A "factory" stroker from Mexico

[guishe]

Hi there. I've been reading different posts in your forum for a while, as I´m cllecting information about strokers, head swaps and all that stuff.

Thing is I have a jeep with a 6l, but is not the "traditional" amc 258, but the one wich was produced in Mexico by VAM
VAM was a subsidiary from AMC. All VAM engines were of AMC design, but built in Mexico. Moreover, VAM added unique engineering features to deal with low octane fuel and high altitudes. This included a 282 cubic inch (4.62 L) I6 engine that was not available from AMC.
I
n 1982, VAM's engineering department worked on a prototype Jeep XJ and fitted VAM's straight six into it (in place of AMC's four-cylinder or optional V6 from General Motors). AMC was impressed by the project, which eventually developed into the 4.0-liter engine version introduced in the 1987 Jeep Cherokee (XJ).

So, the VAM engine was something in the middle of the 258 and the 4.0. The resulting advantage (as I think) is that it haves a reinforced block with a 3.910 bore (a 4.0 + .03) and the 3.895 stroke of the 258 engine.
My problem is that as VAM ceased production in 1982, there is very little information available about this engine, as well as just one manufacturer making parts for it.
I'm in the way of swapping heads for one from a 4.0 (the stock combustion chamber volume of the VAM is 102cc) and the first choice was to look for the pistons from the 4.0 also, as they are easier to find (and cheaper, I suspect) BUT... I found that piston pin distance is less than 4.0... (original VAM has a 0.0 deck clearence).

Question is... ¿could anyone here help me looking for the right pistons? I think I'm making a lot of math but I'm not sure I'm making the "correct" one)

Thanks and excuse the possible failures in my english

Guillermo

[gradon]

Sounds like a rare find! I'm not sure there is info on pin height for the VAM pistons online--you might have to measure what you have and then compare it to what's available--a custom piston might be in order. So these VAM blocks were installed in CJs, FSJs?

[guishe]

Piston pin height is 1.61...

These are some measures I´ve collected:

VAM 282 CI - 4.6 Lt
Bore 3.910
Stroke 3.895
CR 8.500
Horsepower 127 @ 4100
Torque 225 @ 1000

Intake Valve Diameter 2.020
Exahust Valve Diameter 1.675
(both bigger than 258)

Cam Degrees 266°

Pushrod Lenght 5.875

Deck Height 9.424
Deck Clearence 0.000

And this is not a "rare" find.... all 6l jeeps and amc´s (ramblers, gremlins, pacers, etc) in México have this engine!

[guishe]

As far as my math goes (i´m far away of beeing an expert), things go like that:

With that cylinder volume (766.6) and CR (8.5) , the combustion chamber volume results in 102 cc
If I want to raise CR to somewhere around 9.2, I need a combustion chamber volume of 93.5 cc

Swapping to a 4.0 head, with stock gasket, but using VAM´s stock pistons, that volume would be something about 80.5!!!
So I guess I have to find some 4.0 + .03 pistons that allow me to reach the 93.5 cc... I know that with a shorter pin height and a bigger dish I could be around 88.4 cc, (and a resulting CR of 9.6) but some quench issues will came to scene, and I'm not sure about how to interpretate them...

[gradon]

Looks like the stock pistons w/ 1.601" pin height will be the closest match to the 1.610" of the VAMs(deck clearance would go to .009"). You said the bore is 3.910" so that is .035" over and I guess you'd have to get the .040" over pistons and bore/hone the extra .005"(the .030" overs would be too small, right?).

[guishe]

Yes, pistons are +.035, my mistake.
The "original" plan was to rebuild the block to standard size, (I already found a mexican dealer who sells sleeves and pistons)
and, with te head, I have two ways to go: rebuid the original, or swap to 4.0.
The main problem with swapping heads is that at this bore, the compresion is far higher than the one you obtain if the swap is made in the amc 258. Anyway, I live at 6100 ft, that may help, but...
The second thing that makes me think, is that even when flow is better in a 4.0, valves are smaller*, so I don´t know if the real benefit of the swap would be as noticeable as in the case of a 258.

*: I know the head can be machined to accept bigger ones, but that is (I suspect) a "no-so-common" kind of job for mechanics in here. I tried to explain the head swap to a couple of them, and they said that it was imposible. (even when they couldn´t give a reason for that) Something like "you can´t do because... you can´t"

[SIXPAK]

I have an odd request. If you get to the point the head is off and the pistons are out could you please tell me the height of the cylinder wall inside the bore? It would need to be measured on the interior of the bore on the passenger side and the driver side. Thanks JB

[guishe]

Pistons are still inside, just the head is off. Planning to take off the pistons next weekend, and I´ll post the measures you need.
In the book of specs that I have, the block deck height and the stroke lenght are the same as in the amc 258, so cylinders are possible equal too.

[gradon]

I did some searching for pistons with the 1.610" comp. height and only came across some 3.6" for a 4-banger and some 4" ones for the 302.

[SIXPAK]

Pistons are still inside, just the head is off. Planning to take off the pistons next weekend, and I´ll post the measures you need.
In the book of specs that I have, the block deck height and the stroke lenght are the same as in the amc 258, so cylinders are possible equal too.

I believe tha you will find that it may have the same length bores as the 258 but are longer than the 4.0 block. This means better piston skirt control for a stroker=THUMBS up for a nice stroker!

[guishe]

In fact, I have spend a long time studing about what to do with this engine, because I think it haves good potential.
At first, I have just read about the head swap, then I arrive at Dino´s page and now I want more... jeje

Thing is that as 3.910 is the stock diameter, and there are pistons in +o.30, + o.40 and +o.60, makes me think that the cylinder accept that amount of overbore.
I´m conservative, and I want to stay at stock, but I think you are right when you say the engine have potencial for a good stroker.

My problem is that in certain sense I don´t know what to expect: a 258 - 4.0 head swap, with better cam, intake and headers claims about 40 hp gain... a stroked 4.0 gives more than that, but ... ¿which are the possibilities of this 4.6?

I´ve been thinking about the chance of leaving this as stock and after I finish the hole truck (it was unarmed till the last screw) then start with a 4.0 stroker... just for fun...