Compression issue in Farmall Cub Loboy

[TractorBob]

Hi, my brother rebuilt a Farmall Cub Loboy early 60's. Did Values, put on new standard rings, and head gasket. He huckleberry the cylinders. The motor ran for awhile and then cylinders began to loose compression. I took the head off and checked the cylinder size. Took 4 measurements at top and 4 at bottom. Measurements range from 2.627 to 2.629. According to the manual that should still be within acceptable spec. He used standard rings and the ring gap at the bottom of the cylinder is .012 and .024 at the top. Manual calls for .007 to .017.

He does not want to pay for a rebore. Looking for suggestions on what to do next. Is the bigger gap in the rings that big a deal? This is my first time doing this so any input would be helpful.

Thank You Bob.

 

[Fritz Maurer]

How do you know the compression loss is due to the rings?

 

[used red MN]

Hi, my brother rebuilt a Farmall Cub Loboy early 60's. Did Values, put on new standard rings, and head gasket. He huckleberry the cylinders. The motor ran for awhile and then cylinders began to loose compression. I took the head off and checked the cylinder size. Took 4 measurements at top and 4 at bottom. Measurements range from 2.627 to 2.629. According to the manual that should still be within acceptable spec. He used standard rings and the ring gap at the bottom of the cylinder is .012 and .024 at the top. Manual calls for .007 to .017.

He does not want to pay for a rebore. Looking for suggestions on what to do next. Is the bigger gap in the rings that big a deal? This is my first time doing this so any input would be helpful.

Thank You Bob.

Is there scoring (vertical scratches) in the cylinder walls? Usually loss of compression is accompanied by this, unless the rings installed did not fit the piston grooves properly. What were the compression check numbers? You said “Did valu(v)es” what does that mean? You set the valve clearance (lash) correctly?

 

[wore out]

Hi, my brother rebuilt a Farmall Cub Loboy early 60's. Did Values, put on new standard rings, and head gasket. He huckleberry the cylinders. The motor ran for awhile and then cylinders began to loose compression. I took the head off and checked the cylinder size. Took 4 measurements at top and 4 at bottom. Measurements range from 2.627 to 2.629. According to the manual that should still be within acceptable spec. He used standard rings and the ring gap at the bottom of the cylinder is .012 and .024 at the top. Manual calls for .007 to .017.

He does not want to pay for a rebore. Looking for suggestions on what to do next. Is the bigger gap in the rings that big a deal? This is my first time doing this so any input would be helpful.

Thank You Bob.

If it's losing compression past the rings it would have a lot of "blowby". Have you noticed a lot of blowby?

How long was it run and WORKED some to seat the new rings?

Were the cylinder bolts retorqued after it was warmed up, and did the head gasket show any signs of leaking between the cylinders when you took the head off the last time?

What was done with the valves during the "rebuild"?

Has the valve tappet gap been checked after it lost compression? What were the compression readings before you pulled the head?

Had a "cylinder leakdown test" been done before the head was removed you would know where the compression was going.

https://www.ringspacers.com/tips.htm

Above is a link to some info from a piston ring GURU about ring gap and ring groove wear that may be of interest.

 

[TractorBob]

How do you know the compression loss is due to the rings?

How do you know the compression loss is due to the rings?

We dont know for sure its the rings. No sign of leaking around the head gasket. We tested the valves for leaks and they seem to be seating fine. The valves were lapped and adjusted to the .015 gap.

 

[TractorBob]

Is there scoring (vertical scratches) in the cylinder walls? Usually loss of compression is accompanied by this, unless the rings installed did not fit the piston grooves properly. What were the compression check numbers? You said “Did valu(v)es” what does that mean? You set the valve clearance (lash) correctly?

No Scoring. Lapped valves and adjusted to .015 gap. Compression reading were poor 45 lbs.

 

[TractorBob]

If it's losing compression past the rings it would have a lot of "blowby". Have you noticed a lot of blowby?

How long was it run and WORKED some to seat the new rings?

Were the cylinder bolts retorqued after it was warmed up, and did the head gasket show any signs of leaking between the cylinders when you took the head off the last time?

What was done with the valves during the "rebuild"?

Has the valve tappet gap been checked after it lost compression? What were the compression readings before you pulled the head?

Had a "cylinder leakdown test" been done before the head was removed you would know where the compression was going.

https://www.ringspacers.com/tips.htm

Above is a link to some info from a piston ring GURU about ring gap and ring groove wear that may be of interest.

