390 pistons in a 401

JERSEYJOE

Considering the additional stroke and the fact that the 390 has a taller compression height, what is the result of putting 390 spec pistons in a 401? Is it instant BOOM or ???? I am guessing that the pistons will protrude above the deck resultingin an extremely high CR.

jeepsr4ever

The 1970 390 used the same block deck height as a 401, same rod but had a different compression height for the piston.... .052" more for the piston on the 1970 390. You would end up over 16-1 compression :?:

JERSEYJOE

I just got in a 401 that I took apart and it has 390 spec pistons. The tops of th epistons were distroyed due to detonation. They are cast Badger pistons in 390 spec. Piston is not dished like a 401 piston just has valve reliefs

jeepsr4ever

Whew Joe!

JERSEYJOE

I think that they are the old L2380F sealed power units. The tops of the pistons are shattered around the ring lands, the rest of the pistons look like someone took a ball peen and beat the sh*t out of the domes. I'll take some pictures. The other 401 had standard deep dish 8.5 to 1 pistons that some brain surgeon put not only the wrong diameter rings, but they installed # 2 ring in the top groove, and the top ring in the #2 groove. The first thing that I do when I pick up a motor is do a leak down . This motor was reported to be " rebuilt ." As we all know REBUILT means different tings to different people. I was puzzled to the reakdown results of 30+% with good valves. the motor looked ok. The bores were fine at .030 but still the leakdown numbers puzzled me until I checked the ring gaps. Aside from the second rings being in the top groove, the end gaps were over .060. Aside from this all it needs it a clean-up, finish hone and the correct rings.

jeepsr4ever

Sounds like you may have gotten a couple of good builders! Detonation is a real crankshaft twister as well Joe...if you revving check to see if she is strait. I would love to see some pics of that piston :idea: :mrgreen:

JERSEYJOE

I will take some tonight but I don't know how to post them here. The both cranks are at my crank guy. Both need only micro polishing. One is .010 rod and main, the other is .020 rod and main

Mudrat

I was looking for the 'How to post Pics' thread but can't seem to find it. 111!!!

But you can get a free www.Photobucket.com account and post them there. Then copy either the [img] tag (small pics3x4 inches or so) or the [url] tag (large pics Hi-res kinda things) and paste them into a post here on the board. :wink: