thinking about how to trim down an h70 on a budget, (lots of questions)

joshua. c.

Well-Known Member
#1
I have a speedway scorpion project that is nearly done and its now time to look at the engine. the engine is an h70 with points and a lighting coil and a plug in electric starter. I shouldn't need to use the starter very often when I'm done. I have a Tecumseh star billet rod that I heard fit this engine and I plan to sent the cam out to dyno cams for a re grind. and billet flywheels aren't available for this engine or I would have one already. I'm also doing some port work and adding a header and a larger carb. I just took the shroud off to look at why its so big on these engines. and noticed that the fins on this flywheel are enormous and its also the same diameter as an hs50 flywheel. is there any reason this engine needs that much air flow.? and can I chop around 1/2 an inch off these fins to mach the fins on an hs50 flywheel in order to shrink the shroud some? also can I use the early hs40 aluminum flywheel on this engine? it would be allot smaller and lighter. and Ive seen billet crank case covers made for the h50 racing engine to take around 1/2 an inch or so off that side. dose anyone have a source for one of those.?
 

PatrickCraik

Well-Known Member
#2
I have a speedway scorpion project that is nearly done and its now time to look at the engine. the engine is an h70 with points and a lighting coil and a plug in electric starter. I shouldn't need to use the starter very often when I'm done. I have a Tecumseh star billet rod that I heard fit this engine and I plan to sent the cam out to dyno cams for a re grind. and billet flywheels aren't available for this engine or I would have one already. I'm also doing some port work and adding a header and a larger carb. I just took the shroud off to look at why its so big on these engines. and noticed that the fins on this flywheel are enormous and its also the same diameter as an hs50 flywheel. is there any reason this engine needs that much air flow.? and can I chop around 1/2 an inch off these fins to mach the fins on an hs50 flywheel in order to shrink the shroud some? also can I use the early hs40 aluminum flywheel on this engine? it would be allot smaller and lighter. and Ive seen billet crank case covers made for the h50 racing engine to take around 1/2 an inch or so off that side. dose anyone have a source for one of those.?
Call Acme Lawn Mower on Southfield Rd in Detroit. This guy has been very helpful with info. over the years,especially if you spend some money. Good luck.
 

joshua. c.

Well-Known Member
#5
these are the bike and engine in there current condition. the bike is just dusty it will look much better after a good wash. and the engine is just as bad as it looks it needs a good paint job before install. I already set the bike up to center the engine in the frame but its still very wide., so I'd like to make it a bit less wide to fit in the frame better.
 
#6
I respect your "date to be different" approach to using an h70 in your scorpion.

Please consider the following:

The overall width of the engine is basically determined by the length of the crankshaft. You may be able to save a half inch or so by shaving down the fins and chopping the shroud a corresponding amount but there is still the starter cup and nose of the recoil to consider. The starter cup needs to be chopped too.

A shallower side cover will not change the
length of the crank .

Hs40 and hs50 flywheels will not work on any h50 -h70. The taper is not only a different angle it's also a different width.

A certain amount of air needs to circulate as you know these engines are air cooled, reducing the air flow too much could raise the operating temp,lead to overheating and reduce engine performance.

Maybe the fins on the h70 are larger that the hs50 because the engine is bigger and needs the additional air flow.

Regarding the electric starter is it worth the extra weight for only intermittent use?

Check to be sure that your star rod(for an h50?) will actually fit your h70. It sounds like a key component in your choice of build. In my experience with h70 power snowblowers, they are notorious for breaking rods.

You mentioned that you set engine in the frame Consider the balance and ridability of the bike with such a wide engine ie., reaching the footpegs securely and safely . Its one thing to sit in the garage and put your foot on a peg and another to go riding off road and you have to get your foot on the peg.

H50-H70s are substantially larger and heavier than hs50. Any performance gained from the extra displacement of the big block engine is somewhat offset by the extra weight and bulk. Exception being Star racing engines. They can power a small car.

In closing have you thought about an hs50?its smaller ,lighter, dyno cams and billet rods are available .There is also a lot of grass roots development surrounding high performance mods to these engines. You can even find some with electric starters, if your set on that.... Just a thought...

Best of luck with your build whatever you decide.

