Dyno 356 cam Extreme build!

This all started out as guy that had a bunch of old obsolete race parts for an obsolete engine, but turned into a whole bunch more!

If this thing stays together, it may be one of, if the not the, most radical OHV small block Tecumseh ever built.

I've had this 356 cam for while, and some initial test fitting back when I got it showed it was definitely not a drop in deal. The lobes hit the crank, the valve springs coil went into coil bind, and it simply wasn't going to be easy. Even still, I had set out to do it anyway, but this crazy guy known as delray sent me pics of a crazy looking vertical lawnmower head that happened to fit an OHH engine changed everything! The intake and exhaust were in the wrong spot and were swapped, but it had good sized valves and big ports. The bowls are very large and had room to be ported much bigger. I picked up one on eBay and the madness began! Having raced these OHH engines in the past, there's no doubt this head is capable of flowing more air than any conventional OHH head.

It's coming to together nicely, so I decided it was time for a build thread.

Starting at the bottom, the shortblock is brand new stepped shaft generator engine. I picked it up on eBay for $55 and took it completely apart.

The 356 cam has a target RPM range of 9500 RPM, so billet rod and flywheel is obviously a must. I got my hands on a new Tecumseh Motorsports rod that has had bronze bushing in the small end and a Motorsports bearing set. This rod also has a bigger machined step on the cap instead of the normal serrated cap.

I also dug through my old race parts and found a Motorsports ball bearing side cover and bought a new bearing for it.

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Back when I was racing these, the one thing these engines did really well was rev. The big bore and short stroke along with a long rod length (relative to similar sized engikes) makes them zing pretty good. I run to 8000-8100 back in the day and never had a bottom end failure, but never with as much cam and compression as this engine will have. Since this cam is capable of so much RPM, my thinking is that any rotating mass I could eliminate and still be balanced would be a good thing.
Obviously a billet rod is heavier than the stock cast rod, so I decided to lighten the piston to get the static weight of the piston rod assembly down. I also noticed that the 172cc OHH50 crank was lighter and has smaller counterweights than the 195cc cranks. I was able to get the billet rod and piston assembly within a gram and half of stock 172cc piston and rod piston assembly.

This isn't the correct way to do a precision balance by any means, but we'll see how it works out.

For comparison's sake, this is the piston and rod assembly compared to a Predator piston and ARC billet rod that I'm building for a friend.

This is awesome I will be following this build as I also like the OHH engines and would like to hop one up someday
Thanks! I love these engines and so far I have fairly mild Dyno 245 cammed OHH60 and a Motorsports engine with a Dyno 255 cam that is a quite a bit hotter, but this 356 cammed engine will be the wildest one yet.


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The 356 cam requires quite a bit of clearancing on the crank to even make a revolution. Delray and I have wondered how many of these were actually built. The cams are still available on eBay, and I know some people here have bought them, but I've yet to see any running engines with this cam.

The pictures below show the difference between the 356 cam and a stock cam, and the difference of how it looks in the block.

This thang just about don't fit! -lol

Sounds great!
You can def hear the difference in the cam between the 2

Edit: wow that is quite a bit of clearancing nice job
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After the bottom end was sorted out, it was on to the head. This is the oddball head that Delray told me about. The head as we come to find out came on a long discontinued vertical lawnmower engine known as OVRM-40. It was a 4HP OHV engine, but for some strange reason, it had much larger valves than normal OHH heads. The problem was that the intake and exhaust was swapped, so the large valve was on the wrong side. The ports and bowls were very big, and there was room for porting. The port that will now be the intake is huge, and is a straight shot to the valve. The combustion chamber was small and was odd shaped, so I knew it needed work. The head also had shorter valves and pushrods than a normal OHH, and which would be no problem for a smaller cam engine, but also ended up creating a challenge with the 356 cam.

Here's some pics of the more modern OHV Bungholio head! ;)

Here is a comparison between the OVRM head compared to a Predator clone head, and the OVRM head compared to a normal OHH head.

*photo credit on the last 2 pic to delray

You can see difference in valve size to the normal OHH head. The intake is slighter bigger than the big valve Predator head and the exhaust is slightly smaller.
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When I started planning and mocking everything up on this engine, I was also in the process of building a Predator engine for a friend of mine's son. It is pretty stout little engine, and we were going with Champion rocker shafts on it. I had the 2 engines sitting side by side and thought "Man, the rocker spacing and length looks very close." I grabbed two 1/4"-28 bolts and the rocker shafts bolted right on. The spacing is very close, not exact, but close enough to bolt up. The center line of the bolt holes is about .030" off as well, but I saw with some modification that it was a feasible option. (More - MUCH MORE - on that later...)

The rocker shafts are known to be rigid and stable, and since there's no need for ratio rockers with the 356 cam, I thought it was a great upgrade with the RPM this engine will see and the 36# springs required with the cam.

