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Mark D. 
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Joined: 14 Sep 2009
Location: Ubly, MI
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 Topic: decube a 426, how?
Posted: 17 Feb 2022 at 8:21am |
our club has changed the rules for fair play, and are allowing us to replace our 301 in our XT with a decubed 426 . max ci. is 410. what would be the best way to go ? 4.75 stroke? any ideas would be appreciated. the IH and JD boys are just to hard to compete with .we've been running twin chargers on max 3500 rpm limit per class rules. dry head and block, and just keep cracken up heads 3- 4 hooks and so many cracks we can hardly start it. 3 different heads done by reputable builders with AC exsperiance. sorry I can't spell worth a hoot
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NEVER green
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Joined: 28 Feb 2013
Location: MN.
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Posted: 17 Feb 2022 at 8:45am |
I would go with shorter stroke, smaller bore would shroud valves restricting air, the valves on a 426 are already widely spaced. Keep same rod , adjust pin height or shave top of piston. What do you fill you heads with??? I havent tried it yet but next time I will find a foundry and will fill the water jacket with pored cast iron, should strengthen it and will expand at the same rate preventing cracks.
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2-8050 1-7080 6080 D-19 modelE & A 7040 R50
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DrAllis
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Posted: 17 Feb 2022 at 9:38am |
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Press a thin dry sleeve inside the wet sleeve and get bore down to 4 1/8" (I think that works). Make custom pistons. Work over the head/valves . You know what?? That semi-myth of valve shrouding I just don't always buy as fact. EVERY 301 I've ever been involved with that had seats knocked out of the head and 426 valves installed and notches in the sleeves so the valves would clear ALWAYS ran better. In fact, usually shifted up a gear !! So, I'm sorry I don't think shrouded valves are an automatic negative, especially when you've got 80 or more psi of boost.
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LeonR2013
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Posted: 17 Feb 2022 at 5:06pm |
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I don.t think[ that teduceing the stroke is the way to go. You need all the torque you can get, so shortening the stroke is counter productive it seems to me, Leon
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DrAllis
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Posted: 17 Feb 2022 at 6:32pm |
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On the smaller bore, 4.160" will get you 408 cubes with a stock 5 inch stroke, but be sure you can get rings to fit that diameter.
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NEVER green
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Posted: 18 Feb 2022 at 8:05am |
 DrAllis wrote:
Press a thin dry sleeve inside the wet sleeve and get bore down to 4 1/8" (I think that works). Make custom pistons. Work over the head/valves . You know what?? That semi-myth of valve shrouding I just don't always buy as fact. EVERY 301 I've ever been involved with that had seats knocked out of the head and 426 valves installed and notches in the sleeves so the valves would clear ALWAYS ran better. In fact, usually shifted up a gear !! So, I'm sorry I don't think shrouded valves are an automatic negative, especially when you've got 80 or more psi of boost. |
Actually, flow benches prove that valve shroud exists.
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2-8050 1-7080 6080 D-19 modelE & A 7040 R50
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NEVER green
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Posted: 18 Feb 2022 at 8:07am |
LeonR2013 wrote:
I don.t think[ that teduceing the stroke is the way to go. You need all the torque you can get, so shortening the stroke is counter productive it seems to me, Leon |
You are not going to get the torque by reducing the air either.
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2-8050 1-7080 6080 D-19 modelE & A 7040 R50
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DrAllis
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Posted: 18 Feb 2022 at 9:44am |
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How "deep" of sleeve/valve notch makes a difference too. A Dyno run in the end settles the dispute. When you get tired of exhaust seats falling out of the cylinder head, the old school way was to remove the seats and go to a larger diameter valve, notching the sleeves as needed. It worked. Still works.
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bsallis180
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Posted: 18 Feb 2022 at 11:19am |
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Rod end is 4.188 wide on big pin rods
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DrAllis
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Posted: 18 Feb 2022 at 1:44pm |
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I'm sure the big end of the rods would have to be shaved down to fit inside the sleeve, but I feel this is the best way to decube. Work over the head and move the valves closer together a little bit if shrouding is an issue. The 426 head is faaaar better than any 301 head and I don't need a air flow machine to tell me that.
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bsallis180
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Posted: 18 Feb 2022 at 2:41pm |
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If you press a dry sleeves in your sleeve And notch your sleeves you will Be blowing compression between your sleeves. Less you get custom sleeves then weaken your rods at the bolt If you do all that and re work your head that’s not cheap to get your valves closer. with all that just de stroke your crank get pistons and be done.
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DrAllis
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Posted: 18 Feb 2022 at 2:57pm |
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Parent bore blocks have been repaired for decades pressing in a thin sleeve. I agree the notching would possibly cause issues, but again I believe that could be worked around. Many years ago, I was going to do this. I was going to use a STOCK piston and turn it down to the proper diameter and hopefully find rings that would work. It wasn't going to cost very much with old sleeves/old pistons/stock crank/etc. Todays custom built parts are much more readily available, when then they were not. 408 cubes. Either debore or destroke, Your decision.
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injpumpEd
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Posted: 18 Feb 2022 at 5:35pm |
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destroke to 4.8" is 408.57 cid, 4.75 is 404. Either way I'd have custom sleeves and pistons made, so you can get exactly what you need. I know there's less expensive places, but I do like my riverside parts. It's not a low budget deal, but very high quality parts, and if need replacements they can supply within a couple of days! I've had 2 stock pistons fail, causing a lot of damage. Now, they were original 210 pistons cut down as well as running too much timing. But when a cast piston fails, it simply explodes, a forged piston may begin to crack but will get discovered on a freshen up.
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NEVER green
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Posted: 19 Feb 2022 at 7:38am |
DrAllis wrote:
How "deep" of sleeve/valve notch makes a difference too. A Dyno run in the end settles the dispute. When you get tired of exhaust seats falling out of the cylinder head, the old school way was to remove the seats and go to a larger diameter valve, notching the sleeves as needed. It worked. Still works. |
Yes, a dyno also proves that valve shroud exists, its no myth.
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2-8050 1-7080 6080 D-19 modelE & A 7040 R50
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Relayman
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Posted: 24 Feb 2022 at 8:09pm |
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If you are supposedly limited to 410 cubes, why not just make it a 414 cube using the 1% tolerance for "wear" like the IH people do.
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