Thanks Midway.
Thats the same deck ht as for earlier radial-fin engines, but the top ring is lower which would mean one could retard the TRs (& EX) & advance the IN timing by machining some off the base of the cylinder if one so desired (to correct the time-areas) & used a Magnum piston. Of course the head would have to be adjusted to suit. My guess is thats exactly what Keith did on your barrell/head Midway. Would be interesting to know yr cylinder ht Midway; base-gasket to top of sleeve.
Having spent considerable time over the w/end poring over 2 pre75 barrels (73 & 74), its hard not to conclude that Maico should be ashamed of themselves for this engine as delivered. The quality control & attention to detail is atrocious.
In port heights it seems they were content w 1mm variance from side to side.
In port widths (esp TR) there is an incredible 2mm variance one side to the other!!
On the 74 barrell almost every port is diff in size to the equivalent on the other side.
No wonder the factory riders were said to search for a "good" barrell to start with!
As bad as that is, perhaps the biggest stuff-up is the design of the boost port & its effect on the IN area. Because of the angle, the effective area of the boost port as it enters the cylinder above the piston is 2cmx1cm (ie 2 sq-cm) on the 73 barrell. But the hole in the back of the piston is 2cm dia (ie 3.14sq-cm), necessitating the boost port casting intrude considerably (to put it mildly) into the IN tract & port area.
Had they made the boost hole in the piston 2cm wide x 1cm deep, they would have had a matching 2sq-cm area & more importantly been able to make the boost port casting intrude 1cm LESS into the top of the IN port. That would have given an extra 2.5sq-cm of IN port area (cos of the size of the boost port casting) which it desperately needs.
Also, the IN tract simply splays outward once it comes to the boost port casting (which is 3cm wide) which further reduces cross-sectional area as the charge enters the crankcase. There is no attempt to even keep a constant cross-section of the inlet tract, let alone a 10-15% larger one. Lewo tells me there was some attempt to correct this on the AW barrells, but I don't have the dimensions to know if they corrected it sufficiently.
The unneccessarily large boost port casting simply strangles the engine (esp 73/74) at the point of entry. If nowhere else, it desperately needs work in that area. (And a suitable pipe, as per earlier posts)
As a friend (& owner of one) suggested, it would almost be worth making a new sleeve for this engine to correct its inherent design faults. Then wack on a decent pipe (GMC?) & have a hoot of a time racing perhaps the best-handling VMXer out there - at last w the performance to match maybe.