Not sure about blow by. I was run for 5 hours. At start 1 cylinder was not firing and after 5 hours 2 were not firing. Never retorqued head. Valves were lapped and adjusted to .015 gap. No we did not do a leakdown test. Thanks for the tips about piston gaps. I checked the gap with ring in piston. I did not check the spacing between ring and piston sidewall when ring is on piston. What is acceptable clearance for that.

 

[TractorBob]

Not sure about blow by. I was run for 5 hours. At start 1 cylinder was not firing and after 5 hours 2 were not firing. Never retorqued head. Valves were lapped and adjusted to .015 gap. No we did not do a leakdown test. Thanks for the tips about piston gaps. I checked the gap with ring in piston. I did not check the spacing between ring and piston sidewall when ring is on piston. What is acceptable clearance for that.

He said he was getting good spark and put new wires plugs and points in.

 

[Fritz Maurer]

We dont know for sure its the rings. No sign of leaking around the head gasket. We tested the valves for leaks and they seem to be seating fine. The valves were lapped and adjusted to the .015 gap.

Can you just ascertain that the valves are opening and closing at or near top dead center, so we can eliminate any potential timing issue.

 

[Janicholson]

Not sure about blow by. I was run for 5 hours. At start 1 cylinder was not firing and after 5 hours 2 were not firing. Never retorqued head. Valves were lapped and adjusted to .015 gap. No we did not do a leakdown test. Thanks for the tips about piston gaps. I checked the gap with ring in piston. I did not check the spacing between ring and piston sidewall when ring is on piston. What is acceptable clearance for that.

The ring gap is measured as follows: the cylinder (piston in or out makes no difference) is wiped clean. A piston ring from the packaging is placed in the cylinder (not on a piston) and pushed down evenly to a depth just below where the top ring would stop moving. The gap is now measured between the ends of the ring at the gap with a feeler gauge, maybe using more than one blade. .006" pre inch of piston diameter is considered reasonable. I think something other than what is being discussed is the issue. Compression is measured by removing all spark plugs, opening the throttle, making sure the battery is fully charged, and having 6 compression strokes for each cylinder being tested. Next a teaspoon of oil is put in each cylinder and the test made again. Getting to the answer requires real analysis. Jim

 

[grandpa Love]

Not sure about blow by. I was run for 5 hours. At start 1 cylinder was not firing and after 5 hours 2 were not firing. Never retorqued head. Valves were lapped and adjusted to .015 gap. No we did not do a leakdown test. Thanks for the tips about piston gaps. I checked the gap with ring in piston. I did not check the spacing between ring and piston sidewall when ring is on piston. What is acceptable clearance for that.

Wait.....??? At start one wasn't firing? After 5 hours 2 weren't firing?? That's odd? Plugs? Wires? Distributor? I dunno ....not sure about blow by? That's really easy to see......

 

[used red MN]

Not sure about blow by. I was run for 5 hours. At start 1 cylinder was not firing and after 5 hours 2 were not firing. Never retorqued head. Valves were lapped and adjusted to .015 gap. No we did not do a leakdown test. Thanks for the tips about piston gaps. I checked the gap with ring in piston. I did not check the spacing between ring and piston sidewall when ring is on piston. What is acceptable clearance for that.

“I checked the gap with ring in piston.“?? I assume you misspoke here, ring gap has to be checked with the ring in the cylinder bore. As Jim describes.
“I did not check the spacing between ring and piston sidewall when ring is on piston.” Ring to piston groove side clearance should be .002” - .004” that might be a tight spec for your situation. My comment was mostly aimed at not installing as an example a 1/16” ring in a 3/32” groove.
ARE YOU SURE IT’S NOT A PROBLEM WITH YOU COMPRESSION GAUGE?
Use it to test the compression in a cylinder of some other engine. Or test it with compressed air.

 

[rustred]

Hi, my brother rebuilt a Farmall Cub Loboy early 60's. Did Values, put on new standard rings, and head gasket. He huckleberry the cylinders. The motor ran for awhile and then cylinders began to loose compression. I took the head off and checked the cylinder size. Took 4 measurements at top and 4 at bottom. Measurements range from 2.627 to 2.629. According to the manual that should still be within acceptable spec. He used standard rings and the ring gap at the bottom of the cylinder is .012 and .024 at the top. Manual calls for .007 to .017.

He does not want to pay for a rebore. Looking for suggestions on what to do next. Is the bigger gap in the rings that big a deal? This is my first time doing this so any input would be helpful.

