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Old 30-04-2020, 18:03   #1
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Lehman 120 valve stem seals in situ

My Ford Lehman 120 hp 2715 is very old and tired, a victim of abuse by previous owners. It starts easily, but there is a massive embarrassing cloud of smoke for several minutes until it warms up.

There is also strong blow-by which necessitates placing a plug on the oil dip stick tube otherwise oil will spill though it.

Despite that, the engine starts and runs well.

There may be a problem with stuck rings or scored cylinders -- but as an initial step I'd like to replace the seals without removing the head and without stuffing rope or injecting compressed air into the cylinders to hold up the valves.

From pictures I've seen of the pistons at top dead centre, there doesn't appear to be enough space for the valves to fall into the cylinders once the springs and retainers are removed.

So is it therefore possible to replace the valve stem seals on this particular diesel engine without going through the usual methods?

I have no compressor to blow air, and even if I had, no adaptor to screw into the injector holes, and I don't want to remove the injectors and wedge rope into the area.
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Old 30-04-2020, 18:58   #2
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Re: Lehman 120 valve stem seals in situ

I'm going to question this being the right approach. I suspect the entire head needs to be sent to the overhaul shop for new valves, seats, etc. It will come back beautiful. While it's gone you will want to investigate the possibly scored cylinders (hone them), replace rings, or maybe replace pistons.

I'm taking this position because the engine sounds totally clapped out, such that repairing the seals will leave a bunch of other wear uncorrected. If you need a starting point, check your compression. It sounds like it will not be good news. But, given that it's running, you should be dealing with wear, not damage.
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Old 30-04-2020, 23:50   #3
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Re: Lehman 120 valve stem seals in situ

Take tkeithlu’s wise advice and lift off the head. Doing stem seals on the ford really requires you to remove the injectors and leak off manifold as well as the entire rocker assembly so that there’s nothing impeding your progress with compressing the valve springs and popping on the new stem seals. The crank case pressure could be from a blown head gasket leaking compression across to the pushrod gallery rather than worn pistons and rings......... either way , a cylinder leakage test will quickly ascertain the source and severity of compression leakage.
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Old 01-05-2020, 00:19   #4
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Re: Lehman 120 valve stem seals in situ

I concur with tkeithlu and skipperpete.

Doing the valve seals in situ one at a time seems to me to be a lot of work for very little gain. It is unlikely to make a huge difference and will do nothing to identify the source of the blow by.

While it is best to do a cylinder leakage test, you could just bite the bullet and whack the head off to have a good look at the cylinders. If they look reasonable, do a valve job on the head and see how it runs after that.

This assumes you are planning to keep the engine long term and fixing as required rather than replacing it.
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Old 01-05-2020, 11:23   #5
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Re: Lehman 120 valve stem seals in situ

Replacing the valve stem seals will not help your problems. The purpose of the seals is to keep oil from draining past the valve guides into the cylinders. The valve guides keep the compression in the combustion chamber. Blowing oil out of the dipstick tube is indicative of compression getting past the piston rings and or valve guides. The only thing that will cure it is a complete rebuild. Good luck
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Old 01-05-2020, 15:36   #6
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Re: Lehman 120 valve stem seals in situ

I had one of those engines in a Fordson tractor. The 4cyl was swapped for the 6 with extended chassis rails & bonnet.
They were a very cheap motor to do up.
Mine had electrolysis through one liner & leaked a small amount of water into the sump.
Make very sure you keep the right anti-freeze/coolant in the fresh water supply!!! (unlike my previous owner)

Like a Perkins these old Pommy motors always 'smoked' on start up mostly through being cold and sending unburnt fuel out the exhaust.
Are you VERY sure it's oil smoke, not diesel smoke?
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Old 01-05-2020, 16:47   #7
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Re: Lehman 120 valve stem seals in situ

Quote:
The valve guides keep the compression in the combustion chamber.
Umm... I thought that the valve face and seat did that. The guides only keep the valve centered on the seat and are not exposed to cylinder pressures.

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Old 01-05-2020, 17:32   #8
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Re: Lehman 120 valve stem seals in situ

Thanks for the advice folks. Of course a major rebuild is what I am hoping to avoid as the boat is already overcapitalised, and sees little use (especially with this lockdown).

Since the engine runs well, the unsightly smoke is my main concern, rather than the blow-by.

River Cruiser, yes, the purpose of the seals is to stop oil draining into the combustion chamber -- but mine don't, so replacing them will help my problem. Blue smoke which clears later is oil which has drained past the seals, down the valves, into the combustion chamber where it sits until startup, to be burned in a cloud of smoke.

If the rings aren't doing their job, oil from the crankcase can also be burned, but since my issue is at startup and clears later I suspect the stem seals as an initial and cheapest fix.

There is a minor fuel sheen on the water around the exhaust, so some diesel is not being fully burned. Could be injectors, or low compression caused by the valves not seating properly, or stuck rings,or worn cylinders, or all these.

I use a catch-can vented externally through the hull to deal with the excess crankcase pressure.

