Bugout4x4

So I am putting a head together for mine today and could use some technical advise if anyone might find the time. Here's the situation...

When I got this Jeep the motor apparently had been rebuilt not too long before and was just barely broke in and even still has crosshatch on the cylinder walls. But the previous owner got it hot several times and cracked the head. I drove it for awhile because it was not using much coolant and didn't get hot on me until it decided to consume coolant like crazy on one of our off road outings. So it was time to tear it down and make things right from the bottom up.

The head was trash because the crack went from the combustion chamber through the exhaust valve seat and up into the exhaust port but the valves are still like new in this 97 0630 head. So my son in law heard I needed a donor head and surprised me with junkyard pull for it but it is a 7120 head. No big deal and it will work to get me by for awhile but I ran into an issue that I am not too sure about and could use some expert opinions from you guys who know this engine better on which way to go with this in putting it together today.

So I tore this donor 7120 head apart to inspect and lap up the valves and seats and found that at some point the engine this head came off of had been stored upside down outside and water had apparently got into the exhaust. While the seats cleaned up real nice a couple of the valves were pitted too deep to use. Well I have almost new valves in the other head to swap right? Nope, the Valves in the 0630 head have a much larger stem than the ones in the 7120 head and are cut down on the end for the keepers. So that was a no go and I ordered new exhaust valves for it.

Here is my curiosity... I am trying to decide which springs to use. I am going to assume that the springs on the 0630 with the heavier valves have a stronger spring rate than the valves in the 7120 head that has lighter valves, but I have no way to check for sure. And I know that a slightly stronger spring rate can be advantageous but I am wondering how prone the smaller valve stems are to breaking in these engines when a heavier rate spring is installed. Does anyone know how prevalent these are for breaking valves? Should I play it safe or do the springs in both heads have the same rate anyways?

EvstaG

Your 0630 head must've had the valve guides ground and larger stemmed valves installed, because the valves are supposed to be identical from 87-99. Not sure what the "end cut down for the keepers" would relate to, maybe some sort of aftermarket spring? However: The valve springs on the 7120 head are actually supposed be a bit stiffer due to the different cam profile used. For the 0630 in 1996 head they switched to the softer beehive springs.

I would keep the valves that are already on the 7120 head, especially if you're keeping the stock cam, and if the valves on the 0630 head have been modified, potentially for a different spring. In the experience I have I'd say the 4.0's aren't prone to a lot of valve issues.

Bugout4x4

Thank you very much for your time reply! This is what I thought also... maybe they drilled the guides and put in bigger stemmed valves. But when I talked to "The expert Jeep guy" at Summit he said that some of the HO high output motors came with these. The larger stemmed valves and guides and they actually had a part number that applied to these "stepped" valve stems as an OEM replacement on these.

And I do have the same type of spring on both heads. So I would have to ask, wouldn't they need to put a higher rated spring to counter act against this increased mass?

So I'm thinking at that point if they used the same rated springs these heavier valves are going to float at a lower RPM, right? Not that I have too worry much about this because I am easy on the tach, Be nice if I ever needed it, but it raised a question about ratings on these springs and if I could maybe be doing myself a favor by using the springs from the head with the heavier valves. And I will be using the stock grind on the cam.

It is good hear that these are not prone to breaking stems. I think I am
take your guidance and play it safe since I'm not racing this thing and stick with the valve springs that match the 7120 head. But for future knowledge this would all be good to know.

cruiser54

I think your Summit guy may be full of excrement.

Bugout4x4

Honestly I thought so too Cruiser. I think he may have been pulling and looking at a valve for a different engine. But... I have these in hand and and they are bigger stemmed and stepped down at the end to standard size. I'm thinking they drilled the guides and replaced the valves with these larger ones. But personally I would have went with new guides and OEM valves if I had done it.

Know what though? I am going to chase these down to make sure because these would be great for a high performance high lift strong spring build and it would be great to know the optional larger stemmed valves are indeed available. In the mean time I still need to make sure none of these are bent but this whole set I took out are like brand new if someone wants them and the guides for a high performance stronger spring build project. Send them free if anyone wants them.

cruiser54

I wonder if they're out of a 258.

Bugout4x4

Very well could be an AMC application! They have me curious for sure.