Pocketbike Forum banner

1 - 10 of 10 Posts

·
Registered
Joined
·
83 Posts
Discussion Starter #1
browsing through YouTube in the wee hours of the night, i came across a couple videos of a DM thats titled "DM 70ccm".

its pretty baaaad.

does anyone have anymore info on how this was done?

looks like a BZM bottom end, which i know to have a different head bolt width than polini and running either a Polini Evolution 2 head for a moped or maybe an equal Malossi head. 28mm carb.

would be sick to add that motor build to my stable for being a dish!t at the track. would be even cooler to have a few bikes to get built the same way to get a sick fast race class out of it.
 

·
Registered
Joined
·
39 Posts
Forget about it, i made one engine and yes, it has a lot of power but only on the lower band.. maybe with a special made exhaust pipe you will get a great power band, but after all a well tuned 50cc bzm engine will toast you because you will have to work harder to control the bike.
There is also a lot of work to do on the crankcases, exhaust, cooling system...
 

·
Registered
Joined
·
83 Posts
Discussion Starter #3
I'm sure as with all 2 strokes, the expansion chamber and cones have to match the displacement of the motor to get a reasonable amount of power from.

What kind of work is needed on the cases to get be head to fit? Any mods needed to the rod to fit the piston and wrist pin?
 

·
Registered
Joined
·
39 Posts
You have to mill out aluminium from the cases because the skirt of the cylinder is bigger and because of that the reed cage needs a spacer, otherwise the piston will hit it.
New holes and threads need to be made for the cylinder bolts and the piston has to be shortened in the skirt.
The intake of the scooter cylinder didn't match the cases and that's the major problem, to match the cases you need to refill with aluminium outside to mill inside otherwise the cases will end very weak.
I made that to fit the big cylinder with a standard rod, i don't know if there is any longer rod to fit on the crank to avoid the piston modification.
 

·
Registered
Joined
·
8 Posts
looks like a moped barrel bolted to a BZM crankcase, theres a lot of heads that would bolt up to the crankcases but they are mostly from old euro mopeds.
 

·
Registered
Joined
·
83 Posts
Discussion Starter #6
did some more research on this.. and jumped on ordering a cheap Polini 70cc Evo 3 head kit.

fitting the cylinder to the case is gonna be a task on its own. BUT.. being that the head kits is designed for the old Aprillia RS50's, the recommended stroke is that of the stock polini cranks (39.2mm) and the same size wrist pin is used. the difference in bore from the 50cc is 6.5mm (46.5mm bore). using a displacement calculator.. the bore x stroke make an even 70cc. dope.

what seems to be the common way to go about this is to (after getting the crankcase milled to accept a large cylinder skirt and drilling and tapping new holes in the case for the new stud locations) use a 2 or 3mm aluminum plate to fill the void in the transfer ports. the cylinder's transfer ports at the base of the cylinder is much larger than the crank case itself. probably doesn't make sense so far but.. these pictures should help. (not mine.. but this is from a running bike.)
 

Attachments

·
Banned
Joined
·
24,169 Posts
Great project engine. I was going to try something similar with Polini cases and BZM cyld and head off BZM cases w/ different mount holes than Polini and the ports did not match. I decided since I have no machine shop not to try it.
 

·
Registered
Joined
·
389 Posts
No it won't fit directly, you have to got a cylinder for a minarelli engine, build an adapterplat or weld some alu to the top of the crankcase.
 
1 - 10 of 10 Posts
Top