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As mentioned before, this head has shorter valves, springs, and pushrods than a normal OHH head. The install height of the spring is only .800" and the 356 cam would not even make a revolution due to coil bind.

It was clear that that point that I either needed longer valves and pushrods or the head needed machining to increase the install height.
Awesome! I picked one of those up a while back, as well, for one of my HS50 builds. It's fun that the 3.5 Power Sport shroud goes on...
I didn't realize how tight it would be in the bottom end. Likely negates my idea of a cut-down Briggs crank for a stroker.
I hear you on that! I have an early OHH50 blower housing that still has the old Indian head logo that I've had on some pretty hot OHH's over the years!

One of my friends used to look at that 5.0HP decal say it was the biggest liar he ever saw!

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In the midst of trying to decide whether I wanted to go with longer valves or machine the head, I ran across a parts lot of new old stock Motorsports race parts. Some of the parts were said to have been directly from the Motorsports division R&D. I believe this to be true, because I ended up with lot of semi-machined parts (holes not tapped, valves guides never reamed to size, Pistons that were never chrome plated, and bunch oddball stuff). I didn't need all of it, but I bought the whole lot, including a complete Motorsports OHV engine. I ended up with some billet intakes, rods, tons of rings, gaskets, a heat treated Motorsports crankshaft, a bunch of heads with various sized combustion chambers and valve sizes, aluminum Motorsports retainers, various sized stainless valves, a hot coil, and lots of other stuff.

This kind of made the decision to go with longer valves for me, because I had them in hand and had a lot of sizes to choose from.

The decision to use OHH valves resulted in other challengees. They are quite a bit longer than the valves in the OVRM heads, but the plus was I could use OHH pushrods. The OHH are much bigger and beefier than the OVRM pushrods. The longer valves created the condition having too much little of and install height to having way too much.

This also of course completely changed the height of the rocker shaft.

To make this work, I had to machine billet aluminum valve spacers to achieve the correct .900" install height and I had to machine a new yoke for the rocker shaft system. When I made the yoke, I moved the bolt holes to the correct spacing and moved the fulcrum shaft forward .030" to center the rocker over the valve. This was a lot of trial and error, but I eventually ended up with the perfect geometry.

Here's pics of the spacers and the yoke that I machined for the rockers.

Here's a short clip of the rockers working with the big .356" lift cam. It looks simple to see it spin over, but it was definitely not easy to get there. As of yet, I've only spun it over mild springs. I haven't installed the 36# springs yet because I want to heli-coil the rocker threads in the heads. I had issues with the rocker ball studs pulling out the threads back in the day with much less cam and spring pressure than this, and even though the base of the rocker pedestal should be much more stable and have better clamping force a rocker stud flexing all over, I don't want to take the chance.

After getting valve length and geometry sorted out, it was time to start on the head. Being that the exhaust and Intake were opposite, what would now be the exhaust port already had a large 1.080" (27.4mm) valve, so I ready only needed to put a bigger seat and valve in what would now be the intake port. I took measurements and decided that 1.181" (30mm) would be about the max, but 1.161" (29.5mm) would be optimal for the size bowl I thought I could machine without worrying about breaking through the casting. With some welding, a 32mm valve would be possible.

I pressed out the seat and started reshaping the funky shaped chamber. In time, I may weld up the middle area around adjacent from spark plug side, and create a heart shaped, but I have used this semi open design with welded chambers before, and I know it works well. The 3rd pic below is an old OHH head I used to race. It has been welded and it very similar to how I shaped this OVRM head.

Going with an oddball sized intake valve required an odd sized seat and modified valve. I made and heat treated a seat and modified a valve to the size I needed.

I machined the bowl and did the major porting before pressing in the new seat. This thing will flow some air sbd should make a lot of upper end power. I just hope the block and crank are up to the task!

I after getting the bulk of the porting some, I pressed in the seats and ground the new valves to fit. The difference in the between this head, a large chamber, and small chamber OHH are striking!

This is the OVRM head compared to the larger stock valve (27mm/25mm) clone head.

I'm going to be running 36# Dyno springs with this cam, so I ordered the springs and some hard annodized aluminum retainers from EC. The diameter was a little big as well at the step on the inside, so I made 10 degree arbor to machine the retainers for my springs. I have a Heli-coil kit on the way to do the head before I put the big dual springs on it.

The 36 pound springs are massive compared to the stockers!

No, the cam is a normal OHH cam. The ports had to be swapped in order to use the cam. I used the already big factory 1.080" intake as my exhaust valve, put a 1.161" valve in what was the exhaust side as my intake.

To have been able to use the head with the ports like they were, a H35 style cam would have had to have been used. The engine would be a OHV side popper in that configuration.
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