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'68 390 Mods

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Blue68SST View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Blue68SST Quote  Post ReplyReply Direct Link To This Post Topic: '68 390 Mods
    Posted: Dec/02/2017 at 9:28am
I am looking for some help in understanding the thinking that went into the building of the 390 in the AMX I bought 2 years ago. The car is an X code 68 AMX that I bought from the widow of the previous owner, so I have very little on the background of the car, other than it was imported to Canada from the US in 1999. He had an extensive collection of American muscle cars, but his was his only AMC.
It runs very well, except for an oil consumption issue that I think is a bad valve seal, so I am not about to mess with it.
The engine appears stock except for an Offenhauser 360 dual quad intake with twin Carter AFB's. The block has the 390 cast on the sides, with a casting # of 3190806, which I believe makes it a 68/69 390 block. There was no valve cover tag on it. It has "log" style manifolds on it, which led me to believe it was a completely stock engine, except for the intake. Upon swapping out the valve coves, I found that the casting number on the heads was 3233344, which upon looking up, seems to be 1979 360 heads. The rockers are bridged, so this appears to be the case.
My question is what are the performance implications for putting 1979 360 heads on a 68/69 390 block? Would this not lower the compression ratio?
Also, this would mean that the heads are "dog leg", which means the exhaust manifolds are either from a 1970, or are the original 68/69 square port manifolds bolted to dog leg heads. Is the latter even possible? As I said, it runs great, so I am not keen on taking anything apart until I am forced to. I have bought a set of free flow exhaust manifolds, as I am probably going to re-do the exhaust system this winter/spring.
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oleblue69 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oleblue69 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/02/2017 at 10:41am
yes you can bolt square port headers-manifold to dog legs and visa versaThe bolt holes are in same location.
Bridged rockers are later heads. 58cc /lower compression ratio...find a set of 71-73 independent rocker stud heads, still will be 58cc lower compression but you could buy aftermarket pistons and choose your compression ratio. Cheaper then trying to find 3196291C heads which are 52 CC hi compression heads. casting 3212993/3213948/3216090 low compression still independent rockers...360 and 401 heads of those cstings are the same. If you are using 68-69 block you need adapter dowels and use ealier head bolts thru the newer heads.
pistons? try BULLTEAR
JS
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gremlinsteve Quote  Post ReplyReply Direct Link To This Post Posted: Dec/02/2017 at 10:41am
I bet the manifold is not the tin valley pan style
If some one assembled the motor using anything else
It's going to consume oil
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oleblue69 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oleblue69 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/02/2017 at 10:43am
JS
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oleblue69 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/02/2017 at 10:48am
if you need clarification on stuff, cell 303-913-0056
Jeff
including what gremlinsteve said about intake, pan-tin as oppossed to mister gasket type independent intake gaskets....
RE: oil consumption.
JS
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Blue68SST View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Blue68SST Quote  Post ReplyReply Direct Link To This Post Posted: Dec/02/2017 at 11:30am
Just wondering what the thinking was behind why the intake gasket type would affect oil consumption.
My belief that it may be a bad valve seal is that the #1 spark plug fouls badly after afew hundred miles. The drivers side exhaust pipe does put out blue smoke under hard acceleration (I am told!), but otherwise doesn’t smoke visibly. I don’t think it’s a piston ring issue, as the compression is fairly consistent on all cylinders, both dry and wet.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Buzzman72 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/02/2017 at 8:13pm
Dead umbrella valve seals in drainback holes? Had a '70 Javelin 360 with bad valve seals and plugged drainback holes. Used a quart to 50 miles. Used the twisted end of a coat hanger to corkscrew the drainback holes open, and instantly changed oil consumption to a quart every 500 miles. Later I did a valve job and replaced the seals with the kind that required machining the outside of the guide and installing with a driver. Problem solved. Umbrella seals are a waste of time.
Buzzman72...void where prohibited, your mileage may vary, objects in mirror may be closer than they appear, and alcohol may intensify any side effects.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amcenthusiast Quote  Post ReplyReply Direct Link To This Post Posted: Dec/02/2017 at 11:04pm
My first thought was the heads got changed because one cracked due to lack of proper coolant due to poor maintenance. (very likely)
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My second thought is you need to buy the books:

'68 AMC Technical Service Manual

Chilton's Repair Manual '68-'70 Javelin and AMX (hard back light blue book is the best one for your car -this has the gymkanna/autocross chapter in the back)

"AMX: A Source Book"

