Tecumseh rebuild specification questions - HobbyTalk
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post #1 of 8 (permalink) Old 12-01-2008, 09:09 AM Thread Starter
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Tecumseh rebuild specification questions

I’ve rebuilt several engines and had good success. On one though I still can’t get it to run. After lapping the valves and new rings it still leaks air out of the oil filler hole on a leakdown test. The valves and head geasket are fine. I thought the issue might be the rings did not seat so opened it up and rescored the bore. No luck – still getting air through the oil fill hole and won’t start (gas through the plug or in throat). The dimensions of bore, ring side clearance and clearance seem ok.
I'm not so good at getting the bore dimensions though. I have the T gauges and bought a digital bore gage and having trouble getting consistant measurements. With the T-gauge I get inconsistant results With the digital gauge I have trouble even with a machinests L block getting reliable measurements - operator error I guess.

From the Tecumseh 3-11hp L-Head specifications manual.

The bore specification is 3.125 and 3.126. Is that top and bottom factor dimensions or min-max? If it’s top and bottom then what are the wear limits for a good compression rebuild with standard rings? What distinguishes whether an engine will be fine with just rings versus require a re-bore?

Ring Groove Side Clearance .002 .005. Is this listing max-min?

Ring End Gap: .010 .020. Is this min-max? If it is and the reading with the old rings is half the range, .015. would the rings be considered fine?
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post #2 of 8 (permalink) Old 12-03-2008, 11:05 PM
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I don't understand what "rescored the bore" means. If you mean "honed," then I understand and want to ask if you left a 30-degree cross-hatch? Everything you went over seems within spec. Ring end gaps are typically (on OPE engines) considered good up to 0.030". New spec is usually 0.010 to 0.020 depending on the OEM.
If you put new rings into a resurfaced bore, you will not get a high compression reading nor a low leak-down reading until the rings seat (break-in).

Have you checked the piston skirt for wear? There should be machine marks (lathe cuts) somewhat visible all the way around the piston. On the thrust faces of the skirt, a wear pattern of up to 20% is okay, more than that indicates wear. You can use a feeler guage between the skirt and the bore to check clearances, although that method hasn't been used in years. New clearances are typically 0.0015", with a max. wear factore being about 0.005". If the piston is worn, it'll rock back and forth between the compression and power cycles allowing blow-by.
I'd say it's either a worn skirt or the bore surface isn't correct.
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post #3 of 8 (permalink) Old 12-04-2008, 08:08 PM Thread Starter
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>>I don't understand what "rescored the bore" means. If you mean "honed,"
Yes, rescored is honing. No measurement on the degree of the crosshatching. The cross hatching was just whatever was left by running the hone up and down quickly.

How long does it take for the rings to seat – roughly? How much run time? I have run it for a short period a few times but have not re-done a leakdown test yet.

For piston wear you mentioned a “wear pattern of up to 20%”. Does that mean the surface worn smooth for up to 20% of it’s surface and I guess that surface is the surface from the bottom of the oil ring to the bottom edge of the piston. Is that correct? Unfortunatly I did not check that. The engine was slightly low in the leakdown so opened, re-honed and put back together 4 days ago.

>>You can use a feeler guage between the skirt and the bore to check clearances, although that method hasn't been used in years. New clearances are typically 0.0015", with a max. wear factor being about 0.005".
How is that done with a flat feeler gauge i.e. flat feeler gauge measuring a curved surface?

Asking about these two again:

The bore specification is 3.125 and 3.126. Someone told me those figures were the top and bottom of the bore sizes. So for these specs the 3.125 is the measurement expected at the top and 3.126 the size of the bottom. He then went on to say the bottom was usually several thousands larger than the top so the spec’s did not match what he said. So I’m confused if the 3.125 and 3.126 is the min max for the top or what.
Along with this what is the max wear in thousands of the bore. For example if the figures above indicate the rough size of the bore is 5 thousands over too much, 10, 20??

>>Ring Groove Side Clearance .002 .005. Is this listing max-min?
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post #4 of 8 (permalink) Old 12-05-2008, 12:39 AM
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If you move a hone up and down "quickly," you may end up with a much greater angle than 30 degrees. This could certainly create poor sealing capabilities.
To judge the angle you produce, take the up-down hatch marks, and estimate the angle between the two. For example, see

A greater-than and less-than symbol >< back to back are about 30 degrees (each) in this type font and represent a good cross-hatch.

