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| Building a 125 Classic Class legal MX engine By Terry Miller |
Page 3
Top-end and valve train is where the power gains are hidden. Porting is a very hard science to explain or teach.
90% of home ported heads will increase flow in certain area's but tend to hurt or have no gains in other area's. I
spent 2 days on flowbench with one of the premier porting experts to get the flow numbers we wanted. The final
porting involved gains across the board by a significant margin. 40% average everywhere and huge gains in upper
lifts where our new race cams work.
Granted some heads will always flow a little better than others of same design. This is due to casting variences
and port designs. I can keep all within a few cfm on flowbench and the difference probably won't register on dyno.
Most home ported heads do flow better but if short-side radius is cut wrong(commonly done) the low lift numbers
really suffer. Also people tend to focus on runner before bowl and this just creates volume without helping flow.
The "True-Goal" of these heads is to remove the least to gain the most. If done properly you gain power
everywhere and snappy quick power is result. My suggestion is to have myself or someone very skilled do your
portwork.
On all head rebuilds I suggest new valve guides, valves and rocker arms. A quick simple tip on rocker arm shafts
is to swap the right and left side. You will notice they only wear on one side so this allows twice the lifetime of
shafts. I also suggest Kibblewhite bronze guides over stock iron guides. If your guides are good they will work BUT
if running one of my new race cams and high compression I suggest bronze(plus you get to run seals on both
sides). Also I always replace guides on full build because it allows me better access to porting with guide removed.
Stock Honda valves are adequate for most purposes but I had a small batch of special handmade stainless race
valves made and they were excellent and last longer.
Camshaft depends on rest of combination and rider needs and ability. Our race MX cams are very aggressive
and hard on parts. Reason is I kept duration numbers low with very fast ramp rates and high lift. Creates best
mid-range and top without making motor soggy. These cams are only suited for motors with head, compression
and pipe. If you go to a larger off the shelf cam it will have great top-end but won't pull out of corners as quick.
Camshaft design is a touchy subject and something you don't guess at. Put a big cam on low compression or a
very small cam on high compression and combo just don't work. Camshaft selection needs to be based on
headflow, compression, CC's, usage and rider style. Powroll has some cams that work well. Standard 614 cam is
good all around cam for a 145cc or larger. The new 100 Race cam will work in a 145-150 also and will make better
power for MX use but is harder on parts. I am in process of testing some new cams for 150cc race only, will be
available later this spring. Megacycle makes some great cams also. If ever in doubt on cams you should always err
on smaller side rather than larger cam.
When head is ready to be assembled there is much extra work to check. There is valve to valve clearence, valve
to guide, guide to retainer, rocker to retainer and even cam to head clearence to check. DON"T assume it will
work!!!! You will break or explode motor if not right. I can do this checking very quickly as I have done a lot of
these lately. Valve to valve is most common problem when running camshaft and this is UGLY when not correct.
New stock springs will work on smaller cams and even a 614 cam. If you rev motor too high you may still
experience valve float. Powroll's new race spring is suggested for 614 cam or larger. They will not float but
pressures will wear out parts faster than normal. Titanium retainers are nice but not necessary but always run new
valve keepers when ever valves are removed or head rebuilt. I personally replace tappet screws frequently as they
wear and makes valve adjustment inaccurate.
Valve job and even back-cutting valves is the icing on the cake. 3 angles or more on valve seats and a 30
degree backcut on valves can give you gains of up to 10% flow alone. Many motors I build are just play motors and
I have some mild cams that require no special parts and when combined with a simple "clean-up" port and valvejob
really run. A mild motor done like this with good pipe and carb are more than capable of winning in novice
class(also excellent for cross country or road bike). Also these motors with 10-11 compression will run for many
years with no issues. When I go to full race there is only modest gains above these mild motors but nature of
power is completely different. There is a point that any additional power is very hard to find. This is where
"Matching" all components makes the difference
Once head is all finished and ready to install be sure to use O-ring on composite gaskets and newer metal
gaskets don't use o-ring. Also some cylinders use a long thick o-ring sleeve rather than standard o-ring. Wipe any
oil, fingerprints etc from cylinder/gasket/head. Use brake clean or thinner. Put on clean rubber gloves and install
head. Be sure to get camchain tensioner in slot of head. Take copper head washers and heat to dull red and drop
in cold water. This anneals them so they will seal(there is pressurized oil passage under them. Install and torque
head to spec.
