Maico 326 revisited

[Hoony]

hoony, keith did graft a honda topend
onto a maico bottom end
and keith makes all the sleeves  he can do them from scratch
but i am not sure if he is still doing them
and i dont think paul is doing them  
paul can ask and get back to us



and he did transplant a honda engine in a maico frame
it was his trail bike

well there ya go , thanks Honny's Buddy. wonder if any photo's exist. That could be you mission Paul should you choose to accept it

[holeshot buddy]

i think i have a picture somewhere of the engine
and also the honda/maico will have a look

[JC]

Thanks Holeshot & Paul. I tho't he'd be making his own sleeve for the 326.

[firko]

I would have thought that fitting a 250 crank to a 400 and skimming 10mm from the base flange of the barrel would have been the easier option to producing a 326. That's how the factory did it.

[paul]

this what paul stacker said

hi paul year getting ready packed all weekend
326 is a 250 with a 400 piston in it the barrel is sleeved out to fit

[paul]

just came back from stackers and keith  was fitting a 490 sleeve to a 76 cases and 400 barrell  ,you would nt know it was in there if you get my drift

[paul]

but he said rebores were very limited

[JC]

Paul,

Next time yr over at Stackers, can you ask him if he leaves the boost port in the 250 barrell for the 326 conversion or removes it when he resleeves it?
Thanks

[paul]

sure i rang and hes busy but he said they take 10 ml out the barrell for the sleeve and the  boost port isnt that deep so they redo the port  (its chinese to me ) so if you want to  know any thing in particular
 tell me which model were talking exactaly cause from memory in the original thread it was big k 75 model 250
 here my email  paul@pftrees.com.au
  and i will forward it to him and then post the reply im going over tomorrow  any way

[JC]

Paul,

Did you get my email?

Yes that boost port mod makes sense.

I'd also like to know what he does to the inlet also. Enlarged? Widened? Lowered? How much?

I'm still talking about the early radialfin motor. Esp 73, but 74/75 don't seem to be a lot different.

Many thanks

[paul]

yeah i got it and sent a reply  apparently the infomation well has dried up and he said send a barrell in and get it done was the last reply (secret maico bisness)

[firko]

I recieved the below email from American Maico guru Jim McCabe a number of years ago and only just found it while cleaning up my hard drive files. It's an interesting read, even if you don't totally agree with his theories.

MAICO 326 THEORY
I built a 326cc oval barrel in 1967 using 250 -70mm stroke crank
under a 360 oval barrel cylinder for Enduro work. The 360 oval barrel
with a 76 mm stroke vibrated a lot at higher rpm and the 70 mm stroke
326cc I came up with was much smoother. I was using a tucked in
silencer from a Bultaco Madador on the 360 oval barrel at that point
and it and the 30 mm carb carried right over to the 326cc version
with no change. Compared to my buddies 360 oval barrel I lost a bit
of torque but had another 12-1500 rpm on top and actually the bike
became faster in the woods and definitely had a much faster usuable
cruise speed for Enduros. I recently got my original 326 chassis(62-
65 single loop frame) back after 40 years complete with the engine
etc. to restore.
I purchased a factory made 326cc radial top end a few years ago, with
the 70mm stroke, but with a 250 pattern rear boost port inconjunction
with 77 mm bore. I've done a bit of research on various combinations
of 400,440 and 501 engines. Incidently the original 360 oval barrel
cylinder required decking the top of the cylinder and also the top of
the crank case by 1/2 the stroke decrease (3mm off each) to get the
proper degrees of timing. The key to any of these mods is measurement
of the port locations in the cylinder castings to see if the finished
port timings are close to optimum. Perhaps the most interesting swap
is conversion of a 501 back to approximately 460cc via a 250 stroke
crank. Yamaha rods with longer center distance can be employed to
reduce the machining required in some cases, and late model cranks
can be destroked to provide the advanges of 3 row primarys.
Incidently I dropped off a 80+ pound block of aluminum for my brother
to start machining on a 760cc. McCabe top end. 108mm bore planned.


Within limits, I believe bore and stroke relationships are less
important as far as power characteristics/max power than the port
timing and the exhaust expansion chamber of the engine being built.
The biggest advantage of the short stroke versions is they usually
have larger transfer passages (in proportion to displacement) than
the original smaller cylinders and are capable of breathing better on
top end in addition to the increased displacement. Power output is
controlled really by how many cfm can be flowed per minute and the
shorter strokes will allow higher rpm thus more strokes/minute within
which to flow fuel and air.(and many of the destroked engines have
wilder port timing than the original cylinders had) In the past 50
years the biggest change in Maico engines is in increased vertical
transfer port areas. The difference in actual exhaust port areas
between a 68 250 square barrel and a 2000 water cooled power valve
250 is actually not that much but the transfer ports have doubled in
area during that period. My long term rule is to optimise the
transfer port areas for better breathing and more top end power.
(440's in particularlar need increased area as the increase in bore
dia. is obtained by a reduction in transfer port area) All that said,
a longer stroke combined with a shorter (percentage rod
length/stroke) rod will provide better low speed punch than a short
stroke version of the same displacement in my limited experience.
Nothing beats porting cylinders area wise and degree wise for the
rpm/power characterists desired using data available to us today.
Expansion chambers are another matter however and I'm planning on an
inertia type dyno to facilitate tuning of my engines more accurately.


