1.787" exhaust valves?

I am curious as to whether anyone has pulled a set of 1.787" intake valves from 290/304 heads and had them installed as exhaust valves in a set of 401 heads while keeping the stock 2.025" intakes. SS AMX heads had 2.080"/1.740" valves...so 1.787" valves are only a hair bigger, plus with keeping the standard 2.025" intake instead of 2.080" intake it would seem logical that if the valves in the SS heads fit, just going up in exhaust to 1.787" exhaust should work. My interest here is to put together a set of 401 heads for some nitrous work. Nitrous use obviously puts a premium on exhaust flow, and since a 1.787" valve would seem to be the biggest reasonable exhaust valve size, and I just happen to have 4 sets of 1.787" valves(from 304 heads)to play with that are in great shape. Since my understanding is that the valves stem diameter, overall length, keeper grooves, etc are identical to the 1.680" valves, the 1.787" should cause none of the typical issues that the usual non AMC valves tend to create. I'm a big fan of keeping as much factory AMC parts in an AMC car as I can get away with, and from what I have read from some of the head porting threads, factory AMC valves with a good valve job, back cut, and top cut treatment are hard to beat. Are these reasonable assumptions?

George

Great question.

MC, you got the dimensions for AMC Heads
that show the center distance locations
of both valves relative to the cylinder bore center ?
This would indicate valve stem spacing and max size
vavles for a given bore size, I think, wouldn't it

Since I posted this question I went over to my "parts bin" and took out a pair of factory intake/exhaust valves. One obvious thing that came up was that the exhaust valve's head appeared a tad thicker as did the taper in the valve stem, more of a tulip shape. Not being any sort of expert here, I am guessing that the exhaust valves were bult with a little more "meat" in them for a good reason. I imagine it is due to the much more severe heat built up in the exhaust runners than the intake. I suppose a logical concern would be whether running an apparently "lighter duty" intake valve would be strong enough and heat resistant enough to withstand the punishment of exhaust work. Am I overthinking this?

George

Yes, I think heat would definitely need to be considered. The exhaust valves on some of the new engines hollow and filled with sodium to help with the heat?

Seems to me that the seat face on intake is 30 degrees, exhaust is 45 degrees, or vice-versa. Don't know if you'd be able to re-cut the seat/face correctly.

Bob
tufcj

Not a good idea. Exhaust valves are made from a different material than intakes to deal with the extra heat. Intake valves in exhaust location will wear out the seats and possibly warp. They will also get hot enough to deform the head by the valve spring pressure if the engine is run hard. I have seen this on some weirdo dirt track motors I was involved with many years ago.

Sounds like it makes sense to me. If I had taken a good look at the pair of intake/exhaust valves BEFORE I put up the question, I probably would not have asked. I didn't realize that the exhaust valves were made of a different material. Interesting! I just figured that they were formed with more meat to withstand the high temps. Thanks for insight. It's sure nice to have some experienced people to bounce my retarted ideas around to.

Thanks,

George

Not retarded. It is part of the process of learning.

yep learning and innovation.. sometimes no one has a valid idea why it wont work.. then you get to blow stuff up to see for yourself (mythbusters motto)

There are some things that I would considering doing just to "see what happens" but the thought of needlessly dropping an exhaust valve in a freshly rebuilt 401(ouch) is not high on my list. Thanks for the technical support. Better that Onstar!!

George