Re: [AMC-list] I know why 195.6 ohv heads need re-torquing

[Frank Swygert <farna@xxxxxxx>]

Great detective work! I pretty much knew the problem was expansion/contraction, but I always figured it was vertical, never considered horizontal! That makes more sense -- the back and forth movement gradually/eventually loosening the bolts. And that would be the most distance. Still, nothing to worry about, just retorque the head every 2-3 years, 10-15K miles. The valve cover needs to come off so the valve can be adjusted anyway, no problem to just put a torque wrench on each bolt in turn then break it loose, tighten back. No need to worry about order unless all the bolts are loose. The real problem is too many people assume the engine is like more modern designs and they don't maintain it, and think that adjusting valves and retightening bolts shouldn't be necessary. Well, it's a relatively ancient design! 

The cooling explains why the L-head doesn't have the same problem. The head is spread over the entire surface of the block and is cooled more evenly, plus the block gets hotter due to the exhaust ports in the head. Then it has studs on top of that! I figured it was something to do with the conversion. A flatter combustion chamber design might have helped, but it's not like someone is going to make a new head for the thing! Looking at a cross section of the head, there really wasn't much choice. The way the intake is made on to the head dictated the deep wedge so there would be room to fit the spark plug, which is already in a deep well. A separate intake manifold would have solved that part of the problem, probably why the 232 doesn't share the same problems as the 196 though. The 199/232/258/4.0L block didn't have dowel pins either, not until 1996. 96-06 models have a dowel pin on the ends of the valve side (right side). Not around the bolts like the AMC V-8s, the block has 
small solid pins, head has matching socket holes.
See pics at http://www.enginebuildermag.com/Article/1943/perax2019s_core_corner.aspx 

Copper head gaskets are made for racers all the time, just copies of the standard gasket. You'll just have to figure out the thickness before and after compression. Copper isn't cheap, but for a one-off that can even be reused a time or two it shouldn't be bad. Would be for a manufacturer! 

----------
Date: Tue, 2 Feb 2010 14:05:19 -0800
From: tom jennings <tomj@xxxxxxx>

I know why the 195.6 OHV head needs to be retorqued, and maybe how to solve
it.

Thermal expansion.

The head is what, 30" long? (Don't have one here at work...!). It rises 140F
in normal operation (60F to 200F, say, and that's assuming no overheating
and impossibly even heating). The coefficient of expansion for cast iron is
5.8 microinches per inch, per degree F. That's .024" increase in head
length! It cycles cold, hot, cold, hot, ... every time you drive the car.

We know for fact that on this motor the head gets hot VERY FAST. The
combustion chamber is over an inch deep in the head, with a lot of area
(that oddball wedge). The exhaust passageways through the head are long.

Read this and watch the video: http://www.boltscience.com/pages/vibloose.htm

Plus, the head is heat-insulated from the block with the composite gasket
(a copper gasket would be nice!). Therefore, the head grows more and faster
than the block, by a big margin.

All this simply walks the bolts out. 


PRECISE LOCATING DOWELS. I'm gonna ask around about dowels. The differing
head vs. block growth rates though means the head will try to shear them off
the block. That obviously won't do.

--
Frank Swygert
Publisher, "American Motors Cars" 
Magazine (AMC)
For all AMC enthusiasts
http://farna.home.att.net/AMC.html
(free download available!)


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