The area below the oil ring land is called the skirt. The skirt, over time due to contaminants will wear. I've seen engines with 1700 hrs. with less than 3% skirt wear pattern. All depends on environment and maintenance. 20% is considered to be the max. amount before it's over spec. When the skirt is very worn, the piston cocks in the cylinder. Rings can't seat right then, and often skirt cracking occurs.
This is an example of excessive wear, resulting in a skirt crack:

When measuring skirt clearance with a feeler gauge, a flat blade one will flex enough to check the fit. That is, unless there's so much wear you get up to too thick a blade!

I've never heard of bore being intentionally tapered. Unless you loitered with a hone, don't know how you'd make it tapered. You can't do that with a boring bar - at least none that I've used. Taper is a result of uneven wear, not machining. The top end is usually worn the most, just below the "ridge" if one has formed.
Without seeing the manual the specs. (and thus the context) you cite are derived from, I can't tell you but usually a bore or piston will have an original dimension, often with an allowable deviation. However, 0.001" is a lot of deviation for a machining process, but for an air-cooled lawn mower engine, 0.001" is not a lot. So, the orig. spec. could be 3.125", or anything up to and including 3.126" being allowed. Valve guide clearances are a good example of more "slop" being permissable over say a liquid cooled car engine. Almost double the clearance is allowed by spec.

Ring side clearance. This can definitely affect compression, and to my best recollection about 0.005" would be about max. spec.
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post #5 of 8 (permalink) Old 12-06-2008, 09:38 AM Thread Starter
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The spec’s are from the “3 to 11 hp 4-Cycle L-head Engines” Tecumseh Technician’s Handbook.

The bore is listed as 3.125 – 3.126. That confused me because a knowledgeable rebuilder told me the values specified were top and bottom values. And, that the bottom of the bore was larger than the top of the bore in a new motor. He also said the difference was 5-8 thousands. ? We talked about many things that day and possibly I mis-understood some of what he said. From what you are saying and the spec I assume the specification figure is the factory expected range of an acceptable bore. What values would be too far out for a successful rebuild on small engines like 5-12 hp Tecumseh L-heads? From what you wrote, double, I guess that would be anything past 3.127+.

Thanks for the photos. I thought the wear occurred at the bottom of the skirt. I can’t remember the wear on the skirt. The piston is in the motor and will get re-run time and hopefully a better leakdown value. Actually the engine seems very strong and the leakdown very close to a new motor. I thought the rings might not be seated fully and suspected the lack of proper honing I had done so unbuttoned the motor and did it again. My honing on the second pass does not look as good as the picture but I think it’s close enough.

OK on the feeler gauge use for side clearance. I was not doing that measurement but will start on the next rebuilds.

Thank you for the help.
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post #6 of 8 (permalink) Old 12-06-2008, 04:58 PM
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Yes, most skirt wear does usually occur near the bottom - just thought the pic would be a good representation of a wear pattern.
Yes, I would say that the 0.001" deviation of spec. given in the manual would be an acceptable range.

I will restate that I've never heard of a bore being intentionally tapered. Heard of oval cylinders (Honda race bikes) back in the 80's.
Air-cooled engines have higher expansion rates than liquid cooled, especially flat-head air-cooled. Because of these higher expansion rates, the permissible tolerances are higher than that of liquid-cooled.

For bore wear in an air-cooled engine, you can either go by what the factory publishes as acceptable limits, or what you might call a general rule. When I did auto-engine rebuilds, up to 0.002" bore wear was acceptable, with 0.005" being a definite candidate for rebuild. Although, 0.005" wear doesn't necessarily mean there will be problems, such as oil consumption, low power etc. It could run perfectly fine. Ring-end gap and skirt wear will be contributing factors - it all comes together!

The only other thing I can suggest, is to ask if you staggered the rings? Kinda think you did though, given your knowledge.

And again, skirt-clearance with a feeler gauge would probably be considered arcane today. I asked Kohler techs about not publishing this spec. years ago, and they told me with the Mahle pistons they use you couldn't go by that anymore. But then, I didn't always agree with what I heard at the factories.
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post #7 of 8 (permalink) Old 12-09-2008, 10:24 AM Thread Starter
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The rings were staggered when put in so should be ok. I have run the engine a few more times and it seems ok, no smoke or droplets out of the breather. There’s no snow around to load it up but it does pull me off my feet when I struggle to hold it from going forward. The leaddown is 5-10% now so I’m happy with that. The problem must have been the very light honing.
OK on no parameter being a knockout and considering them together. That seems reasonable.
The overall problem here is not being trained in a school or shop so I come across puzzlers all the time.
Thanks for the help.
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post #8 of 8 (permalink) Old 12-09-2008, 07:45 PM
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Glad to hear it. Since this is a snow engine, remember that with the heater box (cover) over the carb. it'll overheat if it's above, say 40 degrees where you're running it. The box keeps the carb. and linkages from icing up.
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