OHSK90 Tecumseh OHV Build

[karl]

Something I have wanted to do for some time, play with a Tecumseh OHV motor. My other OHSK90 is clapped out. So I found a sufficient sacrifice for the build. Bonus, it has the high output duel charge coil, 5 magnet flywheel. I will use a regular flywheel , no magnets or ring gear, and a scatter guard for this build.

Most make fun of Tecumseh engines , and call them junk.

As long as the oil gets changed , and the governor is working right, solid motors.

Anyway, the cylinder head seems promising , large valves, big ports, crossflow head, I think with a large slide carb and around 11 or 12 to 1 compression, 50lbs springs, port it, billet rod , the works. 318cc, should scoot.

Might burn some rubber

 

[karl]

Teardown!

 

[landuse]

Nice motor. It should put out some nice power

 

[JTSpeedDemon]

Besides the carburetors and the governor issues you mentioned above, karl, one of the biggest reasons I would never put much money into one is the (mostly) lack of performance parts. There's just not as many out there as Briggs flatties and even Predators.
Don't get me wrong! If I got a clapped go kart with a Tecumseh on it, I would put a little money into getting it running! But no more than what it would take to get an equivalent Briggs running.

Although now I'm wondering if it would be possible to transplant one of those heads onto another engine......

 

[J.S.@SMS]

Actually tecumseh did that from the factory. The ohv 11hp was a flattie with an overhead valve head bolted to it. I believe it was the hm70-100s.

And I like tecumseh's. Other than over complicated carburetors, they're pretty solid little engines. I'm actually in the process of rebuilding an OHH60 right now.

 

[karl]

Just want to build something different, unique , and a challenge

The budget for the build is $200 total, and hopefully will get close to 20hp. Still cheaper than a predator 301.

The blocks are very similar to the flathead, but not exact by any means. Most parts will interchange tho.

The motor on the right is the crank and side cover donor for the build, a lh318sa, that uses the same rod, piston, and many other parts as the ohsk90

 

[karl]

Test fit the crank and side cover donated from the 8hp flathead, perfect.

Need a sheet of Plexiglass so I can CC the head, and see how much material needs added and removed to reach the compression ratio I want. And needs a good scrubbing.

 

[JTSpeedDemon]

About how many HP are you expecting from this build?

 

[karl]

About how many HP are you expecting from this build?

Good question, I am still doing research, I want to have my cam reground to Tecumseh star 703 profile, RPM 6000+, 265 duration at .003 , .325 max lift.

And with the head work , compression ratio, about a 30mm slide carb, and 110 octane leaded, 20-30hp might not be unreasonable.

I cant find much on the later OHV star engines, but I found this, seems like people agree these heads flow nice.

The successor to the Tecumseh Star, is the OHV version. The 9,10,11 hp OHV Tecumseh with a 3" stroke, provides 380cc's of absolute dynomite.
This is because the crank and valves placement and geometry, is superior to the competitors. It's not magic, it's just basic engineering.

The 11 and 12 hp Tecumseh OHV is a little bit taller block, and has, interestingly enough, the big bore of the Star fhead. This is 3.36 (3 and 5/16 inch bore), where as the 9 and 10 hp
engine is 3.06" bore(these are all approximates).

 

[karl]

Of course, right after I get into the build, I was offered a free gx390 and I could not say no, bonus, it runs

Soon I will have a battle of the big blocks, tecumseh vs briggs vs honda vs clone

Blocks nice and clean, working on the head so I can take some measurements.

 

[J.S.@SMS]

I take it the 390 came off a pressurewasher. And you have quite the stockpile of engines. Did you convert that briggs oppy to a horizontal shaft?

 

[karl]

I take it the 390 came off a pressurewasher. And you have quite the stockpile of engines. Did you convert that briggs oppy to a horizontal shaft?

Concrete buggy, that's nothing, not counting what's in the garage

I did convert the Briggs, couple years ago, fun times.
Held together well for how much time it has spent at 5k

The head is 35cc. So I have a baseline

 

[karl]

The head is 35cc. So I have a baseline

So based on that ,

Stock compression ratio 8.71 to 1

Smacking stock head directly on deck 9.75 to 1

Removing 2cc + no gasket 10.25 to 1

Removing 5cc + no gasket 11.14 to 1

Removing 7cc + no gasket 11.83 to 1

And sending it, removing 9cc, leaving us with 26cc, no gasket,

12.62 to 1

Guess what Imma do lol

So I checked the bore at various points, and the ring gap did not exceed .014, spec is .010 to .020, and the bore looks nice, so It will get sent as is.

Once I get the #6286 ARC billet rod in, and figure the cam out, I can check valve clearance , and know how high of a compression ratio I can get away with on the unmodified combustion chamber.

 

[Haul_A$$_]

Nice to know I'm not the only one who stockpiles engines very nice collection I like what your doing with that engine I have one similar so I'm interested to see what you do to it thanks in advance for all the ideas lol

 

[mckutzy]

Wow this is cool... Idk how I missed this build....

If they can OHV a Ford Model A, then it can be done on a flat head Briggs/Tec.... As long as the bolt pattern matches and you can figure out some push rods... good to go...
Now obviouslly there more to it than that, but this so cool...
Id like to hear this run.....

 

[karl]

Thanks guys ,

idk if you misunderstood, but it is a native OHV engine, sorry to disappoint

I have been working on other things, and the Tecumseh has not got any attention.

But now that im laid off, as lawn mowers are non essential, I might have some time for it.

Today I got my repowered 1969 cub 106 driving with one of the big blocks I fixed, briggs intek I/C 305cc 10hp. It was on a leaf blower, and snapped the crank, but perfect cast iron bore. New crank seals, bushings, and rod, plus donor crank, charge coil , and ring gear flywheel from snowblower engine. I modified a 12v opposed twin starter to turn it, and made the flywheel drive work, shifts and runs great. Starts so easy. Custom motor mount. Total engine cost $80. The old kohler was worn beyond repair, and will be used as parts for other tractors.