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K1 4" Stroke Crank |
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RSZ 
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Joined: Aug/12/2021 Location: Florida Status: Offline Points: 11 |
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 Topic: K1 4" Stroke CrankPosted: Aug/17/2021 at 8:27pm |
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I just bought a 1970 AMX with a blown engine, it is the correct 390 for the car
Crank has some damage but is most likely repairable, dropped it off at the machine shop to see Three of the Rods are pretty much destroyed and most of the others have some battle damage Block appears to have survived, having it magnafluxed to make sure Since I'm needing rods I will most likely end up getting a set of Molnar rods The question I have is since I have to get new pistons, rods, and possibly a crank (or spend a lot on machine work on the one I have) would putting the K1 crank in be worthwhile? Mainly concerned with longevity of the motor after it is built. I assume that if you keep the RPMs reasonable (5500?) it should be as reliable as any other long stroke motor. Thanks
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mbwicz
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Joined: Feb/20/2019 Location: Buffalo, NY Status: Offline Points: 1991 |
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Posted: Aug/18/2021 at 5:08am |
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Take a look in the For Sale section, there is a recent post about shortages of aftermarket rods from Molnar. You may want to check with K1 for parts availability as well. The stroker crank may be in stock or many months away, and that could influence your choices.
Getting a decent used crank for your 390 won't be cheap either, you are probably wise for exploring the option of the stroker crank. You are doing the right thing having your existing setup checked out. What is the primary purpose of your build, what is the rest of your combo? The more info the better the responses will be.
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1970 AMX, one step forward, one step back. Both steps cost time and money.
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Boris Badanov
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Joined: Dec/14/2013 Location: NH USA Status: Offline Points: 4209 |
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Posted: Aug/18/2021 at 5:43am |
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I believe it's Chevy rods journals for the K1
Flip and re-chamfer the Chevrolet rods. They will be shorter than stock Chevy or AMC Early flywheel flange also. I do not remember what my 1970 390 had for a flange. Big increase in compression along with the increase in displacement. Custom pistons are needed. When in doubt stoke it :-) |
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Gremlin Dreams
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RSZ
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Joined: Aug/12/2021 Location: Florida Status: Offline Points: 11 |
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Posted: Aug/18/2021 at 6:37am |
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Car is originally a 390 4 speed car, it now has a turbo 400 with a converter and trans brake.
I am going to go back to a stick (ordered a TKX silver sport transmissions) and also ordered an Edelbrock pro flo 4 fuel injection, probably will go with Edelbrock heads too. I will order a flywheel and harmonic damper when I figure out which way I'm going with the crankshaft Not building it as a drag car but do want it to be a real strong street car that can be driven and won't be embarrassed by late model cars (would be nice to be in the 11's on drag radials but I would be happy with mid 12's) Most likely stick with 3.50 - 3.70 gears (may be able to get by a little less with the stroker crank) Won't be cutting car up for bigger tires, most likely the furthest I will go is Cal trac bars and springs.
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PROSTOCKTOM
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Posted: Aug/18/2021 at 8:06am |
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I seriously doubt your going to be getting a 4" stroke crank from K1 or any manufacturer within the next year. I am personally just not a fan of using a 4" crank do to the required extremely short compression height pistons (1.00") needed in any application other than drag race duty. This combination also has an extremely poor rod stroke ratio that I am not a fan of either. The last set of 1.00" compression height pistons that I acquired from Diamond and rings cost $1800. Getting anything for an AMC from Edelbrock will also probably be at least a year away now as well. Don't get me wrong I am a Molnar dealer and like to sell parts as well as the next guy, but if it were my engine I wouldn't do anything with the 390 except build it back so it can stay fairly correct for the car. If you really want power and reliability I think the best bang for the buck is to go buy a 360 and put a Procharger on it. Even 5-6 psi is just incredible what it does to power levels. If you really want a 4" crank just be prepared to spend $2500 for it and plan to wait up to a year to get it.
Tom
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Molnar Technologies Full Service Dealer - Crankshafts & Connecting Rods
1969 AMC Rambler Rouge Race Car 1974 AMC Hornet Hatchback, Wally Booth Outlaw Nostalgic Pro Stock Race Car Project |
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RSZ
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Posted: Aug/18/2021 at 8:22am |
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You're most likely right about the crankshaft, I've considered just putting together a 70 360 that I have (just put in a good set of pistons, use the cam/lifters that came out of the 390 and put the edelbrock heads and injection on it). That way I can take my time building the 390 without having the AMX just sitting around collecting dust.
Back to the 390: In your opinion would it be better just to find a 401 crank and have it offset ground and a set of Molnar rods to get ~420 CI out of it? (May be able to do the same with the 390 crank that I already have if it checks out alright)
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Trader
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Posted: Aug/18/2021 at 10:49am |
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Very likely the 390 crankshaft would be repairable.
Based on this premise, working you numbers from the piston to the crankshaft will likely turn out better for reliability and power. The best CH of a piston is 1.3" to 1.4". Keeps the oil ring above the pin, provides enough skirt to reduce rocking, provides enough meat at the top to pick your SCR and better rod/stroke ratio. Now, depending on your crankshaft, pick the rod length and stroke. Your machining the crankshaft anyhow and need custom pistons. Machine the stroke for everything else. Just a different approach. |
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79Jeep
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Posted: Aug/18/2021 at 2:23pm |
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I put an order in for a K1 crank in May. It got pushed out til next year. They make them in batches of I think 25 or so. I Think there are at least a dozen people ahead of me in line. You will obviously be behind me and I was told now it will be Jan2022 or later.
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Lenni AMC
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Joined: Jun/05/2011 Location: Iceland Status: Offline Points: 427 |
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Posted: Aug/18/2021 at 2:41pm |
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There are no big issues running a 4" stroke crank on the street.
Don't let people tell you it sucks that haven't done it them selfs. Custom stroker pistons cost excactly the same as any other none stroker custom pistons. Molnar has stroker rods available that are in stock as already has been stated in this thread. People who have never done a 4" stroke AMC street engine are maybe not the best people to ask if it works good or not. My advice to you is. Do what you want no matter if you want a 3.8, 3.9 or 4" stroke crank go for it, lots of people have and have been very happy with the resaults. Lots of people have the stock crank offset ground, so that might be a option for you as well |
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AMC Gremlin w/401
6.212 @ 117.01 - 1/8 Mile 9.785 @ 133.11 - 1/4 Mile https://www.youtube.com/channel/UCikyNqrfpMB8VzLd910Hkhg https://rbjracing1.wixsite.com/website |
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Boris Badanov
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Posted: Aug/18/2021 at 3:11pm |
Stroke it!
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Gremlin Dreams
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Lenni AMC wrote: