em all!
I had two untouched stock cranks and the CB balanced at the same time about a month ago, and the CB was the worst of that lot.
_________________
1974 Chenowth 2RL #1244 Street Legal
My other car isn't ridiculous. |
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Fender38
Samba Member
Joined: April 11, 2015
Posts: 75
Location: Las Vegas, NV
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| Posted: Wed May 25, 2022 7:48 am Post subject: Re: Longer Rods???? |
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| bigworm0860 wrote: |
| What is the difference between "A" and "B" pistons? And how do I know which ones I have? I bought them from LJ Aircooled and the description on the website nor the label on the box specifies A or B. |
The difference has to do with the pin height. The skirts are likely different too, but the A/B designation is related to the pin height. Based on the website description you provided, the pistons for "up to 76mm crank" would be "A" pistons, and the for " 78mm and above crank" would be "B" pistons.
You stated above you bought the B's. You"ll want the B's, with 5.5" rods, which are longer than stock. |
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bigworm0860
Samba Member
Joined: January 04, 2009
Posts: 33
Location: Midland Texas
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| Posted: Wed May 25, 2022 11:29 am Post subject: Re: Longer Rods???? |
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| I'm tracking now. Thanks to all who responded. Much appreciated!!! |
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bigworm0860
Samba Member
Joined: January 04, 2009
Posts: 33
Location: Midland Texas
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| Posted: Wed May 25, 2022 11:37 am Post subject: Re: Longer Rods???? |
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| Do I stay with I-Beam rods with "B" Pistons? |
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Dauz
Samba Member
Joined: January 05, 2010
Posts: 1790
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| Posted: Wed May 25, 2022 12:10 pm Post subject: Re: Longer Rods???? |
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| bigworm0860 wrote: |
| Do I stay with I-Beam rods with "B" Pistons? |
Doesn't matter |
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Pruneman99
Samba Member
Joined: February 22, 2012
Posts: 5013
Location: Oceanside
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| Posted: Wed May 25, 2022 12:24 pm Post subject: Re: Longer Rods???? |
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| Just chose something with a radiused end cap. It will save a bunch of clearance work Vs. the normal style. |
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txoval
Samba Member
Joined: January 23, 2004
Posts: 3552
Location: The Woodlands, TX
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| Posted: Wed May 25, 2022 1:56 pm Post subject: Re: Longer Rods???? |
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| Get a set of CB uni-tech rods…5.5” |
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nsracing
Samba Member
Joined: November 16, 2003
Posts: 9462
Location: NOVA
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| Posted: Thu May 26, 2022 5:55 am Post subject: Re: Longer Rods???? |
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Longer rods? Ask your GF what she prefers - 
The shorter rods will move faster -
You can mix and match pistons rods - all depends how much work you want to do. But the longer rods and high-pin B-pistons will be best.
YOu can trim the cylinders now and not even buy another set of rods. Or buy longer ones and re-do the deck. But you will still have to fit them for best results.
Just some machine work is all. |
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