My 340/420 Stroker Build

20 months ago, I lost oil pressure in my 340 (70 Challenger numbers matching) and found that the oil pump internal components had many scars, the oil pump screen was full of crud, the thrust surface on the crank was ate up, and my main bearings were toast. (Please keep in mind that the bottom end has been together for at least the last 24-25 years and 4 cam changes.)

So, out came the engine and I stripped the block down and sent it to the machine shop as a bare block/main caps. At the time of drop-off, I was warned that there was at least a 12-14 month backlog. This shop I have used in the past and due to him losing some people during COVID, and another local performance machine shop going out of business during the same time frame, I knew I was in for a wait.

Now, about 19 months later, I received my new 340/420 rotating assembly yesterday. I spent last night cleaning the block and prepping it for paint. Next week I will start reassembling with as many parts from the 340 assembly that makes sense for the new block size.

This block was rebuilt to achieve a 10.7/10.8:1 comp. [pending head gasket selection] with my existing Promaxx/Sidewinder alum heads (63cc & ported to 280cfm], with the K1 4.00” forged crank and rods, +.050” JE Pistons, & ARP main cap studs.

I have two gasket matched intakes (Weiand single & a RPM dual), painted and ready to install and I am currently leaning towards the Weiand single plane. As for the carb, I am re-using my Quick Fuel SS-750-AN-DP and tuning with two FAST analog A/F meters.

I will be reusing my TTi 1-5/8” step headers, 2.5” x-pipe, and 2.5” Hooker Max Flows, then 2.5” exhaust over the rearend to a set of Classic Chambered 2.5” bullet style mufflers, then out through the 2.5” RT exhaust tips.

Since this is the numbers matching block, I don’t want to spin it up past 62-6400rpms. Plus, I don’t think my 280cfm heads and my two intake choices will allow the rpms to go up past that range on 420ci, as a guess.

Since I blew most of my budget on the rebuilt short block yesterday, I am now looking into a solid flat tappet cam. I am aiming for 515-525’ish range on the torque and hp if possible. I do not have AC or power brakes, and I never will. As long as I can tune this new combo to have a decent idle quality at a stop light (in D) with my foot on the brake, I will be happy.

The 727 has a 9.5” Dynamic converter. It flashed to 3800 with the 340 and a 226/238 hydraulic flat tappet cam. I did not get a chance to check the flash with my last 234/238 solid flat tappet cam, before I lost oil pressure, which caused the block re-build. I am assuming that the flash will move up a few hundred rpms with the 420. I am running 3:91 gears and Nitto 275’s on the rear.

I will have many questions to come, but for now, I’m just excited to have it back in my garage!