Thank You Bob.

first thing is , yes the cylinders are worn,and have some extra taper, but that is not enough to make it not run. u said a valve lapping was done, well thats not a valve job. to lap valves you must see that the valve seats are in spec , and if they are wide and pounded out lapping does nothing. also was everything cleaned spotlessly clean. all it takes is a spec of carbon to get under the valve and stuck in the seat and there u go,... loss of compression. not there to have eyes on the subject . i would run it at a fast idle and see what happens. then recheck that compression as 45 lbs is poor. jim gave you the procedure. plus a recheck of valve setting to make sure none are tight causing this problem.

 

[used red MN]

We dont know for sure its the rings. No sign of leaking around the head gasket. We tested the valves for leaks and they seem to be seating fine. The valves were lapped and adjusted to the .015 gap.

“We tested the valves for leaks and they seem to be seating fine.”? How was the valve test accomplished?
Adding to Fritz’s number 9 reply here’s what you need to do. Turn the engine to the point that the exhaust valve is closing on number 4 cylinder. Turn it to the point that the intake and exhaust are opened the same amount. At this point the number 1 and 4 pistons should be at the very top of their bores. (TDC) If not your cam timing is off. This requires the #4 cyl. valve clearance to be set correctly.

 

[used red MN]

Wait.....??? At start one wasn't firing? After 5 hours 2 weren't firing?? That's odd? Plugs? Wires? Distributor? I dunno ....not sure about blow by? That's really easy to see......

Which cylinders were not firing? 1&2 or 3&4? A vacuum leak on the intake manifold gasket would cause this, particularly noticeable at an idle.

 

[rustred]

Hi, my brother rebuilt a Farmall Cub Loboy early 60's. Did Values, put on new standard rings, and head gasket. He huckleberry the cylinders. The motor ran for awhile and then cylinders began to loose compression. I took the head off and checked the cylinder size. Took 4 measurements at top and 4 at bottom. Measurements range from 2.627 to 2.629. According to the manual that should still be within acceptable spec. He used standard rings and the ring gap at the bottom of the cylinder is .012 and .024 at the top. Manual calls for .007 to .017.

He does not want to pay for a rebore. Looking for suggestions on what to do next. Is the bigger gap in the rings that big a deal? This is my first time doing this so any input would be helpful.

Thank You Bob.

a person needs to find out if this is a spark issue or compression. your brother did the job, now u took over saying its your first time doing this. so that leaves lots of room for different explanations. no idea if this tractor is running at the moment or not. if its running pull the wire off the plugs one at a time and actually see if you have spark there on each plug. if so remove them plugs in order and post a picture of them, just reading the plugs u can tell what the engine is doing. and do you compression test and post both results . also is this new plugs? show the old plugs also. plugs tell the story if an engine was burning oil. and last you need to get that engine hot and under a good load to seat them rings. "cylinders began to loose compression" is confusing and is not the normal. its the other way around.

 

[Janicholson]

a person needs to find out if this is a spark issue or compression. your brother did the job, now u took over saying its your first time doing this. so that leaves lots of room for different explanations. no idea if this tractor is running at the moment or not. if its running pull the wire off the plugs one at a time and actually see if you have spark there on each plug. if so remove them plugs in order and post a picture of them, just reading the plugs u can tell what the engine is doing. and do you compression test and post both results . also is this new plugs? show the old plugs also. plugs tell the story if an engine was burning oil. and last you need to get that engine hot and under a good load to seat them rings. "cylinders began to loose compression" is confusing and is not the normal. its the other way around.

If there was no ridge reaming done, the top rings may have crashed! Jim

 

[Fritz Maurer]

explanations. no idea if this tractor is running at the moment or

The head isn’t on it , I would guess ‘not’

 

[used red MN]

a person needs to find out if this is a spark issue or compression. your brother did the job, now u took over saying its your first time doing this. so that leaves lots of room for different explanations. no idea if this tractor is running at the moment or not. if its running pull the wire off the plugs one at a time and actually see if you have spark there on each plug. if so remove them plugs in order and post a picture of them, just reading the plugs u can tell what the engine is doing. and do you compression test and post both results . also is this new plugs? show the old plugs also. plugs tell the story if an engine was burning oil. and last you need to get that engine hot and under a good load to seat them rings. "cylinders began to loose compression" is confusing and is not the normal. its the other way around.

ur brother did the job, now u took over saying its your first time doing this

My guess is he is the more “computer literate” brother relaying the information to try to solve their problem. I agree it is always nice to know the full story.