# Tecumseh OH318?



## ELaw (Feb 4, 2015)

Hey... does anyone here know anything about the Tecumseh OH318 engine?

There are a few people selling them at very attractive prices online (example: https://www.smallenginesuppliers.com/shop/html/pages/products/tecumseh_horizontal_shaft1926.html). It's an odd design for an OHV engine, as the cylinder is vertical and not at an angle. It really looks like they just took one of their old medium-frame block designs, tweaked it a little, and stuck an OHV head on it.

The question I'm really trying to get answered is what parts might interchange between it and an HMSK110. The flywheel and alternator I'm most curious about. The two engines take the same magneto so that says to me at least the OD of the flywheel is the same. I'd also be interested if the crankshafts interchange?

The cheap ones online are made for generators so they have a tapered output shaft. I have someone that might be able to machine the shaft to be more suitable for use with my snowblower, but I'm curious if just swapping the crankshaft from my current engine might be a possibility.


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## Kiss4aFrog (Nov 3, 2013)

Have you thought about dropping a note to the seller and asking about the crankshaft swap ?? I'm guessing they might have the information available to figure it out.


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## AriensPro1128 (Nov 9, 2014)

Tecumseh Parts

All sorts of Tecumseh specs and info here.


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## AriensPro1128 (Nov 9, 2014)

Tecumseh Service Manuals

Should have provided this link to make it easier.


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## ELaw (Feb 4, 2015)

Hey thanks!

I actually looked at a few places online and you'd think the parts (flywheel in particular) are not compatible because they have different numbers. But I know sometimes there can be other factors... like the HMSK110 flywheel has magnets for an alternator but the OH318 does not. But the l-head flywheel might otherwise be the same and fit.

I actually emailed smallenginesuppliers and someone replied to me indicating he thought the alternator and flywheel would swap over since there are variants of the OH318 and HMSK110 that used the same flywheel.

I may just have to buy one of the OH318s and find out for myself... :icon_whistling:


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## dbert (Aug 25, 2013)

ELaw said:


> I may just have to buy one of the OH318s and find out for myself... :icon_whistling:


I thought about it while reading this thread. Thinking there must be a way to fit some pulleys on that crank. 
Keep in mind this is heavy. $40 shipping to my house.
pic of shaft from another site.


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## ELaw (Feb 4, 2015)

dbert said:


> there must be a way to fit some pulleys on that crank.


Well luckily I've got an ace up my sleeve in that respect! 

I've got a friend who's a machinist that I barter with, and I'm 99% sure he could turn that crank into one identical to what I've got, which is the 3/4" x 2-1/2" variant. It probably could be turned down to a straight 7/8" too, as long as you need about 2-3/4" length or less.

Actually I think the biggest challenge at this point is getting the accounting department (my wife) to agree to putting *another* $160 (cost of motor with shipping) into this snowblower... :icon_whistling:


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## ELaw (Feb 4, 2015)

Just an update...

I received an email reply from the other seller I contacted, indicating that the alternator, flywheel, *and* crankshaft can be swapped over from the l-head engine.

We'll see if that's the case because... I ordered one of the OHV engines! 

There will be a lot of challenges... in addition to the stuff mentioned above, I need to figure out how to attach my gas tank to it (it comes with none) and make the throttle adjustable. My research indicates there are variants of this engine with throttles and fuel tanks similar to the ones on my old engines, so I'm hoping with some creativity those things can be swapped over.

*Since this has gone from theoretical to real, I'll post any further updates on my ST1032 build thread
*EDIT: I changed my mind. Since this thread is about the engine specifically, I'm going to continue posting stuff that's relevant to the engine here... it just seems to make more sense.
http://www.snowblowerforum.com/forum/ariens-snowblowers/69882-st1032-restoration.html


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## Shryp (Jan 1, 2011)

You could always mount the fuel tank to the handlebars.


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## ELaw (Feb 4, 2015)

Hey just for the benefit of anyone searching in the future, I thought I'd post the results of my attempt to interchange parts with my old HMSK110...

Basically what I was told was correct... but there's a catch. The alternator and flywheel fit just fine. And the crankshaft fits! 

But... there's a hole through the crank pin that I assume is for balancing... to offset the weight of the piston and/or connecting rod. And while the stroke is the same (I almost had a heart attack when the thought popped into my head they might be different), the bore is different between the OHV which is 318cc displacement and the l-head which is 358cc. Different bore = different piston and... the hole in the crankshaft is a different size. The hole in the HMSK110 crank is larger which makes sense because the piston is larger and thus they removed more material to compensate.

