Porting old heads

[Chancer]

I was having a conversation with a mechanically-minded friend the other night, and mentioned that I was keen on porting the head at some point. He asked how many miles the engine had done (159,000) and said that it would probably need bottom end work carried out at the same time. Now, this is the first time I've ever heard of that being an issue, and since he can be pretty full of it, I'm taking it with a pinch of salt and asking you guys - the people who know jeeps - is he talking BS, or is it a factor that should be taken into account before going up the porting path?

 

[75SV1]

My opion is it depends on how the engine was taken care of. If it is in good shape, then just do the head. The other thought is while its down why not freshen up the engine. I know money. So I wouldn't say your friend is full of BS. THe 4.0L does have a good rep for running a long time.
Tom

 

[rsalemi]

If the oil pressure is still good (+13 - +35 idle and cruising) and its not using excessive oil you should be fine.
Most hi-mileage early 4.0's could use rod bearings and a rear main seal - something pretty easy to do (especially if a lift is available).

 

[Toolmaker]

In THEORY if your engine has a lot of miles then your valves are likely not seating & sealing completly so some compression is lost. If you do a head rebuild / valve job on an otherwise worn engine then it can lead to blowby and ring seating problems because the old, worn rings are not up to handling the increase in compression. This problem would be compounded when porting the head to allow more flow because the more fuel & air that enters the combustion chamber, the harder it is to compress.

On an engine with 159,000 on it I would plan on at least doing a hone & ring job along with the head work and new bearings would be good idea.

 

[Chancer]

OK, to update the info, it has good pressure - 35-40 at idle, 60 cruising. My overheating problems are now non-existent, what with a new 2 row rad, thermostat and hoses, and oil consumption is good. One other thing to bear in mind is that it runs on LPG. I don't really see how that would affect it, but just so you know...

 

[75SV1]

How long have you run it on LPG? It does make a difference. I've heard they run cleaner, so less wear and tear on the engine. You might want to check with someone a bit more knowlegeable with LPG setup though. I thought one of the benifits of LPG is it burns cleaner, so your oil doesn't get contaminated as much and the fuel is a gas so no wetting of the cylinder walls. Thus not as much wear. Something to look into. You could look at your cylinder walls or have them measured. Then decide is just new rings, bearings and gaskets would do the job. Still at 150K I would hold my breathe.
Tom

 

[Chancer]

I must admit, it runs really nicely on the stuff - negligible loss of power and like you say, it's clean. The system's been fitted for 2 full years now, and I've run it for 18 months with absolutely no problems. Once I had piston knock on starting up, but I reckon that since it's not done it again that it was an aberration. I know of a few people who've uprated filters, exhausts etc on LPG jeeps but no one as yet who's gone in for more advanced/ hidden stuff.

I'm only really asking the question for information's sake at the moment - I've got no immediate plans to do engine mods - I've got to sort the body and drivetrain out first, and get my interior back together. Who'd have thought that putting leather seats in would become so complicated?!

 

[Toolmaker]

Even though LPG runs cleaner, wear inside an engine comes from friction. With over 150k miles I'd say that engine probably has a fair amount of wear even though it still runs like a champ. All the components of that engine are worn in together. If you remove one component and rebuild or modify it without addressing the rest of the engine then you will be asking for trouble.

 

[Dr. Dyno]

Two years ago I bought a '95 4.0 head from a junkyard and ported it myself successfully. The valves and seats were in perfect condition and all they needed was lapping. Once it was ready, I had it swapped onto my 163k mile old 4.0 (this was before I built the stroker). At the time, it was still running like a champ and oil consumption was negligible. With the ported head it ran even stronger from 3500rpm upwards. I ran it that way for 12k miles without any problem and it was still running perfectly when I swapped in the stroker. I kept the old '92 head as a spare, ported it, and bolted it onto the '00 block that I used for the stroker project.
So why build a stroker when you've got a great-running 4.0 you might ask? The answer was easy. I'm a nutcase. I'd already taken the old 4.0 to the max performance-wise and I still wanted more torque, so a newly-built 4.6 stroker seemed the best way to go especially since I wanted the engine to remain naturally-aspirated. I've been loving it ever since.