2N Rebuild: Low Compression

breese

Member
I checked compression on my rebuilt 6V system 1945 Ford 2N engine: only 40 PSI at each cylinder. This is after new sleeves/pistons/rings/valves. I used the original connecting rods, cam-timing shaft, and drive shaft since all gear teeth & journals looked great & measured in spec. The 40PSI is consistent but far from the 95 PSI target. The ring gaps were all in-spec prior to installation of the pistons with the new sleeves. All 8 new valves were lapped and seals verified prior to adjustment.

My initial concerns are with the head-gasket & would like feedback on these 3 next steps.
1) I installed a head gasket (the type with metal rings around each cylinder) and have seen other bloggers complain about this design. Has the type of head-gasket really been an issue for others? Now is the time for me to try another type.
2) I installed the head-gasket dry: others claim to use copper spray for a better seal. I have a can of the Permatex copper gasket-seal spray: does this stuff really help to improve compression?
3) Lastly, I torqued the nuts on the head stud bolt to 55 ft/lbs: that seems low given the diameter of these grade 8 stud bolts. I've seen conflicting (higher) stud bolt nut torque specs but don't want to go above 55 ft/lbs unless others have.

My thinking is perhaps a consistently under-torqued head gasket (or poor-design, or without sealant) loses compression equally across all 4 cylinders. I have a running 2N (12V system) also 1945. For reference, I'm going to use my torque wrench and see where my 2nd 2N tractors head stud bolt nuts are set (where my torque wrench trips). Compression readings on two of the four cylinders on my running tractor are 90/92PSI so the compression gauge seems to be working.

Any other ideas/suggestions would be appreciated. Clearly, I don't want to start pulling new pistons/rings unless I absolutely have to. Thanks!
 
"new valves were lapped"-----valves not sealing are most likely some, if not all, of the problem.
 
Compression tests are not always conclusive. Since the numbers are consistent, with all due respect, was the compression test done with WOT, all plugs out, sufficient cranking speed, fuel off and five "hits" per cylinder on the gauge. Is it a known good gauge? I would address these issues, and if ok and compression is still very low, it's time for a cylinder leakage test.
 

I would wager your cam trimming is off a tooth are so... Check valve overlap... This would affect every cylinder... I seriously doubt its a head gasket issue at this time... Stop and confirm the root cause and quit treating the symptom...

Does it run :?:
 
I gotta agree with Hobo;
First of all does it run?
If so then your compression readings are wrong.
Secondly, if it wont run it's likely your cam timing.
 
If you try to torque on the other tractor, it will tell you nothing as they are (set) and will take a lot to break loose unless you back them off first.. Pull the plugs and put air pressure to each cylinder & see where it goes. Crank breather is rings, carb is intake valves & tailpipe is ex valves.
 


If rings were not offset would that cause low compression? Also wondering if installed without a generous oiling of everything then pressure would be less? - after running a bit maybe everything will seat properly and pressure increase?
 
without knowing more..and a new rebuild.. I'd have to third you and hobo's advice as well.

bad test or bad cam /valve timing.
 
First time posting to any tractor forum: I greatly appreciate everyone's response. Most of them were quite helpful.

I really like teddy52food's suggestion of pressurizing each cylinder and checking for leaks.
I'll look for a 1/4" quick-release to spark-plug thread adapter later today: I assume that's the most effective way to pressurize cylinders.

Additional background:
a) no - the tractor does not start
b) compression checked with all plugs removed

I'll post my findings in the next week or so: thanks again!
 
Follow-up to the “put air pressure to each cylinder” advice from teddy52food...

I researched leak down testing and bought a tester. As you may recall, my symptom is only 40 PSI of compression across all cylinders on my rebuilt 1945 Ford 2N engine (6V system). New sleeves, pistons, rings, and valves. The tractor does not start. Here are the (cold) leak down results; 2 readings for each cylinder, taken one day apart:

Cyl # & leak% result 1/2
1 – 35/35
2 – 35/30
3 – 40/35
4 – 25/30

All the relative leak down % values show up in the 'green' region of the tester gauge. I don't know how accurate the readings really are. However, it's a great troubleshooting tool. I did not detect any major leaks: a very faint flow from the oil breather. Vales seating looked good. No leaks until I intentionally cranked each cylinder into the exhaust/intake phases: then tons of air passing through raised valves.

My first question: for “cold” values – are these numbers OK?

My next step is to do a wet oil test & re-measure leak down %. Other input/questions welcome...
 
My next step is to check leak down with ~2ml of engine oil in each cylinder.
Any preference between 5W, 10W, and 20W weights?
 
(quoted from post at 07:40:29 01/13/14) Follow-up to the “put air pressure to each cylinder” advice from teddy52food...

I researched leak down testing and bought a tester. As you may recall, my symptom is only 40 PSI of compression across all cylinders on my rebuilt 1945 Ford 2N engine (6V system). New sleeves, pistons, rings, and valves. The tractor does not start. Here are the (cold) leak down results; 2 readings for each cylinder, taken one day apart:

Cyl # & leak% result 1/2
1 – 35/35
2 – 35/30
3 – 40/35
4 – 25/30

All the relative leak down % values show up in the 'green' region of the tester gauge. I don't know how accurate the readings really are. However, it's a great troubleshooting tool. I did not detect any major leaks: a very faint flow from the oil breather. Vales seating looked good. No leaks until I intentionally cranked each cylinder into the exhaust/intake phases: then tons of air passing through raised valves.

My first question: for “cold” values – are these numbers OK?

My next step is to do a wet oil test & re-measure leak down %. Other input/questions welcome...

If I were a gambling man I would feel good doubling down my wager rite now...Even with those leakage #' it should start and I would guesstimate 70 lbs of compression...

I have only run a leak down test on one fresh engine. Leakage ran 20/25% on the engine stand the best I remember... I did suspect leakage # to be on the high side till the rings broke in... Its normal for the rings to break in on the initial test drive if not I have always found you are in deep do-do...
 

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