The manual i have covers all L head engines up to 11 HP but i don't know if it includes engines that go back that far or the 12 HP. It seems about right though.
I can email you a copy if you want (i also have one for the OHV engines), just PM with an email address.
The valve guides are permanently installed in the cylinder block. If they get worn excessively, they can be reamed
oversize to accommodate a 1/32" (.793 mm) oversize valve stem.
The guides should be reamed oversize with a straight shanked hand reamer or low speed drill press. Refer to the
"Table of Specifications" (Chapter 10) to determine the correct oversize dimension. Reamers are available through
your local Tecumseh parts supplier. Consult the tool section in Chapter 11 for the correct part numbers.
The upper and lower valve spring caps must be redrilled to accommodate the oversize valve stems.
After oversizing the valve guides, the valve seats must be recut to align the valve seat to the valve guide.
neat old engine. I had a 16hp 1-lung tec from the 70's (I think, I bought it in the mid-80s needing repair) and it was extremely well-built, solid iron, OHV, counterbalanced, roller-bearing'ed, and it was fun to tinker with.
Yes, I do believe that they spec'd an oversize valve stem for those engines, but check if they are available before doing anything. Knurling is also a good fix if they aren't TO bad. Are the valve stems also worn? Usually about 1/2 of the wear is on the valve itself. Possibly knurl, with a new stock valve if oversize is not available.
The valve was changed several years ago, along with a rebuild. I wouldn't be working on it but one of the screws from the throttle blade came loose and did a number on the piston, rings,exhaust valve and guide.Last rebuild, the guide was reamed and an insert was installed by a small shop. They not around anymore.
I restore once high end communications gear (Collins, Drake, ect) as a hobby and there are no "new" replacements which are as well built. Work better, but just not the same materials quality. Rebuilding can be fun, but in your case I'm sure you can find an up to date replacement engine from Northern Engineering or like vendor and avoid a lot of agro. I still have a Tec lawnmower engine I bought in 1975 which ran (on 20W - 50) until I bought a new Honda mower in 2000 and stopped using it , so I have some respect for the brand.