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| Building a 125 Classic Class legal MX engine By Terry Miller |
Page 2
With rebuilt or stroker crank finished and set aside there are things to do before assembly. First is to up-grade to
a xr-200 oil pump. You will have to remove cover and replace driven gear with one from your 100/125 original
pump. Also you need to get countersunk Allen head bolt of proper length to fasten pump to cases. On gasket set
be sure to have correct case gasket( some of the Chinese/Japanese ones are wrong!!!). There are two types,
one with small oval and one with large round hole for oil pump. MUST use oval as oil leakage will occur from oil
pump. Not good as pressure is lost and all your new parts just got fried.
Install gear set, drum and shift forks with all shims in proper order. A good Honda dealer will give you a print of
photo of gear set assembly if you don't know where everything goes. Drill hole on top of case where oiling hole is
next to cylinder stud, use a #53 drill to slightly enlarge hole. ONLY do this if you run 200 pump assembly. Be sure
to replace crank bearings with high quality C3 clearance bearings(PS some have a roller bearing on one side, you
may replace it with ball bearing and sleeve will drive out from case). I personally replace all tranny bearings every
time I split cases as I feel better with new bearings. Install kick start mechanism in case and when everything is
there bolt cases together. Replace all Phillips screw with Allen head bolts as easy to remove and torque (plus they
look better) when tightening.
Now install all clutch, oiling, shifter shaft, shifter stars and springs etc. Leave flywheel loose but install key in
crank. Install covers leaving flywheel side off for now. 2 types of cam chain tensioner systems. One is spring
loaded and other uses plastic block and is mechanical. I prefer spring as with full race parts the clip on manual
adjust type has been known to come apart. You are pretty much stuck with style you have as cases are slightly
different for each type. If manual type BE SURE E-clip is perfect and tight. You can drill and install a pin instead
and this is better.
Spin crank and shift through all gears to verify working properly. Also verify kick start is working as it in no fun
splitting cases to repair. If everything is great it is time to finish up bottom end.
Install Heavy-Duty timing chain(available through Powroll) as stock chain will not last with race cam and high
RPM's. Set cases aside and cover so no junk gets in it. With fresh bored cylinder take your new rings and check
ring gaps. .007-.008 is safe and if running"uber" high compression .009 is safer. When running 12 or more
compression it is advisable to have piston crown and combustion chamber coated with a ceramic heat barrier. I
also coat sides of skirts with a friction coating and will even do exhaust ports(I will coat parts for you if sent to me,
also have a thermal dispersant coating for outside of cyl/head to help keep them cool, it is a graphite gray color).
Suggest new friction and metal clutch plates along with heavy duty clutch springs.
Take two wooden blocks and fit under skirt of piston to hold it square in cylinder. With a well lubed and clean
cylinder slowly lower on studs and very carefully pinch ends of rings and slide cylinder down. Pay very close
attention on oil rings and don't force it. If any concerns remove and try again. Be sure ring gaps are staggered
from each other. Once cylinder is on, set assembly to side and time to focus on cylinder head/cam. Till next time.....
Bottom-end build.