Grinding a Tecumseh Cam

cfh

Well-Known Member
#1
I've been playing around with Tecumseh HS40 and HS50 motors (for example see my document at http://pinrepair.com/minibikes/tecumsehmb.htm ). And I'm always interested in getting a bit more power from these vintage motors. Open exhaust and a 22mm clone carb (or Dellorto 16-22mm) seems to be an easy way to get some more power with low cost.

Watching on ebay, every once in a while you'll see a Tecumseh Dynocam for sale. There seems to be two available: the 245 and 356 models. The 245 is a mild cam, where the 356 is pretty aggressive. Here's the numbers:

Stock Tecumseh HS40/HS50 cam lobe lift: 0.975"
Dynocam 245 HS40/HS50 cam lobe lift: 1.050". Really kicks at 3000 rpm.
Dynocam 356 HS40/HS50 cam lobe lift: 1.150". Really kicks at 5000 rpm.

I've tried out the 245 cam, and it is very nice. Not aggressive. Works fine with stock springs. Tops out at about 5000 rpm (where the 356 can allow an HS50 to 8000+ rpm, as a comparison.) If you examine the Dynocams they are identical to the stock Tecumseh cam, except for the lobe lift.

Note with ANY modified cam you need an Arc billet connecting rod! this is mandatory! And also available through our sponsor OMB warehouse.

The problem with these Dynocams is they are discontinued. I called them and asked why. They said getting cores was the main problem, and demand wasn't really high either. Would they grind a Tecumseh cam if you supplied the core? Yes they would. But the cost at $75 for the grind plus shipping both ways (and the price of the core if you didn't remove it from a motor) seemed to make the idea a bit expensive.

So how can you make your own version of the 245 cam? Well you need a cam grinder! It's something you can make. Much like a hardware store key duplication machine. I made one with a small belt sander and some barn door hinges.

First you need to have a cam to grind. The stock HS40 and HS50 cams are the same. I use one of those stock cams. But you have to add material to the cam. They are cast steel, so I just TIG welded new material on the top (high side) of the two lobes. A little down the side too, so the transition to the higher lobe can be cut.

You also need a "master cam". That's the cam you will be using as the master for the duplication. I'm using a Dynocam 245 which I bought off ebay some time back.

Next you need to link the two cams together. I used some 1/2" ID tubing with two nuts welded to it. The cams can be linked through the hole that already exists in the gear (1/4" diameter rod through the two holes.) This allows the cams to be "in sync" with each other. The welded nuts allow a couple cap screws to tighten the tubing link, so the cams won't slip out of position.

 

cfh

Well-Known Member
#2
Next you need to modify the belt sanded. The craftsman belt sander I was using had a cast aluminum shell. I welded a piece of angle aluminum to the housing, and tapped it with a 3/8-16 thread. This allows me to put a bolt through it. This is what walks the master cam and reads it for duplication to the slave cam.

The cam holder are barn door hinges, welded together. Two brackets were welded that hold the cams together. The barn hinges swing into place. The 3/8" bolt reads the master cam, and allows the slave cam to be cut on the belt sander. After one lobe is cut, the barn hinge device moves over to cut the second lobe.

I know it does not sound very precise, but amazingly, it works quite well! I have made a new cam and will be trying it out.

If anyone has comments about this, i would love to hear them. Thanks!