AM's "Performance American Style"
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Sorry guys, please take no offense:

'66-'91 AMV8 has Buick V8 style valley pan intake manifold gasket to provide a lifter gallery baffle for three main reasons:

1) allows adding oil to running engine by diverting oil back to rear of block, not to pour oil into a spinning assembly (so it doesn't spit oil on the person's face when they add oil to a running engine)

2) separates oil mist from the PVC vacuum line and oil pan/reservoir vent cap

3) helps prevent oil scalding by preventing oil contact to exhaust manifold crossover between #4 & #5 cylinders -to prevent overheating the oil.                 (engine oil also doubles as a coolant)
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It's ok to use the two piece intake gaskets with the no exhaust crossover aftermarket heads and intake manifolds.
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Sorry Buzzman I always like your comments but the cracked valve seals generally indicate one of two things: the engine has been overheated to cause embrittlement of the seals, then they might crack ~or~ someone tried to run a high lift cam and crushed the seals for lack of proper clearance (AM PAS book says all stem heights need to be checked on cams having more lift than .440" lift)

This was actually a positive feature of AM engines because Chevy only used an o-ring with a deflector = no valve seals whatsoever on the Gen 1 SBC V8s (almost unbeleiveable but true)
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If it makes some smoke after cold start up this indicates valve seals are likely gone bad.

If it makes smoke during full throttle acceleration this means the ring seal is less than optimal.
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IMO, no matter where you are (especially in USA) you should be able to make contact with a local AMC club. The local club may have 'AMC angels' to guide you to all the right people you need in your area -for mechanical work, parts, photographic proof/identification for theft or vandalism crimes etc etc.

Search the car club links on the bottom of AMC Forum home page.
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If your car is in good running condition, one of the worst things you can do is start taking things apart not knowing all about what you're doing; this can kill a perfectly good running car and turn it into another pipe dream/basket case disaster. (so typical it's pathetic)
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The best thing a person can do for their vintage AM car is to simply make up for lost maintenance and resume a maintenance schedule to return the car to like new condition. This includes cleaning all the drain holes under the unibody cavites so their self rinse technology works as designed.
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SAE gave the highly coveted "Best Engineered Car of the Year" award to AMC AMX two years in a row ('69 &'70) ...this says same thing for '68; this is an excellent, very well engineered car.
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One more thing: don't put the square top cheapie valve covers on your '66-'91 AMV8 engine.
The stock curved top valve covers are specially made to deflect oil spurts from the pushrods to the rocker arm tips for superior lubrication. (the Chevy style square top valve covers forfeit the optimized curved shape to reduce lubrication to the rocker arms tips)

Strip the paint, buff them out and spray clear coat paint to prevent corrosion if you must have 'bright metal' valve covers. (buffed out they will look like chrome if you can't afford to have them chromed)
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Please pass these tips on to the next person.
443 XRV8 Gremlin YouTube video: https://www.youtube.com/watch?reload=9&v=2DmFOKRuzUc
XRV8 Race Parts website: http://amcramblermarlin.1colony.com/
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ken_Parkman Quote  Post ReplyReply Direct Link To This Post Posted: Dec/03/2017 at 9:00am
The stock valley pan gasket and stock intake PCV baffle are critical for PCV oil control. The system seems to be OK with at least one, but it is best to have both. It is a very well engineered system from the factory, and often intake changes screw it up as the guy does not understand the system; using paper gaskets and not bothering to rivet the intake baffle on. IIRC the Offy intake has a threaded port at the front, and if the PCV is plumbed to that it's entirely possible that is your oil problem. Nothing wrong with the actual engine.

360 heads on a 390 are a great improvement. Actually the 74 and up 360 heads were 60.1 ccs; some of the sites have this slightly wrong. The drop in compression make pump gas easy, and those flow better than the higher compression heads. There should be no significant difference in power, but a lot less grief to tune on pump gas.

Let me know if I can help; I'm just up the road from you. Think we met at the Milton cruise?
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Blue68SST View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Blue68SST Quote  Post ReplyReply Direct Link To This Post Posted: Dec/03/2017 at 9:53am
Hi Ken.
Thanks for the reply.
There is obviously no way to tell what gasket was used unless I pull the intake, which may be worth doing.
The PVC valve is routed to the front base of the forward Carter carb. I will try to post a picture.
Yes we have met at both the Milton and Mississauga CC cruises. I have the gold AMX, used to have a blue 68 Javelin.
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