406 with 6 inch rods
Thread Starter
Melting Slicks
Joined: Jun 2002
Posts: 2,844
Likes: 99
From: Orrtanna Pa.
406 with 6 inch rods
I have a 406 now with a SCAT crank and 5.7 inch rods. Still runs great, but I have been toying with trying an internal balanced setup. The current assembly is externally balanced, and as far as I can feel runs smooth well up into the 7000 rpms range. In looking at internal balanced sets with 6 inch rods I see the oil ring pack and wrist pin intersect. Would this be a huge deal on a street engine? But the big question is cam clearance. For now, I'd like to keep my existing solid cam, and need to know if there will be clearance issues with the 6 inch rods. The set I have in mind has the stroker clearance rods,with cap bolts so I think I should be OK. Just like to hear from anyone that's done this before.
Also on the balancer.....I know it needs to be neutral balance, but what diameter would you recommend. Pretty sure mine is an 8 inch, but there are lots of options....( SFI rated )
If money were no object,.....I'd be doing a 427 next, but I'm getting ready to retire and need to keep an eye on the prize........
Also on the balancer.....I know it needs to be neutral balance, but what diameter would you recommend. Pretty sure mine is an 8 inch, but there are lots of options....( SFI rated )
If money were no object,.....I'd be doing a 427 next, but I'm getting ready to retire and need to keep an eye on the prize........
Burning Brakes
Joined: Nov 2011
Posts: 1,170
Likes: 201
From: Willoughby Ohio
With the better brand crankshafts there should not be a clearance issue between cam and crank. The 6 inch rods and shorter pistons handle rpm better than the 5.7 rods. Call the crankshaft company of choice to verify any potential clearance concerns. I know Callies has many offerings where it will not be an issue.
Pro
Joined: Jul 2021
Posts: 719
Likes: 156
3.75" stroke & 6" rods ... most pistons w/ thinner metric ringpak put the pin below oil grooves
e.g. see mahle motorsport catalog
https://www.us.mahle.com/media/usa/m...-file-size.pdf
** some manufacturers' cranks specify 6" rods (or longer). Those have larger diameter counterweights; making it easier to balance (less mallory metal) and without external balanced damper/flywheel.
e.g. see mahle motorsport catalog
https://www.us.mahle.com/media/usa/m...-file-size.pdf
** some manufacturers' cranks specify 6" rods (or longer). Those have larger diameter counterweights; making it easier to balance (less mallory metal) and without external balanced damper/flywheel.
Last edited by ebbnflow; Sep 3, 2021 at 05:54 PM.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
It's the big end of the rods that cause cam clearance issues. big rod end ARP bolts cause pan rail clearance issues. Most common sfi dampers are 6.250 or 6.125.
Years ago I bought what was called a 391 sbc with a 3.800 stroke. They were offset ground internally balanced 400 cranks. Just an all around bad deal I think that it had 5.565 rods with heavy pistons. because of the low rod length ratio my block was junk when it came to rebuilding day. the cylinders were worn out of round and couldn't be cleaned up with the next at max over bore of .060 total.
I don't know enough about 400/406 as to whether they have block ware problems.
Years ago I bought what was called a 391 sbc with a 3.800 stroke. They were offset ground internally balanced 400 cranks. Just an all around bad deal I think that it had 5.565 rods with heavy pistons. because of the low rod length ratio my block was junk when it came to rebuilding day. the cylinders were worn out of round and couldn't be cleaned up with the next at max over bore of .060 total.
I don't know enough about 400/406 as to whether they have block ware problems.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
I hope that you were buying a whole rotating kit. It's cost prohibitive to have to add heavy Mallory metal to your existing crankshaft in an attempt to balance it.
Good time to buy the 3.875 crankshaft with 6 inch rods and lightweight pistons
Good time to buy the 3.875 crankshaft with 6 inch rods and lightweight pistons
Le Mans Master
Joined: Mar 2004
Posts: 7,502
Likes: 1,510
From: Little Rock AR
I got a good deal on a set of Mahle 383 pistons for 5.7" rods and it was a no-go for internal balancing. Made no sense to do internal so we got a external flywheel and an external SFI degreed balancer. Much better to use the 6" rods and pistons to match so you can internal balance the assembly whenever possible. SCAT has some affordable stroker clearanced rods that are pretty hard to beat.
