Top end conversions for the humble 250

[JC]

Gents,

Back in the day when my 73 250 radialfin was past its competitive day, I often tho't of putting a Yam 360 top end on it, but never had the resources.

Why? For me the 400 Maico was the quintessential MXer, even tho I was only a fly-weight at about 6-7 stone. I couldn't afford a new bike, but the Yam 360 had same 70mm stroke & centre-port exhaiust, so I tho't it could be the next best thing.

I still wonder.... but now I think of other options also.

The prerequisite is simply something w a stroke near 70mm & a centre-port exhaust. I'll post several options one at a time w a few tho'ts/observations I've made over the years.

You'll still have to check a lot of things yrself tho', eg piston deck heights, barrell hts etc to get the piston to line up with top of cylinder at TDC, & whether there's enough meat in the cases to relocate the studs etc etc.

Pls consider this thread as a floating of ideas/possibilities - alternatives to the 326 Maico topend route, altho that could be the easiest/best route.

Option 1 is 360 Yam top-end.

Forget the 74 DT360 & MX/MXA360 of 73/74. These barrels had 6 studs in a weird pattern, w weird location of transfer ports at the barrell base - shared only by the SC500 barrell. It would be a massive job to make them fit.

Consider using the RT1/2/3 or RT1/2M barrells. Stud spacing is of course diff to Maico & would have to be adjusted, but at least there's only 4 studs. Skirt dia is 89mm as opposed to 85mm in Maico 250 cases. I don't know if there's enough meat in the early (pre-440 Maico) 250 cases to allow milling out to 89mm & I susppect the liner/skirt on the barrel would be barely strong enough if turned down to 85mm. Perhaps need to meet somewhere in the middle - a bit off both.

440 Maicos must have had larger spigot in cases to suit 82mm bore, so later cases may have more meat there.

Even the trail-bike 360 Yam barrels have decent porting specs, if you raise the ex & perhaps lower the inlet - both easy enough to do. Most of these barrels also have a compression release facility that exits into the exhaust tract. Very neat & clean.

The RT2/3 barrels only have V4 reeds, but V6 reeds can be found or made to fit, if careful. From memory, YZ A/B reeds (if you can find them) or later IT175/200 reeds either fit or can be made to fit. (Early IT175s had V4 reads if my memory serves me correctly).

Transfers cutouts at the base of the cylinder are more angled towards the front (exh port) than on the maico cases but that should be fixable. Piston pin dia is same 18mm for both Yam & Maico.

Capacity would be 352cc w std Yam piston.

[JC]

I should perhaps have said at the start of this thread: Maico puritst read no further!

Option 2 is 74-75 KTM 400 top end (if you can find one). With the radial-finned KTM head, this option would still look somewhat Maico-like. Stroke is 69mm - close enough to Maico's 70mm. I'd run the extra 1mm at the top of the barrell if the top ring allowed it (with a 1mm spacer under the head), as this would give a little more inlet duration.

Bore is 81mm. Gudgeon pin same 18mm dia. Capacity would be 360cc on 70mm stroke. Need to carefully check stud spacing - KTM typically used diff offset for front & rear studs (ie from centre of cylinder). Need to bore out cases to accept larger liner too, if there's enough meat.

Option 3 is 74-75 Suz TS400 (centre-port) top end. This is a bit more involved as gudgeon is larger & stroke is 5mm longer so need to compensate. Barrell also has quite wide stud-spacing. With 5mm taken off top of barrell (if that is manageable) it gives decent exh & trans port timing on a 70mm stroke, tho inlet duration would have to be increased.

Suz Gudgeon is 20mm dia, not 18mm, but 440 Maico pistons are same 82mm dia w 18mm gudgeon. Suz skirt dia is 92mm. Suz barrel also has comp release facility. Capacity would be 370cc on 70mm stroke. Combustion chamber volume would need to be reduced, probably by milling the head, to maintain sufficient Comp Ratio. Need to check squish-band clearance.

[firko]

John...As much as I dig this "nutty Professor" stuff, and understand that you're just throwing hypotheticals at us, I have to take a devils advocate (no pun intended) stance and throw the rulebook at you. Rule 14.2.6.1 (a) states...."Engines must remain externally unchanged". As I state elsewhere on this forum when discussing pipes, sometimes rules shouldn't be there at all and sometimes rules are underworded. This one could do with some further surgery, perhaps changing it to "engines must use only period (prior to Dec.31, 1975) external parts". That would bring the aftermarket head and added on reed situation into legality. As the rule is as written, you can't fit aftermarket heads or anything that changes the outward appearance of the engine. That would also go for your "Frankenengines".

I attended the "CZ World Championships" in California in the late 80's and saw a single shock KTM250 fitted with an aircooled CZ top end and CZ drum brake wheels. It was weird but it actually went quite well. I guess it was some sort of backwards engineering to fit into the rules for the event. Weird but interesting.

[JC]

Mark,

Thats a very good point.

Rule-book spoils a lot of ingenuity sometimes. I think yr suggested rewording of the rule would be excellent.

Seems to me anything from the era should be allowed, but yr right, as the rule stands, such top-end conversions are not kosher.

For what its worth, I'll finish off the various options tho, in due course. Might be useful for Vinduro.

[JC]

Just to finish off this "nutty professor" stuff w a few more options for "frankenbike"-types:

Option 4 - 400 Yam top-end. Of course this wouldn't be pre-75 legal as they came out in 75. Barrell has same 70mm stroke as Maico & gudgeon has same 18mm dia. 85mm bore gives 396cc. 4 studs w fairly wide spacing. MX barrells produced nice power, tho there were some overheating & piston-skirt breakage probs w MX400 barrell (fixed on YZC barrells). DT400 barrell would need some porting to be competitive in VMX.

Option 5 - 250 Yam top-end.  There are 2 diff types of Yam 250 barrell - DT1/2/3 & later DT/MX from about 73 onwards. The later (post 73) ones have wider reedvalve (V6 not V4) & wider stud spacing. Even the DT barrels had decent porting specs. Gudgeon is same 18mm dia. Stroke is 6mm shorter (64mm) than Maico  so needs to be accounted for. The top ring on most Yam 250 pistons is 3.5 - 5mm from top of piston so you can run the piston higher in the barrell & use a spacer under the head. Adj ht of barrel for porting that suits. Capacity is 269cc w std Yam piston, which is obviously not legal in 250 class. Metal spraying the bore could be investigated to reduce it 1mm to make it legal if desired.

Option 6 - 250 Kaw F11 top-end. Same 18mm gudgeon dia. Stroke is 68mm, but bottom of Ex/tran ports is just over  70mm anyway. Capacity would be 254cc & still legal in 250 class. Stud spacing is nearly identical to DT1/2/3 Yam barrells giving alternate options for this set-up. Would need some port work for VMX, esp on Ex & In which are both easy enough.

Happy 'franken-biking'.