The dilution of the sump oil with blown-by combustion elements doesn't really worry me as the engine is already very old, does few hours, and over the last 13 years we have had it with this problem it still runs and does its job when called upon to do so.

It is all a pity because the massive 6.3 litre engine is otherwise bullet-proof, so the prior owners should have looked after it better.

Once again, thanks for the replies.
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Old 01-05-2020, 17:55   #9
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Re: Lehman 120 valve stem seals in situ

The role trick is a gas motor trick as you have a spark plug hole.
Here is a Lycoming procedure that lays out what you want to do if your curious, aircraft engine but similar issues.

https://www.lycoming.com/sites/defau...20Sticking.pdf

A problem with doing it that way you want to is holding the valves in the fully up position so that you can compress the valve springs and install the keepers, without being fully seated your not going be able to do that.
Your initial smoking is most likely valve guide seals, but you’re wasting your time, you really need of overhaul that motor before it spins a bearing or similarly and is beyond economic repair.
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Old 01-05-2020, 18:47   #10
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Re: Lehman 120 valve stem seals in situ

Worn guide seals usually mean worn valve guides. Pull the head and have it done right. IF the cylinders look good here is a ring test. Hand turn the engine until the pistons are all 1/2 way up or down in the cylinder. Fill all the cylinders up with diesel fuel right to the top. Come back tomorrow. 25% loss is OK. An empty cylinder requires a re-ring job or Cylinder /piston replacement.
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Old 02-05-2020, 07:20   #11
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Re: Lehman 120 valve stem seals in situ

I am going to give the OP some support on this one. I think he has a good handle on his situation and his original question seemed to me to be addressing one of his two issues. I am a mechanic with a lot of experience on quality repairs and "let's suck it and see repairs" for desperate owners. I am encouraged that the engine starts well and runs well. Remembering the diesel is a COMPRESSION IGNITION engine, the engine can't be too bad if it starts well and runs well. The compression can't be too low, so there is hope for it and that is what he is asking for.
He has clearly described oil burning and not fuel, so I agree his smoking issue is the valve stem seals and if he can do them, I believe he has a real chance of resolving this one problem. However, having no experience on this particular engine I cannot answer the question if the valves would drop or not. Me? I would remove the injectors, probably buy an old one, cut it up? and tap a thread into it to fit an air fitting and use air to hold the valves up. Bunnings or Ebay sell small compressors for under $100. The time saved would pay for this and who can't make regular use of compressed air? I personally would not risk trying it without knowing the valves are seated tight, however worst case, you'd have to pull the head.
I have another tip which might encourage the removal of the injectors. Often poorly serviced and lightly used engines will suffer stuck rings in the pistons. We once had a Cat diesel that wouldn't even start because this was so severe. The injectors were removed and the bores were filled with diesel and left in there - you could keep topping it up if it drains into sump (yes, you will need to drain sump and to be safe, do two oil changes to ensure the diesel is removed from the sump.) After a day or so, with injectors still out, drain the sump, leaving plenty of time for all to drain out, refill with a cheap diesel oil (it will only be in there for minutes before you change it to good quality oil). Crank the engine to blow out any residue fuel until NOTHING is coming out anymore. An old folded towel/s over the holes would be a GREAT idea! ;-)
Refit injectors and fire it up. If the rings were stuck, you have a very good chance the excess blow-by will be fixed. If you have no improvement, then a rebuild will then certainly be needed. (head off, bare minimum to confirm)
P.S. Our Cat ran like a new engine after only one night with diesel in the bores, because it obviously was a good engine before being left unused for a long time. Best of luck.
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Old 02-05-2020, 10:08   #12
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Re: Lehman 120 valve stem seals in situ

Quote:
Originally Posted by Jim Cate View Post
Umm... I thought that the valve face and seat did that. The guides only keep the valve centered on the seat and are not exposed to cylinder pressures.

Jim


When the guides wear some compression that leaks past valve & seat will get past them. Having to put a plug in the dipstick tube to keep it from blowing oil out is a definite indication of worn hard parts. I would do a rebuild now instead of waiting for additional damage.
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Old 02-05-2020, 15:23   #13
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Re: Lehman 120 valve stem seals in situ

Quote:
Originally Posted by River Cruiser View Post
When the guides wear some compression that leaks past valve & seat will get past them. Having to put a plug in the dipstick tube to keep it from blowing oil out is a definite indication of worn hard parts. I would do a rebuild now instead of waiting for additional damage.
Any compression getting past the valve seats has two routes of escape. One is via the relative manifold and the other is via the valve guides.

The cross sectional area of the escape routes has a major bearing of where the pressure dissipates. The cross sectional area of the gap in a worn valve guide is far far smaller than the cross sectional area of the manifold.

Any pressure escape from the cylinder via a worn valve seat mostly goes out the manifold.
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Old 04-05-2020, 02:06   #14
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Re: Lehman 120 valve stem seals in situ

Thanks again guys, good advice here and filling cylinders with diesel or proprietary solvents to free any stuck rings is worth a shot.

Valve stem seals being made from a synthetic rubber-like material can loose their elasticity over time, much like oil seals, and this hardening can diminish their ability to perform their intended purpose.
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