Jn amateur's high compression Mod 2 Hemi build

Daniel Coop

Well-Known Member
*Should read "An amateur's high compression Mod 2 Hemi build."
I've yet to do an engine build thread, because some of these guys like Ole4, Trinik and a few others, just make me feel stupid. I try reading threads, but often get lost and or bored with all the technical jargon and mathematics. I feel like a hack in comparison, a tinkerer... Yet my motors have proven to be pretty​ fast and reliable so far. I'm​ building this motor for my buddy Jose who supplied the practically new Hemi and some gently used parts he got for a steal locally. He brought me, a stock length ARC billet rod, an ARC billet flywheel, a Mod 2 cheater cam and some 18lb Hemi springs.

I talked him into buying a milled Hemi head and some shorter Molly pushrods from my buddy Dan at Childish Concepts Racing.

This Hemi head was milled .075 but is otherwise stock. The NR racing chromoly pushrods are 5.440 in length...
I replaced the bearing inserts in the ARC rod and knocked the governor gear off the crank.

To my benefit, this Hemi crank won't require much clearancing for the cam if any at all. My Hemi was a different story...

Due to my lack of proper measuring equipment, I use plasti-gauge to check for bearing clearances.

A shade on the loose side (just between .051-.076) after torquing the cap and rod to the crank at 170 inch pounds, but everything is new, and nothing​ can be done about it really​, so I'm running with it.
Since it's a new engine, I left the factory piston in it's hole and carefully worked around it as to not disturb the rings. I pulled out and installed the billet rod through the bottom...

I then used my angle grinder and small carbide bit to open up the lower hole on the left in the .010 metal head gasket to fit the larger dowel pin on the Hemi block.

Then torqued the milled Hemi head to the block at 19 foot pounds and installed some used 22lb springs from my blown Grayhound clone instead of my buddies supplied 18's.

I then inserted the Mod 2 cam in it's position at TDC before putting the sidecover back on
and installing the ARC billet flywheel.
I set coil gap around .040, doubling up .019 & .020 feeler guages and set valve lash at .002 then checked it all again after turning the motor over by hand twice. Tomorrow I will finish buttoning it up and run it. Try to include a little video...
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Good thing you changed the springs... From my experiences those 18's are junk and like to coil bind... Granted they would've been fine on the mod2 most likely.

Good build so far!

Rocker arm geometry looks good?


Active Member
Sounds pretty good. Should be a nice strong motor & pull well. How many rpm's are you planning on turning it once it's in the frame, any idea?

Daniel Coop

Well-Known Member
I'm hoping it's a solid 7,000 RPM motor... It's out of my hands at this point. My buddy just stuffed it in a Motovox MBX11 roller, so we'll see what he can do with it this Sunday.

Daniel Coop

Well-Known Member
How'd it turn out? Are you running a clutch or TAV2?
Nobody out here is really running torque converters on their minis. He ended up using a 15t Max Torque clutch on it in an otherwise bone stock MBX11 roller he had been sitting on.

His bike was out today at the meet,
and was easily second fastest out today next to my old beater ARCO Easyrider.
We went on a long group ride and I rode my daughter's OldMiniBikes Build-Off Motovox MBX10.
As I expected, his bike was faster than my daughter's stock rod GX200, but when we lined up to do a little drag racing back at the park, niether him or my new buddy Mike with his big block (GX390) mini chopper had much for my beastly little 196cc ARCO Easyrider. That thing hauls... It's an 11:1 compression, stainless small valve Grayhound clone with Dyno's 275 cam.
Here's another little video done by my buddy Walter with his GoPro... I'm the ugly mullet haired dude with the dark red hoodie. Lol. About 2/3's in is the little bit of footage of us racing. I'm the guy out front every time. https://youtu.be/6YfUJaNLmwc
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That little pine tree blowing in wind cracks me up. Looks like a blast running down those streets.


Daniel Coop

Well-Known Member
That little pine tree blowing in wind cracks me up. Looks like a blast running down those streets.

Man, you ain't lying... I have had more fun this year with this group of guys than nearly any other year I can think of. I will truly be sad when it's too cold to meet up, and hope next year will be even more fun. I'm amazed week by week at the shit we have gotten away with riding on the streets of Denver, but we do ride on Sunday through mostly industrial districts where ain't much going on...
Our crew is getting faster and I have landed myself 2 more paying engine builds to keep me busy in the next week or two, so I will definitely be looking forward to next season where there'll be more opportunities for some good old fashioned, heads up street racing and better competition.
As an afterthought​ to this build, I don't think I would build another Mod 2 motor... Its pretty anemic in the lower RPMs, not making much power until after 4,000RPM where it does really come alive. Not an ideal cam for a mini bike in my opinion. I was wrong about my buddy Jose's clutch. It's a 13t not a 15t and he was running a 70t Doodlebug rear sprocket. You'd think with that gear ratio, there'd be plenty of torque, but it definitely left something to be desired off the line. Even with the healthy bump in compression, my daughter's 11:1 compression, stock rod, PVL flywheel GX200 with new design CL1, 1.2 ratio rockers and a 12t to 58t gear ratio seemed to have a slightly better take off.
I have to say, from my experience the Dyno Cams CL3 is my favorite cam for a budget build, mini bike motor and either the Dyno 275 cam or Isky Black Mamba JR cam, my choice for a higher end build. Low end grunt is just as important if not moreso with a mini bike with all the stopping and starting from dead stops. Mod 2 might be great in a kart on a larger track where you can keep the RPM's up, but is kinda lame off the line.
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