So for now in case the weather takes a sudden turn and we get two feet of snow, I"m putting the engine back together with the HMSK110 crank. But I'm also sending the LH318 crank out to my machinist friend to have it modified so the output shaft is the same as the other crank. When I get it back, I'll install it so the engine will be balanced correctly.


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## dodgetrucker (Jan 15, 2016)

well I am in the middle of a similar swap, I have a 1990 Craftsman trac drive snowblower with an 8hp that is dead. extended cam shaft is scored where it passes thru the side cover, the side cover is also trashed. rest of engine looks fine. 
this is an "HM-80" medium frame. 

I have had an OHSK110 with extended cam sitting here for a couple years collecting dust, just got the snow blower last weekend. my 11hp is a 1998 engine. 
Yes the 11hp is an overhead valve. and yes there are alot of "same parts" between them. 
but the crank is vastly different between them. (Tech made several variations of each engine they ever made with different carb linkage configurations, and different crankshaft configurations, otherwise same engine) 

My 8hp is also 3/4" dia X 2-1/2 long. but the 11hp came with a 3-1/2" long, 1-3/8" (same as main bearing diameter passing thru block) diameter, stepped down to 7/8" for the last inch or so. 

I pulled both engines apart, and looked online at parts diagrams for both engines; 
and I looked up the parts breakdown on both, I also have the original manual dated 1990 from the 8hp for those original part numbers; here is what I found. 
--The rod, the piston both supersede on the 8hp to the exact same part number as the 11hp. 
--the flywheel is the exact same part number on both. 
--both have the exact same bore, stroke, and displacement. 
I put the 8hp crank into the 11hp block and it fits fine, endplay is well within tolerance. 

I have seen this before, with tecumseh. how they can get 8, 8.5,9,10,10.5 OR 11hp out of a block with the same displacement is beyond me. 
If I swap a carb with too lean of jets it just wont barely run. If I swap one that is too rich it coughs and belches. so how do they do this? Just print a different sticker and slap it on depending on what the customer wants? 
so I am swapping cranks, all it will cost me is the price of a side cover gasket and 2 oil seals to have a running engine that will directly bolt onto my machine. 
Plan B was to take that 11hp crank and have the end of it machined down to fit. as the 1-3/8 portion of the current 11hp crank doesn't even have a keyway in it, so no simple pulley swap. I'm sure that somewhere along the line, for some equipment maker, Tecumseh made that same 11hp engine, with a 3/4" crank.


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## dodgetrucker (Jan 15, 2016)

my 11hp that I am getting ready to use is an OHSK110 as well; the HM80 that is donating its crank to it is an HM80. Both are 318cc engines. that OH 318 is probably the same thing as your OHSK110.


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## ELaw (Feb 4, 2015)

Interesting!

But my old engine isn't an OHSK110, it's an HMSK110 - an l-head engine. And its displacement is larger: 358cc instead of 318.


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## dodgetrucker (Jan 15, 2016)

oh I got it. I do know that they made alot of different HP engines out of that 318cc block, I wonder how much they REALLY get out of it, besides the cheap cost of slapping a different sticker onto it? Evinrude/Johnson outboards are the same way... that V4 they had could be anywhere between 60hp to 115hp....


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## dodgetrucker (Jan 15, 2016)

you need to do like I did, check the bore and teh stroke of both, before you pop that crank into the other block; the bore isnt a big deal but if the stroke is different then youre screwed as far as the crank swap. But if it does work out for you and the crank does work, I'm not so sure that you would be obligated to swap the original crank back in once machined.


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## ELaw (Feb 4, 2015)

Hey some more random updates...

The first photo below shows the difference between the holes in the two crankshafts.

And... I got the fuel tank mounted! The bottom bracket from the old engine just swapped over. The top one I had to fabricate. A photo of the bracket, and one of the tank installed, are below. As you can see in the tank photo, the electric starter also was able to be swapped over.

The engine's actually almost ready to go on the machine at this point. I drilled out and tapped the hole in the block and installed the tube for the rear oil drain. I had to create a little indent in the flywheel cover to clear it... the old engine had such an indent from the factory but this one didn't.

I think I've figured out how to set up the throttle/governor but I need to make a new linkage wire and that stuff is seemingly hard to come by! The old wire measured 0.090" diameter and was presumably zinc-plated since you rarely see those rust. The best match I could find was some 0.081" diameter stainless wire from McMaster... we'll see how that works.

