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Old 03-06-2003, 07:16 PM
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Serious Johnson Serious Johnson is offline
AMC 4 OH! 1
Join Date: May 19, 2002
Location: Pumpkintown, South Carolina
Posts: 3,831


If I had a line on a decent motor and my current one still got me around, I think I'd try to nurse it until I could be sure about the replacement. Like Andy said, it's of course best to do as much diagnosis as possible before the new motor leaves it's home, but I suspect that it's already out if it's for sale.

My situation wouldn't lend itself to bench running the motor on a stand, so I'd invest in a gasket set and pull the pan & heads for a look-see. Actually, I'm so poor that I'd just stick the thing in there and hope for the best, but presumably most folk have a few more bucks than I.

Check rod side clearances and crank endplay against specs, and look for uneven wear in the cylinder bores. Pull a main and rod bearing cap(someone please jump in here with which ones on an AMC wear first -- I'd guess #4 main & #8 rod) and Plastiguage them for a rough idea of the general health of the bottom end.

Uneven cylinder bore wear (crosshatch gone in places) is a good indication that you'll need at least new pistons, if not an overbore. A piston top with carbon on the center, but clean at the perimeter is an indication of poor ring seal.

A quick-easy on the valves is to fill the ports with gasoline and watch for leakage into the combustion chamber. Before doing that, look for dark, damp deposits on the back of the valves. This can indicate poor oil control from cheap stem seals or more expensive valve guides.

Bottom line, if the cylinder walls seem good, then it's easy to renew the bottom-end bearings if needed, and not terribly expensive to have a valve job done. If it seems that the motor needs pistons, then your budget and timetable will determine whether to slap it back together with the new gaskets and hope for better from the original motor, or to go on with a full build.

As I thought I'd finished typing this hyperbole, I recalled that you said the replacement motor purportedly just passed smog. I'd think that a fair indication that the cylinders are building decent pressure. Here's my new analysis: Stick the other motor in and forget about it.

"Carpe MaƱana".

'83 Wagoneer
360, .030-over, K8600 cam, Crane springs, ported heads, Edelbrock Performer, G.M. TBI, TFI, 3" exhaust,
T-18a/208, D44/AMC 20 w/ limited slip in both, 3.73s, 33s, BDS 4" springs, Rancho 9000 shocks, etc., etc.
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