640: to rebuild or not rebuild?

Cpenn

Member
Hi everyone,

I been running my 56 640 for 10 years, mowing and plowing snow. But in the past two years I have found cylinder #4 buggers up the spark plug with oil every 6-8 months, causing it to not fire in that cylinder. So I did a compression test, dry and wet. Here are the results:

Dry (not warmed engine, gas and ignition off, throttle completely open): 107, 107, 93, and 103.
Wet (warmed engine, small squirt of oil in cylinder, gas and ignition off, throttle completely open): 120, 120, 120, and 115.

What do you think? tractor has no other issues, no major leaks. Should I rebuild motor? If so, should I do an in-frame rebuild?

Thanks!
Chad
 

Your compression numbers look ok to me. You might want to make sure you're getting a GOOD spark to all 4 cylinders. Plug wires may be getting old. Also, might be worth pulling the valve cover off and making sure the oil is draining back to the sump easily. Is your breather on the valve cover ok? You might have a bad valve seal on one of #4's valves.
 
No, it does not use much oil. Once I change the oil in the spring, I never even need to add any. I always put in 1 qt of lucas oil when I change the oil. Smoke is not blue. It gives occasional grey "puffs".

I run Autolite 437 plugs (working off memory here). What do you recommend?

Thanks
Chad
 
Plugs and wires are only 16 months old. I clean the breather on valve cover every spring when changing oil. I'll take a look under the valve cover. How do I know if the valve seals are good or not?
thanks,
Chad
 
(quoted from post at 17:24:21 04/29/19) Plugs and wires are only 16 months old. I clean the breather on valve cover every spring when changing oil. I'll take a look under the valve cover. How do I know if the valve seals are good or not?
thanks,
Chad

You know, thinking about it, those engines don't have valve seals. They just use a rather long valve guide with keepers. Sounds like you're doing everything right. All I can think of is to make sure the oil for the rocker arms is draining back into the sump, and not sitting on top of the valve guides. You're not burning oil, that's a good thing. The only other thing I can think of that would foul #4 is that it somehow is getting too much fuel, and I really don't know how that would be happening.
 
Compression is not too bad.

I would probably put an anti foul adapter on #4 and continue to run it.

It probably will not foul the plug again.

Dean
 
Can I simply pull off the valve cover while not running and see if there is any standing oil as a check for the drain-down to sump? Or should I pour some oil on top? I assume that I should not run the
tractor with the valve cover off.

Thanks again
 
(quoted from post at 17:01:15 04/29/19) Compression is not too bad.

I would probably put an anti foul adapter on #4 and continue to run it.

It probably will not foul the plug again.

Dean
That's what I did on my 640, no more fouled plug.
 

3 is on the low side #4 right behind it... A cylinder leakage test will lead you in the direction as to where the leakage is at... Once you know where the leakage is at that will lead you in the direction you need to go...

About the only thing mechanical you can eliminate as a suspect is valve adjustment...

Cylinder leakage anything beyond 20% is considered worn to worn to be efficient, From 20% plus its all down hill from there...

I don't like what i see all I see is $$$ leaving my pocket book...
 
Hobo,

This is great. I will try and borrow a leak down tool from someone, perhaps the loaner tools at the parts store. Could you be more specific about what you said about valves? Can I check to see if the valves
are sealed well, or is that determined from the leak-down test?

Thanks,
Chad
 
I just went through a 640 from front to back (4 month of working on it when I can) and I still have to put some things back on it because I just got it running this week end. That is not to mention I need to redo the sheet metal and paint it.

Knowing what I do now---I would just fix what is required and use it unless you want to restore it or have plenty of spare time and nothing else to do or spend your money on.
 
Okay Hobo, that is very helpful. I'll do the test. If I were to do a re-build at some point in town, do you think that an in-frame rebuild is suitable?