I'll probably end up going my own route, but if anyone has any photos of the throttle/governor setup on an OHSK110 (or similar) engine I'd be interested in seeing them! I get the sense it's quite a bit different from the l-head one... mine will probably be more similar so I can reuse the speed control from the old engine.

Oh and one other thing... I was hoping to reuse the side cover from the l-head engine and the tall oil fill/dipstick setup that went with it. That's definitely not going to happen... when I tried installing it, the cap was resting on the exhaust manifold.


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## ELaw (Feb 4, 2015)

dodgetrucker said:


> I do know that they made alot of different HP engines out of that 318cc block, I wonder how much they REALLY get out of it, besides the cheap cost of slapping a different sticker onto it?


That's a good question!

One thing I can see making a difference: camshafts. If you look at Tecumseh's old engine catalogs, the HMSK110 and OHSK110 have very different torque curves. The l-head's torque peak is at 2500 RPM. And amusingly, even though at the top of the page in large letters it says "11 HP winter application", the maximum HP shown on the graph is 10!

The graph from the OHSK110 page doesn't have an RPM scale but if you assume it's the same as for the l-head engine (1800 to 3600 RPM), the torque peak is at 3500 RPM and max horsepower is actually 11 (at 3600 RPM).


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## dodgetrucker (Jan 15, 2016)

sounds like you and me have a similar project in the works. Being that (according to your posts above) the L head "11hp" has a larger bore and the 8hp L head and 11hp OHV have the same bore and stroke as each other, that shows there is something to the OHV being more efficient... 
I just had to take my extended cam from the 11 to a neighbor that works in a factory to have a keyway cut into it as the 11hp had 2 flats on the extended cam and no keyway like the 8hp had. ... I just went today and ordered a side plate gasket, and new oil seals for mine (might as well change those while apart) , those will be here Thursday... 
on the 8hp, the only thing bad about it that I could find (it had been previously disassembled for inspection and reassembled finger tight w/o gaskets by a past owner) was that the extended cam was scored where it went thru the cover and the bearing surface in the cover was likewise scored.


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## ELaw (Feb 4, 2015)

And we have progress! Some forward, some backward, then forward again.

I suppose that requires a bit of explanation, huh? :icon_whistling:

About a week ago when they were saying we could get a foot of snow this weekend (the forecast is now down to about an inch) I decided to temporarily put the engine together with the l-head crankshaft. It won't be balanced quite right but at least I'd have a working snowblower while the other crank is out being machined.

There were two challenges that I expected, and one that I didn't. First was mounting the fuel tank. The bottom bracket from the HMSK110 bolted right up to the new engine, but the top bracket wasn't even close so I had to fabricate one. The result is in the photos below - it's not the prettiest-looking thing but it works. And it's surprisingly solid!


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## ELaw (Feb 4, 2015)

The next challenge was the throttle setup. The engine came set up for fixed-speed operation as it was intended for use with a generator. But I like to be able to vary the speed of the engine, both so it's not being started at full throttle, and to slow it down when I stop snowblowing for a minute or two.

To make a long story short, I cut one of the "ears" off the HMSK110 speed control that interfered with the carburetor, and made some new mounting holes for it in the blower cover. There were holes present from the factory but their location didn't really work.

The other half of the job was making a linkage wire to go between the speed control and the governor arm. The one I made has more twists than a bad novel, but it works and doesn't hit anything!


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## ELaw (Feb 4, 2015)

Then things took a turn for the worse.

The HMSK110 had a metal tube out the "back" (flywheel side) of the engine for draining the oil - a feature I really like.

The OH318 did not have such a pipe, but there was a place for it in the block - a hole that was drilled partway in, of the correct diameter to be threaded for 1/4" NPT.

So I grabbed my handy-dandy drill, drilled the hole the rest of the way until it met up with another passage that brings oil from the crankcase, tapped it, and screwed the pipe in. I tapped it a little crooked but some creative tweaking of the pipe fixed that issue. And everything was good, right?

Well, no, everything was not good. It turns out the other passage in the block that that one meets up with is offset... if you drill it straight at full diameter, it meets that passage but also breaks through to the outside, underneath the engine! I found that out when I filled up the oil, went away for a bit, and when I came back the engine was sitting in a huge puddle of oil. 

So I drained the oil, flipped the engine over, identified where the leak was, cleaned the daylights out of it, and plugged the hole with J-B weld. Time will tell, but so far it *seems* to be fixed.

The photo below shows the drain tube and the repair (before I machined it flat).


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