Tore down my 401- some questions...

Conflicted4WD

Well, this is my first post so I'll say hi to all. I live in Colorado Springs (Ft. Carson) and am in the process of building a 1975 Jeep CJ-6. I was all planned out to do a Chevy swap until I started reading this forum and some of the posts over on Pirate 4x4. Went to the junkyard and poked around a bit and on the first trip came across a 401/TH400 in a 74 Jeep J10 (Camper Special). I tore the motor down this morning and the crank and rods look good- very good- but I'm worried about the cylinder bores. The motor appears to have been sitting for a while and the rings were rusted to the bores pretty well. I was C-A-R-E-F-U-L getting them out, soaked in penetrating oil, super-gentle removal, etc., so the damage to the bores is minimal. What has me worried is the ".030" on the piston crowns. I remember reading a while back about issues with overbored blocks but can't find the thread/article where I saw it. There's no ridge at all and I don't have a dial bore gauge anymore so I haven't measured taper, but I doubt the rebuild was all that long ago. .010" would probably take care of anything a hone can't. I was hoping (still am) that things'll clean up with a good hone job, but if they don't, how much can you safely overbore a 401? I'm sure this is just the first of many questions I'll have- built more than a couple motors in the past but this is my first AMC. Lot of good ideas in the design, but different from what I'm used to. Thanks, all- Adam EDIT: P.S.- anybody know a good machine shop in the Colorado Springs/Denver area?

AMX69PHATTY

Best way to know is to have the cylinder walls sonic checked for thickness. Are the pistons cast or forged ? Forged pistons use .006 or so piston to bore clearance on dia. whereas cast pistons use .001-.002 clearance on diameter. So if those pistons are cast, may be able to fit forged pistons with only a hone.

Conflicted4WD

AMX69PHATTY wrote
Best way to know is to have the cylinder walls sonic checked for thickness.
Are the pistons cast or forged ?
Forged pistons use .006 or so piston to bore clearance on dia.
whereas cast pistons use .001-.002 clearance on diameter.
So if those pistons are cast, may be able to fit forged pistons with only a hone.

Well, the pistons appear to be stock replacement castings. I cleaned enough carbon off of one crown to find the overbore marking- have to investigate a little more before I can be certain. I was figuring on needing to sonic-check the block before doing any machine work. Just wanted to check if there was any hard rules or experience with overbores past .030".

Thanks,
Adam

1980_Cj7

We've been running one .040 over, highway and off road, for about a year now, and no troubles.

dwg86

Mine is also .040 over 10.5:1 compression, iron heads, aluminum radiator, electric fan, 180 thermostat. Never got over 180 degrees.

Conflicted4WD

Thanks, y'all. That's the kind of first-hand info I needed. The parts are going out to get hot tanked (or whatever EPA-approved process there is these days- I think you get fined for even saying the words "hot tank" back in California :mrgreen: ), Magnafluxed, and the block sonic-checked this week. Hopefully I'll have some good news.