Thanks
 
(quoted from post at 13:43:41 04/30/19) Okay Hobo, that is very helpful. I'll do the test. If I were to do a re-build at some point in town, do you think that an in-frame rebuild is suitable?

Thanks

I'm sure that Hobo will chime in, and he knows his stuff.

I can relate a little experience. I have a 2N that had similar (actually, a little lower) compression numbers.

It cost me about 600 dollars in parts for an in-frame rebuild. That was with all brand new valves, new rings, new rod bearings, gaskets, seals, various other parts.

While the hood and fenders were off, I had a friend of mine give them a decent de-rusting, painting and clear coat for about 200 bucks.

I didn't fiddle with "restoring", per se. I didn't paint anything on the engine or frame...wait...I did put some muffler black on the exhaust manifold.

That 800 bucks got me a very, very usable and reliable tractor.
 

PS... I'm not saying to rebuild now. That depends on how well it's running. My 2N was to the point where starting and running was an adventure. It started hard, it skipped, it smoked, it constantly hunted for a stable idling RPM...

If mine was running better, I would have put it off; since I always have something that needs to be fixed, and I would have prioritized it down the list.
 
Yes, I hear what you are saying. I have five children from 2 to 11 yr old, and I restored two tractors before they came along (never rebuilt an engine). Those two restored tractors are long gone and my
tractor restoration hobby turned into tractor maintenance. But now that the children are older, this might give me an excuse to get another antique to utilize while restoring the motor on the 640! I'll do
the leak down test proposed by Hobo, report back, and then make a decision.
Chad
 

Its not my call to do a in-frame... Years ago we did but they were not 60 years old at that rime... The labor rate has got so high now its not worth the gamble for me to do a in-frame. The oil pumps on those engines are robust and can mask crankshaft ware so you would have to see the crank and bearings to determine if an in-frame job would provide you with useful service life...

A in-frame would bring the compression numbers back up IF a quality valve job was also done along with the rings, pistons, sleeves...

Quality means done by a professional (you local engine/machine shop)
 
if it were mine and it wasn't smoking or burning a lot of oil I might change to a hotter spark plug but would not overhaul it. your compression is slightly low but not low enough for me.
 

I must have done a "quality" job, because my numbers came back up.

:)

I bought all new valves (along with the other items mentioned) and I lapped the valves in place.

Maybe not quite as good as a machine shop, but I didn't have to pay machine shop labor...which actually doesn't bother me as much as... I would have needed to figure out how to haul that entire block to a machine shop, for my 2N.

For more modern machines, with the valves in the head, I've taken the head to the machine shop and let them do their magic.
 

I am not gonna argue your shining the seats BTDT and I am not gonna say I am not going to do it again but its not as good as new unless you got real lucky and the seats were not worn/pitted...

Sealing the valves the most important processes 99.9 % is not gonna cut it its 100% its not its close is good enoufh...

Lets teach a man the correct way to do the job and understand it...
From there its is call on how he proceeds with are with out special tools needed to bring it back to OEM spec with a long service life...
 
No argument Hobo. I'm an Electronics Designer first, farmer second and a mechanic as a means to support the farm habit.

That's why I try to Express my points with numbers... for this much time and money... you can get x.

I take your opinions as well informed and representing the ideal that people like me...who aren't mechanics as a first priority...should strive for.

Our cattle are in tip-top show condition every day. Bedded, combed, fat and happy. Our machinery is a little more utilitarian.

I'll also admit that, more than once, I've read your (and others') advice here and though.. "crap...didn't think of that...but I've already put the engine back together and it's running well...so I'll leave it alone and learn for next time."
 
Hi Everyone, I took the advice of conducting a leak down test. Borrowed one from the auto parts store. This is what I found:

55, 25, 20, and 20% loss for cylinders 1, 2, 3, and 4, respectively. I could only hear the air through the oil fill, not through intake, exhaust, or radiator fill.

So based on that with the compression numbers, now what do you think?

Thanks
Chad
 

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