 
#3
I have a dyno 255 if you are interested. Your ingenuity is amazing. I had tim isky weld and grind a cam and he mentioned that it was difficult on some of the tec cams to tig the hardface material without getting it porus. Mine welded fine but some did not work at all.
 

delray

Well-Known Member
#6
little off on your rpm range for your dyno cams. the 245 is good to 6000+ and the 255 is 7000+ and the 356 is 9000+ (dyno cam spec's)
I have use all three cams in well set up motors and I have found the 245 in my hs40 comes on strong at about 3500-3800 and go's easy up 7000+
255 in a ohh motor and pulls good up to 7000 and with just a few more mod's and 1.3 rockers it go's easy up to 8200
and for the 356 it will not just fit in any motor without a lot of mod's done first. i'm not even sure it will go in a flathead without maybe hitting the head when the valve open? I have tested the 356 in a heavy mod ohh motor and that comes on at about 4500 to 9000+
also if I am correct dyno base there rpm ranges on a ohv motor and not a flathead. for example hs40 rpm range/power band is going to be totally different from a ohv. that I end up finding out after building my hs40 with a 245 and running a 22mm carb with a crazy large 125 main jet.
do you plan on trying to harden the cam first before running it or you just going to wing it run a lot of Zinc...lol
 

cfh

Well-Known Member
#7
Don't need to harden anything. The stock cam is cast, and is pretty soft. The TIG welding material is a lot harder than the stock material. I asked Dynocam about this too. On the 245 they do not harden. The guy that works there (Randy), who gave me the numbers I posted, said that hardening is not necessary unless you are running vicious valve springs. The 356 for example is both welded and hardened. The 245 is neither (and he recommended stock valve springs with the 245). I did not ask about the 255 (as I didn't know it even existed until this thread.)
 

delray

Well-Known Member
#8
that's interesting. magic number that dyno cams told me and they had it posted in the pass on there site was 18 pound springs for the 245 and 20 pounds springs for the 255 and 40-44 for the 356 also seen them post that number at 36 pounds too. I personally would not trust the stock valve springs at 6000+ rpm's or stock retainers.
I really like that 245 cam. it's a strong little cam in a hs40 motor when setup correct.
I ran 16 pound springs on a stock ohh cam and spun it all day at 4700+ rpms and had good luck with no cam damage. right now I am running a set of 1.3 ratio rockers on a stock cam with 18 pound springs and is working good. Tecumseh cams do seem to be harden enough to run a stiffer valve springs. not like the clone stuff. they tend to be kind of soft.
here is a photo I saved from another member that had this posted in the pass on the dyno spec's
 

delray

Well-Known Member
#9
maybe look into using a grinding wheel stone. for example the type that are use for grinding valves. they come in all different finishes. using a grinding wheel you could get a more accurate grind(spot on) with that belt your getting more inaccurate finish with it flopping around. also with a stone wheel you could mount a diamond cutter and true it up once in awhile and also set it up so you can use a dial indicator on your stone and your 3/8 bolt to get both spot on. all that should be easy for you to do..........

hey, maybe you could sell some cams.
 
#10
That’s such an awesome idea! Kinda reminds me of how they would use something similar to make wooden rifle stocks. They would use 1 stock as a pattern and the lathe would follow it and make an exact copy.
And you did it on the cheap which is also great.
I like the belt sander, I really need to get one it’s something I never think of, until I need one lmao
 

cfh

Well-Known Member
#11
The problem with using a grinding wheel is as it wears it changes the cut. That’s less of a problem with the grinding belt. I’m working on trying to get better accuracy but I think if I just get it dialed in that will sorted it self out
 

delray

Well-Known Member
#12
Yes,thats where you would have to come in with a rough stone first and then clean up the last .010-.020 with a finish stone so it's spot on and you have makesure all your pivot points are true and accurate. For example your door hinges will need to be up graded.
 

cfh

Well-Known Member
#13
Here's some more pictures....

this is the Dynocam 356 versus a stock hs50 cam. Del is right, there's no way you could have this much lift without a modified head. You can also see how the 356 (right) is welded and then ground. The casting marks on the stock cam (left) are obvious.

 

cfh

Well-Known Member
#14
Here's a picture of the Dynocam 245 versus 356. You can clearly see the 245 is cast, and the 356 welded (and then cut.) Dynocam must of found some sort of larger HS50 cam and ground it to the 245 shape. Not sure how that happened, unless they got Tecumseh to cast them for their 245.

 

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