Head machining question (7120)

[jeep.kevin]

**My question is in the first paragraph, read on if you want the full story.**

Ok, so my machinist just called me and said there is a chance that when he grinds the valve seats that they might hit a water jacket? any truth to this on a 7120 head? it was my understanding that the 7120 head had the best flow and was a good one to get.

I know the 0331 is pretty much trash for various issues attached to them. He said that on the past he has done 50+ jeep 4.0L heads and never had an issue, but recently he had three heads in a row with bad castings that exposed a water jacket when they ground for new press in valve seats. I asked if he had casting numbers for those, he said they were all 0331 heads.

If someone with some knowledge on this or experience with getting a head rebuild let me know what you've got. He is waiting for me to respond about to go forward with the machining or not.

Thanks,

-Kevin

 

[cruiser54]

Time for a new machinist.

 

[jeep.kevin]

Time for a new machinist.

what makes you say this? It just seems like he is overly cautious. He runs a pretty popular machine shop out in AZ, I cant imagine he is crappy if his business has been around for years.

 

[cruiser54]

what makes you say this? It just seems like he is overly cautious. He runs a pretty popular machine shop out in AZ, I cant imagine he is crappy if his business has been around for years.

7120s are fine. 0331s are not so good.

But 0331s had an issue on TOP of the head......

 

[Russ Pottenger]

I wouldn't have a problem putting in exhaust seats (and have) in both 0331 and 7120
heads.

The key is to use a seat depth of 7/32, not 1/4. The shallower seat will help keep it from going into water.

Tell your machinist to look into getting 1625-9 seats:
OD 1 5/8
ID 1 1/4
WI 7/32

Hope this helps

 

[jeep.kevin]

I wouldn't have a problem putting in exhaust seats (and have) in both 0331 and 7120
heads.

The key is to use a seat depth of 7/32, not 1/4. The shallower seat will help keep it from going into water.

Tell your machinist to look into getting 1625-9 seats:
OD 1 5/8
ID 1 1/4
WI 7/32

Hope this helps

This does help Russ, he had mentioned to me about some tech data he had that told him to only grind to .187 or something like that. ill tell him about the 7/32 and the 1625-9 seats and see what he says. I'll take as much info on this as i can get.

 

[Talyn]

Hitting water while installing valve seats has happened before. As Russ said, go with a shallow valve seat.

 

[md21722]

For my own knowledge, why would a difference of 1/32 be acceptable for something like this? How much cast iron is enough to protect the integrity of the head?

 

[jeep.kevin]

Thanks for the help guys. I just got off the phone with the machinist. He is going to bore more shallow to .187 to avoid the issue as much as possible.

 

[Russ Pottenger]

For my own knowledge, why would a difference of 1/32 be acceptable for something like this? How much cast iron is enough to protect the integrity of the head?

Don't have the perfect answer to your question, but like most things it comes
down to experience and a bit of intuition.

Cut a seat pocket too deep for a wider seat, you go into the waterjacket.
Cut too shallow for a thinner seat, it falls out.
Put too much interference fit, cracks head. Not enough press, it falls out.

Same applies to a aluminum head. Because of the materials growth characteristics,
the seat needs to be much wider and another .002 to .004 interference fit over a
cast iron head.

 

[jeep.kevin]

Don't have the perfect answer to your question, but like most things it comes
down to experience and a bit of intuition.

Cut a seat pocket too deep for a wider seat, you go into the waterjacket.
Cut too shallow for a thinner seat, it falls out.
Put too much interference fit, cracks head. Not enough press, it falls out.

Same applies to a aluminum head. Because of the materials growth characteristics,
the seat needs to be much wider and another .002 to .004 interference fit over a
cast iron head.

I know what you mean about that. I spoke with him and he is going off of a bulletin posted by Jeep about the head castings and recommended specs for how to proceed.