P.S.
Please refer to original tecumseh reference materials for engineering specifications and interchangeablilty of parts. These will answer most if not all of your questions.. Especially good are the old hardbound manuals with the interchangeable pages They are a little cumbersome to work with but they contain information you just cant find online.
 

delray

Well-Known Member
#7
i'm with oldcarcbx josh, he's pretty much spot on with the problems you may incur. one thing I would avoid using is that lighted coil flywheel. I have couple of them and they weigh a ton with the extra mags in them and that steel ring on it. I hate to see that ring gear come loose. that for sure would be a meat grinder. I personally would boot the ideal running with a lighted coil on a hot rod engine and find a early aluminum non-lighted coil without the extra dead weight to spin. technically you could mill down the fins little and not hurt the engine and you would create less drag too when it's spinning. ARC racing has just recently started building shorter fins for the clone stuff used on racing applications. if indeed it's on a minibike you would constantly have air moving around it and the few minutes it might just sit and idle it won't over heat with short fins. only time it might over heat, if it was a stationary engine and ran wide open in hot weather. besides it would be sacrilegious to cut up a lighted coil flywheel for a H-motor...lol. now could you run a hs-flywheel on a h-block...maybe? well we know the mags are the same for starters. without looking I am going to assume the taper is smaller on a hs. so the taper would have to be enlarge on the flywheel or the taper on the big block would have to be turn down. now if this could be done? does the flywheel still fit correct over the mag? lot of if's and I hate to say this,but way beyond your capability for you to do this josh. for a cam I would send them a stock steel cam with a compression release setup and have dyno grind the 245 cam profile. not sure if you have the resources to build a 255 or higher lift profile cam. it may take more mod's to take advance of the bigger cams/rpm's..etc...
 

joshua. c.

Well-Known Member
#8
thank you guys for the responses. to PatrickCraik at first the name acme made me think you were making a joke about wile e coyote and the acme catalog he always ordered from, where things always went wrong. but I looked it up and its a real place if acme deals with a lot of high performance racing engines I might contact them.

and to oldcarcbx there is a reason for my engine choice. the firs is the bike came with the big engine and it looks rather good and beefy in the frame. and the engine is more powerful than an hs50. which is important to me. I go to allot of mini bike racing events during the summer and my built up briggs raptor just couldn't keep up with the built up over head valve engines that were there. so a beefed up hs50 wouldn't do any better. I just plain need more power. most of the events around hear have a 7hp and under limit for engine size and this engine is right at that limit. and its got more displacement than any other Flathead choice in its size category. also the Tecumseh star engine is based off the hm80 engine and the rod fits the early model h70 with points but not the shorter stroke h50.

and to delray I would like it keep the lighting coil working if possible considering the bike has a headlight that I might be able to get working. I might be able to chop the fins down on this flywheel and remove the ring gear if the gear is a hazard. and my raptor that I built myself is running a 107ss cam that I really like that is an aggressive duration and timing 315 lift and I was thinking of getting a similar grind for this engine. I tried a 275 lift mild grind for my ohh55 in my trike and was not happy with the performance and top end. and as far as adapting the flywheel off an hs40 I know the taper would be different and there may be fitment and cooling issues so I was wondering if anyone had tried it yet with some info. since it apears not to be the case I'll stick with the stock flywheel for now.
 

PatrickCraik

Well-Known Member
#9
LOl, I thought the same when first hearing about the place. Well,I dont know if he does HIPO stuff but I do know he's done a lot of new/old engine conversions for guys I talk with. He was stocked up pretty well last I spoke with him a few years ago.
 
#10
I respect your "date to be different" approach to using an h70 in your scorpion.

Please consider the following:

The overall width of the engine is basically determined by the length of the crankshaft. You may be able to save a half inch or so by shaving down the fins and chopping the shroud a corresponding amount but there is still the starter cup and nose of the recoil to consider. The starter cup needs to be chopped too.

A shallower side cover will not change the
length of the crank .

Hs40 and hs50 flywheels will not work on any h50 -h70. The taper is not only a different angle it's also a different width.

A certain amount of air needs to circulate as you know these engines are air cooled, reducing the air flow too much could raise the operating temp,lead to overheating and reduce engine performance.

Maybe the fins on the h70 are larger that the hs50 because the engine is bigger and needs the additional air flow.

Regarding the electric starter is it worth the extra weight for only intermittent use?

Check to be sure that your star rod(for an h50?) will actually fit your h70. It sounds like a key component in your choice of build. In my experience with h70 power snowblowers, they are notorious for breaking rods.

You mentioned that you set engine in the frame Consider the balance and ridability of the bike with such a wide engine ie., reaching the footpegs securely and safely . Its one thing to sit in the garage and put your foot on a peg and another to go riding off road and you have to get your foot on the peg.

H50-H70s are substantially larger and heavier than hs50. Any performance gained from the extra displacement of the big block engine is somewhat offset by the extra weight and bulk. Exception being Star racing engines. They can power a small car.

In closing have you thought about an hs50?its smaller ,lighter, dyno cams and billet rods are available .There is also a lot of grass roots development surrounding high performance mods to these engines. You can even find some with electric starters, if your set on that.... Just a thought...

Best of luck with your build whatever you decide.