On the damper, just get a nice degreed one. I would not worry about the diameter. I also don't like having to field machine the hub bore to size so I avoid any machine-to-fit dampers. Just not needed for what I build.
On the damper, just get a nice degreed one. I would not worry about the diameter. I also don't like having to field machine the hub bore to size so I avoid any machine-to-fit dampers. Just not needed for what I build.
Thread Starter
Melting Slicks
Joined: Jun 2002
Posts: 2,844
Likes: 99
From: Orrtanna Pa.
The heads will be the AFR 210 set I baught about five years ago...still in the box sitting on my bench
Cam is the same Isky Z35 I have run for the last decade. Love the 7000 rpm top end.
Last edited by The Money Pit; Sep 3, 2021 at 09:35 PM.
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Verdad Gallardo
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
The kit I have in mind is a SCAT internally balanced kit..fully balanced with 6 inch rods and light forged pistons. Vender won't promise cam will clear, but said it should be fine. A 415.....hmmm
The heads will be the AFR 210 set I bought about five years ago...still in the box sitting on my bench
Cam is the same Isky Z35 I have run for the last decade. Love the 7000 rpm top end.
The heads will be the AFR 210 set I bought about five years ago...still in the box sitting on my bench
Cam is the same Isky Z35 I have run for the last decade. Love the 7000 rpm top end.
The AFR 210 heads are really a great flowing head. I don't know anything about Isky Z35
Thread Starter
Melting Slicks
Joined: Jun 2002
Posts: 2,844
Likes: 99
From: Orrtanna Pa.
Last edited by The Money Pit; Sep 4, 2021 at 07:47 AM.
Advanced
Joined: May 2018
Posts: 75
Likes: 12
I have a 406 now with a SCAT crank and 5.7 inch rods. Still runs great, but I have been toying with trying an internal balanced setup. The current assembly is externally balanced, and as far as I can feel runs smooth well up into the 7000 rpms range. In looking at internal balanced sets with 6 inch rods I see the oil ring pack and wrist pin intersect. Would this be a huge deal on a street engine? But the big question is cam clearance. For now, I'd like to keep my existing solid cam, and need to know if there will be clearance issues with the 6 inch rods. The set I have in mind has the stroker clearance rods,with cap bolts so I think I should be OK. Just like to hear from anyone that's done this before.
Also on the balancer.....I know it needs to be neutral balance, but what diameter would you recommend. Pretty sure mine is an 8 inch, but there are lots of options....( SFI rated )
If money were no object,.....I'd be doing a 427 next, but I'm getting ready to retire and need to keep an eye on the prize........
Also on the balancer.....I know it needs to be neutral balance, but what diameter would you recommend. Pretty sure mine is an 8 inch, but there are lots of options....( SFI rated )
If money were no object,.....I'd be doing a 427 next, but I'm getting ready to retire and need to keep an eye on the prize........
The kit I got balanced and clearance. So that was extra. Upgraded to sfi etc and steel crank.
It was like 1700 bucks if I recall.. but they went up..
6" forged scat rods Weisco flat tops were .010 in hole on a vigin deck so if you shaved 025 off deck you may have to be careful.. I was planning 10.5/1 and went with cometic .027 gaskets and 71cc flotek 220s.. (flow on super flow bench 315 cfm qt .600 lift) cam is custom Schneider. 584 but 1.6 makes it like 622 lift.
The cam is a small base.. but to answer your q. No problem with clearance.. but was tight. The skip white guys clearance the rids for me.
So now I'm 10.9/1
I got the motor installed in April in the c3 and now pushing it more. Love it. 390 gears
I set the 6al to 6800 and hit that once oand was still pulling hard AF.
I'm using Howard's max effort lifters that supposedly have anti pump up lifters good to 7k. But I am conservative on that and only would push it if I had to.
Love the 6" scat rods.
Last edited by SeanGalt; Sep 4, 2021 at 09:59 AM.