Some of the 370 Suzuki Savages had very violent power characteristics as
well. I believe there was eventually found a solution to some of the
violence thru the use of an altered ignition advance cure. The best
example of over square under square engines power output I'm
experienced with, is the 125 thru 350 Bultacos where the 125 despite
it's long stroke was a very sucessful high out put engine while the
350 short stroke Astro's are also very very competitive. And the 350
singles went very well on Road racing as well. I firmly believe it's
port timing/expansion chamber design that's the predominant
contolling factor, although most 250 motocross 2 stokes became
slighter under square (strokes became longer) in recent years. I have
to admit that my 499 has grown to be some what of a light switch
engine as far as how it hits compared to power delivery of a stock
400/440/490. as I've strived for more top end output.---

[JC]

Thats very interesting from one who has a lot of runs on the board w Maicos (ie Jim McCabe). Also interesting to see that the factory did produce a radial-fin version based on the 250 boost-port barrell.

While it would be difficult to go past a more-or-less 'proven product' like Keith Stackers version of the 326 if one wanted to go the 326 route, its worth throwing in a few other ideas. Maybe its a bit abstruse/esoteric, but then we do well to have theory push the boundaries of practice. Perhaps there's a 'better mousetrap' around the corner!

The limitation as I see it is the boost-port/Inlet strangulation. Its bad enough on the 250 - how much more so on a 326 version. With a larger dia piston, the IN port can of course also be widened if there's enough meat in the barrell (which there appears to be), but would it be sufficient to let the 326 breath to its full potential?

It would be tempting to remove the boost port when making the 326 sleeve, then remove the boost port casting in the IN tract & widen the TRs to compensatee & give sufficient TR time-area for 326cc. There appears to be enough room to do so (w/out having the TRs too close to the EX), & enough meat in the TR castings. (You lose about 1mm in ht - which aint a bad thing on this engine - but gain about 1mm in width in all the TRs just by boring for a larger piston/sleeve.)

That would give a nice unobstructed IN port, & perhaps an even better 326 version.  But you're still limited by 326cc, which is kind of no-man's land.

I vaguely recalled that the factory experimented w a "revvier" 352cc GP engine in the 70's. Yesterday I found that (brief) article. It was in 74. Weil commented that it had about the same peak power (as the long-stroke 386cc '400'), but vibrated less & was "easier to ride".

My guess is that it was an 80x70 engine (which is exactly 352cc) probably using a 250 crank, & the pic showed the shorter 7fin barrell on the bike. Obviously the factory never developed that engine to production. Not surprising since most of a small factory's limited development resources were in suspension at the time, & other major competitors were increasing capacity not reducing it.

I can't help wondering tho what might have been if they'd used the 82mm 440 piston w a 70mm stroke. That would have given 370cc, almost 20cc more than the 352 & perhaps more competitive at the time.

And thats a conversion worth considering, as Jim McCabe suggested above. LA Sleeve's 82mm 'blank' sleeves are only 86.6mm OD - just a fraction more than the spigot in the 250 cases.

[JC]

Actually, having checked LA Sleeve's catalogue, its the 80mm bore blank sleeve thats 86.4mm OD. The 82mm bore blank sleeve's OD is 88.4mm  (My memory aint always what it used to be)

Here's a few tho'ts on using a 400/440 barrell & head on 250 bot end - just floating ideas.

Apparently the 73-74 250 rod is 5.5mm shorter than 400's while piston deck hts are the same. So w the 400 engine having a 13mm longer stroke, if you bolt a 400/440 barrell & piston on the 250, the bot of the Ex/Tr ports is about 1mm below the piston-top at BDC but the top of the cyl/sleeve is 12mm above the piston-top at TDC. (The 400 barrell appears to be about 12mm taller than the 250's). Which of course is useless & the port timing is ridiculous.

Now if you can drop the barrell 4mm (by machining bot of barrel &/or top of cases), based on the 400 SQUARE-BARRELL port hts, the EX & TR timing are very nice for a big-bore (c 89.5deg & 117.5deg ATDC) tho IN is a little shy (c 74deg BTDC) w an 82mm skirt. Incidently, that is almost identical to KX400 timing (except IN is 76deg).

However you still have 8mm above piston at TDC. Somehow you would have to lower yr head about 8mm, perhaps by machining some off the top of the sleeve (w/out removing all the top flange of the sleeve) & some from the bot of the 400 head (if there's enough meat there) so that the combustion chamber sits down inside the top of the cylinder like the 501 & 250 Canam. 

Or make a new liner to suit & machine 8mm from top of barrell (which could remove the top fin). If you did that, it would be tempting to consider using a 440 piston & a thin-wall sleeve in a 400 barrell machined to suit. Jim McCabe mentioned that the std 440 Maico is a bit restricted in the transfers, but it may be fine for 370cc which is what 82 bore x 70 stroke makes.

Of course, you may have to rebalance the crank & adjust the volume of the combustion chamber too.

Wonder if someone (like Jim McCabe) has already done it.

[paul]

ive been in formed that one off the stacker 326 barrells and head are in with all the ,maico parts i bought .

 ;Dtheres another one there ill have a look .what should it measure