P.S.
Please refer to original tecumseh reference materials for engineering specifications and interchangeablilty of parts. These will answer most if not all of your questions.. Especially good are the old hardbound manuals with the interchangeable pages They are a little cumbersome to work with but they contain information you just cant find online.
way beyond your capability for you to do this josh. for a cam I would send them a stock steel cam wi
 

joshua. c.

Well-Known Member
#11
it may be beyond my ability but I know people who can do some of this stuff who can help me out. also I figured if I could get a smaller crank cover I would just chop the crank down. chop one end and mill the other down to the crank size. in the end a shorter crank with the same length PTO. but I would need to find a pre made crank cover that would be way to expensive to have custom made. and if I try to shorten the fins on the flywheel and shorten the starter cup. I know someone who can do that for me.
 

delray

Well-Known Member
#12
minibike drags,are they on dirt,clay,pavement? how long are the drags?

not sure how you are going to reinventing the wheel. either way good luck josh, I will be watching for this mod's you plan on doing.
just remember the real power in a flathead will be to run methanol...that's if you class will allow it.
 

joshua. c.

Well-Known Member
#14
I was just planning on removing what I can from the width of the motor, I never said it would be as narrow as an hs50 that's impossible.

also no draggs the last couple of years delray, I said races. the drag racing group used to race up to 3 times a year 100 foot drags in east providence Rhode island but they kept getting the police called on them over noise complaints from an old folks home nearby. so they gave up on that spot and are being to picky about finding the perfect spot to pick a new one. they haven't raced since. several of the members have big money stretched out low to the ground drag bikes with extremely pricey motors that can't run on anything even slightly bumpy, so they do have there reasons. most of the races I now attend are dirt track, usually very small and then you have gomas lol, biggest and best race of the year, well worth the trip. a weavy windy track several football fields in size with man made obstetrics to overcome running around the access paths to cranberry bogs. gomas mini bike massacre look it up on youtube over 150 racers last year its the biggest attendance I've seen yet. also I race go kart type trikes (back end go kart, front end trike) with a group that mostly joins up with dirt bike flat track races at Winchendon mass. there's a hole thread about those in the mini bike events section that has posts from now all the way back to when we started. unfortunatly least year was a bad year for us and we never attended a single race at Winchendon this year but we did meet up at other events.
 
#16
Joshua,
If you're going to race this bike, I have to say the H70 may not be the best choice. You might be able to find a vertical shaft aluminum flywheel (H50-60) that will help spin up the engine, but this engine is designed for torque (see my Gilson Night Rider project). I built an H60 a couple of years ago that was bored to accommodate an H70 piston--incredible compression with the shorter stroke of an H60. It's mated to a TAV20 and pulls like a bear but takes awhile to spool up. The spinning mass of the big block engine just prevents quicker acceleration without the TAV and good gearing.

I did build a Gilson Trail Bike with a stock H60--the final gearing and TAV let this bike clock 49mph top end...but again, it needed a bit of time.

My two cents....Good luck!
 

delray

Well-Known Member
#17
yes pete I mentioned that too about using a flywheel with the lighted coil and steel flywheel/teeth. the lighted coil will make it a much heavier flywheel to spin with all the extra mags casted into. you will not gain anything by using that type of flywheel on a motor that you will need it to spin some good rpm's.
josh I don't get you bud. you come here for advise,but at the end of the day you go with your own theory.
I wish you luck and I will be following this thread and asking questions.
if you plan on using that speedway bike for circle track racing it will throw you off the bike when you ride it hard in the corners. so that too will need a ton of mod's done to it also...wait you already know that ...right, ok then tell use all about you mod's to your bike also.
trikes are alot easyier to race then a minibike. i seen alot of guys and girls crash there bikes last year. not that they can't ride. some bikes are just not made to turn left at high speeds without a ton of mod's done first.
 

joshua. c.

Well-Known Member
#18
I know there are better engines, in fact I have a modified briggs animal but that's going on my bonanza.I also have all the parts to build a 6hp briggs intec into a race motor but I have a nice trike for that one. and to be clear delray I have considered everything I've herd and I only asked the specific questions I wanted answers to. and I got the answers I needed so thank you guys, you have been a big help. I didn't ask any questions about engine choice or the type of racing I was doing I just told you about the races I do because you asked. and I'm dead set on this engine so nothing anyone says is going to change my mind. so far I've assessed that no one has tried the aluminum hs40 flywheel yet or tried chopping the fins down on the stock one. and it shouldn't overheat but its a gamble. and no one knows where to currently get one of those thinner billet side covers. so that idea is out for the time being.

maybe I should get a spare flywheel and shroud in case chopping the fins down doesn't work. or maybe try mounting a less valuable non lighted hs50 flywheel first and see how it runs.
 
#19
I think your mixing up the H70 with the HM80 as far as width because the H70 is basically the same block as an H50 just bigger bore and stroke and on the PTO side they are not wider than a HS50 so just build that bad boy and lets see some pics.
 
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