| Building a 125 Classic Class legal MX engine By Terry Miller |
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This series is a basic guide to building a AHRMA legal Classic 125 Honda 4-stroke. There are many parts and
combinations available for these. The problem is mismatched components or failure to do a complete build. If
motor is purpose built and all components are matched then you will get a powerful and long lasting motor.
It all starts with a core motor. Rules state a 1971 or "like-design". This means a 1970-74 SL/CB/CL 125. Note a
100 motor may be used but requires more machining. Unless you have a perfect low mileage well maintained
motor, I recommend an entire rebuild. I suggest crankshaft be inspected and rebuilt and a heavy duty rod doesn't
hurt.
Buy a stack of zip-lock bags, a good "long" impact driver phillips bit and a hand held spot blaster from Harbor
freight. Clean outside of motor well before tearing apart. Remove head, cylinder, piston and side covers. DO use
impact driver to loosen screws as they tend to be stuck and if you strip them it is difficult to remove them. Continue
to disassemble cases and lay out all parts on a blanket or towel. Be careful to note where all shims are located on
kickstart, shifter and gearbox.
Now clean and inspect all parts. Check camshaft journals and journals on head. If oil was neglected these are
first to wear out. Heads are available on ebay and NOS new heads usually run around 100.00. This is a great deal
in that valve guides are new and valve seats are pristine. Be sure to use either a 125 or 100 head. XL-125 heads
are illegal due to valve size. 27mm intake and 21mm exhaust are legal classic heads. A 100 head is identical to
125 except has smaller combustion chamber. This is easy way to bump compression.
After checking and cleaning all parts, bag and label them. You will thank yourself later.
Use spot blaster with baking soda to clean cases and all parts. Don't be tempted to use bead blaster as nearly
impossible to remove all grit from oiling passages. A good soaking in hot water and all soda dissolves and rinses
away.
Now the rules allow 150cc +.020. Two basic ways to do this. One is Big-bore kit and other is "Stroking"
crankshaft. A bore kit is slightly cheaper and is great if you have a perfect bottom-end or lack skills for case
assembly. After many years of experience I highly recommend you purchase a Powroll kit. These have a "forged"
piston, race rings and are pre-machined for race cams and stroker cranks(175cc when combined). Another benefit
is different compressions are available.
combinations available for these. The problem is mismatched components or failure to do a complete build. If
motor is purpose built and all components are matched then you will get a powerful and long lasting motor.
It all starts with a core motor. Rules state a 1971 or "like-design". This means a 1970-74 SL/CB/CL 125. Note a
100 motor may be used but requires more machining. Unless you have a perfect low mileage well maintained
motor, I recommend an entire rebuild. I suggest crankshaft be inspected and rebuilt and a heavy duty rod doesn't
hurt.
Buy a stack of zip-lock bags, a good "long" impact driver phillips bit and a hand held spot blaster from Harbor
freight. Clean outside of motor well before tearing apart. Remove head, cylinder, piston and side covers. DO use
impact driver to loosen screws as they tend to be stuck and if you strip them it is difficult to remove them. Continue
to disassemble cases and lay out all parts on a blanket or towel. Be careful to note where all shims are located on
kickstart, shifter and gearbox.
Now clean and inspect all parts. Check camshaft journals and journals on head. If oil was neglected these are
first to wear out. Heads are available on ebay and NOS new heads usually run around 100.00. This is a great deal
in that valve guides are new and valve seats are pristine. Be sure to use either a 125 or 100 head. XL-125 heads
are illegal due to valve size. 27mm intake and 21mm exhaust are legal classic heads. A 100 head is identical to
125 except has smaller combustion chamber. This is easy way to bump compression.
After checking and cleaning all parts, bag and label them. You will thank yourself later.
Use spot blaster with baking soda to clean cases and all parts. Don't be tempted to use bead blaster as nearly
impossible to remove all grit from oiling passages. A good soaking in hot water and all soda dissolves and rinses
away.
Now the rules allow 150cc +.020. Two basic ways to do this. One is Big-bore kit and other is "Stroking"
crankshaft. A bore kit is slightly cheaper and is great if you have a perfect bottom-end or lack skills for case
assembly. After many years of experience I highly recommend you purchase a Powroll kit. These have a "forged"
piston, race rings and are pre-machined for race cams and stroker cranks(175cc when combined). Another benefit
is different compressions are available.
| Honda 4 Stroke Engine Specialists PH. 503-333-4543 t.raymiller@yahoo.com |
Page 1
.com
Stroking crankshaft tends to build more bottom and mid range and if done right revs just as well as bore alone.
