Primary differences between HM100/Stars?

Boomstick

Boomstick

New Member
#1
#1
I recently picked up a very nice HM100 for free off of my neighbor who just wanted it out of the garage. It actually runs nicely, but could probably use a good carb cleaning. It's an iron bore, and gets great compression.

Now if I remember correctly, the Star is a (albeit heavily) modified HM100.

Now by that same token, what are the primary differences? Is there any way to convert an otherwise box-stock HM100 into a Star?


Likewise, what's the parts compatibility between the HM100, and the all cast-iron HH100? I have a very nice, running (almost NOS) HH100, and a couple ok runners (although they are worn & fussy). Would it be possible to build a Star out of an all cast-iron HH100?


Having a few ideas. The Baja Warrior/HH100 conversion is going along very nicely, it would be kind of neat if I could build a tough-as-nails Star out of the engine that's on there now (or just build the HM100 if possible).


This will also run a Comet TAV set up for 40mph max. (maybe even lower once I'm done) The rear tire is going to be very... um... "agricultural" for local off-roading & climbing adventures. Yes, it will have disc brakes. Frame is gusseted & reinforced. Considering a KX/CR/YZ80 front end swap if possible.

I just want to build a punchy, offroad bike to doodle around on.
 
ole4

ole4

#2
#2
Star had a billet rod, different Cam, Stainless valves 1.250 intake and 1.125ex with 1/4 inch stems good valve springs and aluminum retainers. If yours has a ball bearing PTO the rest is the same.
 
vwmopar

vwmopar

Member
#4
#4
This is for VKE's catalog.
"COMPONENTS FOR THE HM100 TECUMSEH ENGINE
WE DO NOT OFFER COMPONENTS SPECIFICALLY FOR THIS ENGINE. ALTHOUGH IT APPEARS SIMILAR TO THE MOTORSPORTS STAR ENGINE, THE INTERNAL COMPONENTS ARE DIFFERENT AND WILL NOT INTERCHANGE. HOWEVER, SOME PERFORMANCE STEPS CAN BE TAKEN. PLEASE CALL BEFORE PLACING AN ORDER"
I would talk to them before you start buying parts.
 
MB165

MB165

Active Member
#5
#5
the crankpin/rod journal is smaller than stock....I found out the hard way...:laugh:

Man I hope you do it Ive been dreaming of putting a HS80 in a minibaja for years that would look cool. its wide youll need to offset to the right a bit...but its possible.
 
roccosbike

roccosbike

#7
#7
I can tell you that the star engine was a racing engine made just for that, they had more webbing inside the block than an HM100...the crank was special too and so was the piston. I do know a little bit about the star motor and also modifying the big Tecumseh engine to run.
This is an HM80 with a star rod in it lightened piston,ported,milled head,bronze valve guides with 1/4"swirl polished undercut stainless valves split keepers and the block had work done to it for flow...it also had the stock cam with the comp release welded and ground to a star cam grind with springs to match. Its a nice powerful motor now:devil2:
 
Boomstick

Boomstick

New Member
#8
#8
Very nice Rocco :thumbsup:


Do you think an HH100 would be stronger than a straight aluminum HM100 block? I really like the HH100 I have on the bike now, it weighs just right, and the weight distribution is almost perfect (it feels great).

If the cast iron HH100 would be significantly stronger, I'd be more than willing to build that instead. It's going to be a low-ish speed crawler that will need piles of torque. :scooter:
 
roccosbike

roccosbike

#9
#9
You need to measure your crank journal and post it up then I can tell you if you can use the billet rod or not...that will be the main factor in a motor that will run and a motor that you don't want to waste your time with. I know how to build a BB Tecumseh to run but the parts to do it with don't always come from one engine...its a recipe that I have perfected and it aint cheap. An HM100 block should be steel sleeved and lighter is better.
 
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Boomstick

Boomstick

New Member
#11
#11
Gotcha, I have a good runner that's a duplicate of the HH100 I have on the bike now, I'll pull that one apart & take measurements (it's in good shape, I really don't want to crack the cover off of the really really nice one, LOL).
 
massacre

massacre

#12
#12
I am interested in this as well, I am looking at building a drag bike for the 100ft. And do NOT want to build a Honda or a clone. The Star engines look like a great option.
 
Boomstick

Boomstick

New Member
#13
#13
I'm really more interested in doing the cast iron HH100 up into a Star-esque torque monster.

I'm not chasing revs (I'd be happy with 4000-5000rpm, which I believe is reasonable) or maximum HP numbers, I just want gobs & gobs of old fashioned torque that's going to go through a 1'' TAV (will be a genuine Comet) to a really agricultural rear tire. As is, they make a good deal of torque, but I want a little more just so I can crack the throttle & get on top of stuff.

I'm thinking about gearing it for 35mph & calling it good, I just want a bulletproof woods/crawler bike, especially when I can swap a small dirtbike front end onto it (and I can either set aside or sell the chintzy forks it came with).
 
David wulf

David wulf

#15
#15
Boomstick said:
I'm really more interested in doing the cast iron HH100 up into a Star-esque torque monster.

I'm not chasing revs (I'd be happy with 4000-5000rpm, which I believe is reasonable) or maximum HP numbers, I just want gobs & gobs of old fashioned torque that's going to go through a 1'' TAV (will be a genuine Comet) to a really agricultural rear tire. As is, they make a good deal of torque, but I want a little more just so I can crack the throttle & get on top of stuff.

I'm thinking about gearing it for 35mph & calling it good, I just want a bulletproof woods/crawler bike, especially when I can swap a small dirtbike front end onto it (and I can either set aside or sell the chintzy forks it came with).
Click to expand...
A stock HM100 with a comet torque converter will climb anything . The super bronc i just sold , a VT10 would climb any thing in front of it . A motor this size is a torque monster .
 
Boomstick

Boomstick

New Member
#16
#16
David wulf said:
A stock HM100 with a comet torque converter will climb anything . The super bronc i just sold , a VT10 would climb any thing in front of it . A motor this size is a torque monster .
Click to expand...

David, while I agree with that (I've had a few tractors with the HH100's, that's why I love them), I really want to squeeze every bit of torque & horsepower I can out of one I can within reason (pump gas & reliability oriented).

In doing so, I'm hoping that I can hit 4000-5000rpm's, and do 35-40mph, but still have absolutely ridiculous ability off-road. These are big, fat, very agricultural tires that I want to grab the earth, rocks, & general debris & still hook up.

I'm debating whether to fab up mounts to fit a dirtbike swingarm, just for the extra suspension travel (and lift it a little bit at the same time).
 
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