This began as a question about ideal ring end gaps for a Tecumseh HH120. Ben kindly provided that info.
I then decided to convert the thread into my Tecumseh HH120 build. I won't be covering every detail; just anything that seems maybe interesting along the way. The following goes back to the original text:
Ok, I've spent some time searching this here and elsewhere. Most of what I find is centered on how big the ring end gaps might be permitted in a worn bore before reboring is necessary. Of course, there are other influences for when to rebore besides ring end gap. But that is not what I'm looking for.
The Tecumseh manual simply says .007" and .020" for ring end gaps. They don't specify whether that is a range for the top ring or all rings. Or maybe 20 is for the top, 7 for second ring since it runs a lot cooler?
This block has just been bored and honed to .020" over with .003" piston skirt clearance. I'm looking for the ideal ring end gaps for all 3 rings for when I check and/or file fit as needed.
This engine is for my Sears Super Hydro-Trac 12 and is intended to be my main snow pusher. However, assembly is going to be delayed until we get out of arctic temps. No sense trying to heat a garage in zero degree ambient temps. Although I have 2 artificial hinges and am in danger, I'll just have to use the snow thrower until 'el tractorsaurus' is back together.
The old (automotive water cooled) formula for generic low perf ring end gap is bore size in inches times .004 for the top ring. So with a 3.5" bore, that would be an ideal top ring end gap of .014".
However, that formula is for water cooled engines. Air cooled engines run a lot hotter, right?
The formulae online suggest the 2nd ring should run looser than the top ring. I really don't get that since the 2nd ring absolutely runs at lower temp than the top ring...
Anyway, trying not to overthink this.
Can anyone please provide ideal ring end gaps for top, 2nd and oil rings on an air cooled Tecumseh HH120?
p.s.: Valve work ramble: This is too lengthy so nobody should read it.
After 2 months of whining, I finally had to take the block back from the machine shop after they had only completed the bore and hone... and do the valve work myself.
Even though I provided the Tecumseh reamer for the 1/32" oversize valve stems, you'd have thought I was asking for cold fusion. Turns out even though this later model reamer was made in India and probably not nearly as good as the earlier spiral flute reamers, hand reaming going slow and careful took 10 minutes on the intake guide, 20 minutes on the exhaust guide. No huhu. If there was any challenge, it was this: Valve guides wear out in a so called "bellmouth" manner. That is, very loose at top and bottom; little wear in the middle. The challenge was this: The reamer's pilot was tight at the top worn out area. Getting it through the MUCH tighter unworn center area was gummy and scary tight at times. I twisted/removed the reamer a dozen times to clean out cuttings and to re-oil with ATF. Although the reamer pilot is fluted, it doesn't cut much. Nothing like the main cutter flutes cut. Anyway, it is totally doable and there is no excuse for 2 months of whining.
BTW: The newer Indian reamer lacks a square cut on the end. The machine shop balked about this. I spent 10 minutes on a bench grinder and carefully cut flats for a square end. These turned out better than most squares I've freehanded but in any case worked just fine with a tap handle. Because of no flats on the drive end, some may assume that it is a "chucking" reamer. However, my machinist and I agree that making a .030" reamer cut in one pass is an awful lot and must be done SLOWLY by hand. My suggestion would be: Please do not chuck this in a drill or mill. Especially with the binding of the pilot near the center of the valve guide, I suspect using power drive would result in a broken reamer or a badly tapered bore (from heat), etc.
I then used my Dad's left over valve equipment from when he had a machine shop in the 60's and 70's. Such as: Sioux and K.O. Lee valve grinding and hard seat equipment and did a fair dinkum 4 (yes four) angle valve job. Things lapped in super fast and easy so I think everything was probably concentric. Forced to DIY, I didn't have valve seat runout dial indicator, etc.
The exhaust valve from Argentina had an enormous stem underhead. Close to 1/2 inch in diameter if you can believe that... I couldn't believe it. It would have choked the poor little engine to death so I undercut both intake and exhaust valve stems back to 5/16" (not in the guide area; only in the port area) and polished them in the portions of the stems that live in the ports.
New valves should always be reground. In this case, the Argentinian exhaust valve wobbled by 30 thousandths. Holy cow?! The NOS Tecumseh intake was "only" out by 8 thousandths. The legendarily accurate K.O. Lee Valve grinder brought them both to true although the exhaust took quite a long time to get right. I was shocked. Glad I took the time, though. The machinist was going to trust the factory grind. Yikes.
Since I was forced to DIY on the valve work, I took a couple of decades of porting experience (former speed racer type here) and went after the low hanging fruit. I studied up on garden tractor puller mods beforehand. The intake short side radius was a train wreck OEM with a razor sharp 90 degree corner in there. There were a few other obvious enormous defects. 80% of my time was on intake, 20% on exhaust. There were some obvious ridiculous defects and I went after mostly just the crazy stuff. About an hour of porting time between the 2 ports.
I live at 7150 feet elevation, so every engine loses a lot of power up here. Hoping to get a bit of the high altitude losses back.
The intake seat cut about like normal. The exhaust seat required white stellite stones and was harder than the hinges of hades. Although, it was softer over by the combustion area. Had to diamond dress even stellite stones after every single cut. Tedious, but ya' know. I back cut both intake and exhaust valves with a 30 degree cut up within .030" of the lap line. That's supposedly worth some 10% of 'mousepower' on small flathead type engines just by itself. Yeah, I know. "Wish in one hand..."
One other rumor I'm afraid I must confirm: The NOS valves with 1/32" oversize valve stems are LONG. About .060" long. As you may or may not know, the butt grinding stone on most valve grinders including my Dad's antique KO Lee are very fine. It took about an hour each to sneak up on the proper valve lash. I got them to within about a half a thousandth but it just about cost me my sanity. Ok, now I'm whining. mm out
Just my experience; your mileage will vary.
edit: I found the Tecumseh reamer for this PN 670284 in stock at Equipatron. Although it is the newer Indian reamer, it worked ok with some minor mods and patience as outlined above. Those guys shipped super fast and were very friendly. It was roughly 40 clams shipped.
Edited by MountainMichael, November 14, 2014 - 10:48 PM.