Stroked engines still require a bore and some additional piston work but can be used with stock or race 125
pistons. I personally prefer stroking as I have crankshafts rebuilt on any motor I build for customer. If a person
was to use these for age/enduro or playbike then I throw in a big-bore kit for 175 or even larger cc's.
Have gear set, forks and shifter drum inspected for unusual wear or bent forks etc. Bikes usually used by kids
tend to have gearbox troubles due to stomping on shifter or poor shifting by rider. Also inside "Oil slinger" it is a
sludge trap. If you see a lot of metal or a lot of sludge you have a motor that requires extra care in checking. Very
dirty motors will wear out bronze bushings and require loads more work and money. You can find core motors on
e-bay or craigslist for 50.00 or less if you look hard. Also this gives you extra head and other hard parts if
problems are found. The extra head comes in handy if you plan on trying your hand at porting(I'll discuss that in
future articles).
Piston choices depend on type( bore or stroke ) motor you build. If doing big-bore(actual 145cc 61mm) then go
with Powroll. Stroker pistons can be stock 125 pistons up to 4th over(2nd if using XL-125 style). I have a very
small batch of special forged lightweight race pistons being made. Not cheap but extremely well made and
absolutely require race fuel.
Compression is a good thing on little motors as this helps build power everywhere. More is better BUT tuning and
parts selection are critical. If motor is to be used for any races or riding that last more than 30 minutes keep
compression ratio reasonable. 11-1 is good compromise. Reliable and only requires slight mixing of race fuel and
pump gas. Anything above 11-1 is a short duration use motor as heat builds up and will "melt-down" if tuning is
wrong or used in trails or long events. Stock compression is 9.5-1. Anything less than 10-1 hurts overall
performance and makes a "soggy" feeling motor. Low compression also limits camshaft choices. If you have a
"Rocky or similar" 150 piston kit then a 100 head is simple and easy swap. This will raise compression 1 full point
above stock. You can tell if it is a "100" head in that chamber is a perfect circle. 125 head has extra cut that
makes it slightly oblong chamber.
Remember this is a guide not gospel as far as article is concerned. There are many different components from
different year models. If you follow basic specs I will outline you will have a balanced and complete package that
works great. A big cam on stock motor or other individual parts won't work correctly if rest of package isn't correct.
Happy motoring and any questions direct them to myself at t.raymiller@yahoo.com
Stroked engines still require a bore and some additional piston work but can be used with stock or race 125
pistons. I personally prefer stroking as I have crankshafts rebuilt on any motor I build for customer. If a person
was to use these for age/enduro or playbike then I throw in a big-bore kit for 175 or even larger cc's.
Have gear set, forks and shifter drum inspected for unusual wear or bent forks etc. Bikes usually used by kids
tend to have gearbox troubles due to stomping on shifter or poor shifting by rider. Also inside "Oil slinger" it is a
sludge trap. If you see a lot of metal or a lot of sludge you have a motor that requires extra care in checking. Very
dirty motors will wear out bronze bushings and require loads more work and money. You can find core motors on
e-bay or craigslist for 50.00 or less if you look hard. Also this gives you extra head and other hard parts if
problems are found. The extra head comes in handy if you plan on trying your hand at porting(I'll discuss that in
future articles).
Piston choices depend on type( bore or stroke ) motor you build. If doing big-bore(actual 145cc 61mm) then go
with Powroll. Stroker pistons can be stock 125 pistons up to 4th over(2nd if using XL-125 style). I have a very
small batch of special forged lightweight race pistons being made. Not cheap but extremely well made and
absolutely require race fuel.
Compression is a good thing on little motors as this helps build power everywhere. More is better BUT tuning and
parts selection are critical. If motor is to be used for any races or riding that last more than 30 minutes keep
compression ratio reasonable. 11-1 is good compromise. Reliable and only requires slight mixing of race fuel and
pump gas. Anything above 11-1 is a short duration use motor as heat builds up and will "melt-down" if tuning is
wrong or used in trails or long events. Stock compression is 9.5-1. Anything less than 10-1 hurts overall
performance and makes a "soggy" feeling motor. Low compression also limits camshaft choices. If you have a
"Rocky or similar" 150 piston kit then a 100 head is simple and easy swap. This will raise compression 1 full point
above stock. You can tell if it is a "100" head in that chamber is a perfect circle. 125 head has extra cut that
makes it slightly oblong chamber.
Remember this is a guide not gospel as far as article is concerned. There are many different components from
different year models. If you follow basic specs I will outline you will have a balanced and complete package that
works great. A big cam on stock motor or other individual parts won't work correctly if rest of package isn't correct.
Happy motoring and any questions direct them to myself at t.raymiller@yahoo.com
| Powroll |
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Core & components