Top-end is now finished so next article will be cam installation and preliminary tuning.
90% of home ported heads will increase flow in certain area's but tend to hurt or have no gains in other area's. I
spent 2 days on flowbench with one of the premier porting experts to get the flow numbers we wanted. The final
porting involved gains across the board by a significant margin. 40% average everywhere and huge gains in upper
lifts where our new race cams work.
Granted some heads will always flow a little better than others of same design. This is due to casting variences
and port designs. I can keep all within a few cfm on flowbench and the difference probably won't register on dyno.
Most home ported heads do flow better but if short-side radius is cut wrong(commonly done) the low lift numbers
really suffer. Also people tend to focus on runner before bowl and this just creates volume without helping flow.
The "True-Goal" of these heads is to remove the least to gain the most. If done properly you gain power
everywhere and snappy quick power is result. My suggestion is to have myself or someone very skilled do your
portwork.
On all head rebuilds I suggest new valve guides, valves and rocker arms. A quick simple tip on rocker arm shafts
is to swap the right and left side. You will notice they only wear on one side so this allows twice the lifetime of
shafts. I also suggest Kibblewhite bronze guides over stock iron guides. If your guides are good they will work BUT
if running one of my new race cams and high compression I suggest bronze(plus you get to run seals on both
sides). Also I always replace guides on full build because it allows me better access to porting with guide removed.
Stock Honda valves are adequate for most purposes but I had a small batch of special handmade stainless race
valves made and they were excellent and last longer.
Camshaft depends on rest of combination and rider needs and ability. Our race MX cams are very aggressive
and hard on parts. Reason is I kept duration numbers low with very fast ramp rates and high lift. Creates best
mid-range and top without making motor soggy. These cams are only suited for motors with head, compression
and pipe. If you go to a larger off the shelf cam it will have great top-end but won't pull out of corners as quick.
Camshaft design is a touchy subject and something you don't guess at. Put a big cam on low compression or a
very small cam on high compression and combo just don't work. Camshaft selection needs to be based on
headflow, compression, CC's, usage and rider style. Powroll has some cams that work well. Standard 614 cam is
good all around cam for a 145cc or larger. The new 100 Race cam will work in a 145-150 also and will make better
power for MX use but is harder on parts. I am in process of testing some new cams for 150cc race only, will be
available later this spring. Megacycle makes some great cams also. If ever in doubt on cams you should always err
on smaller side rather than larger cam.
When head is ready to be assembled there is much extra work to check. There is valve to valve clearence, valve
to guide, guide to retainer, rocker to retainer and even cam to head clearence to check. DON"T assume it will
work!!!! You will break or explode motor if not right. I can do this checking very quickly as I have done a lot of
these lately. Valve to valve is most common problem when running camshaft and this is UGLY when not correct.
New stock springs will work on smaller cams and even a 614 cam. If you rev motor too high you may still
experience valve float. Powroll's new race spring is suggested for 614 cam or larger. They will not float but
pressures will wear out parts faster than normal. Titanium retainers are nice but not necessary but always run new
valve keepers when ever valves are removed or head rebuilt. I personally replace tappet screws frequently as they
wear and makes valve adjustment inaccurate.
Valve job and even back-cutting valves is the icing on the cake. 3 angles or more on valve seats and a 30
degree backcut on valves can give you gains of up to 10% flow alone. Many motors I build are just play motors and
I have some mild cams that require no special parts and when combined with a simple "clean-up" port and valvejob
really run. A mild motor done like this with good pipe and carb are more than capable of winning in novice
class(also excellent for cross country or road bike). Also these motors with 10-11 compression will run for many
years with no issues. When I go to full race there is only modest gains above these mild motors but nature of
power is completely different. There is a point that any additional power is very hard to find. This is where
"Matching" all components makes the difference
Once head is all finished and ready to install be sure to use O-ring on composite gaskets and newer metal
gaskets don't use o-ring. Also some cylinders use a long thick o-ring sleeve rather than standard o-ring. Wipe any
oil, fingerprints etc from cylinder/gasket/head. Use brake clean or thinner. Put on clean rubber gloves and install
head. Be sure to get camchain tensioner in slot of head. Take copper head washers and heat to dull red and drop
in cold water. This anneals them so they will seal(there is pressurized oil passage under them. Install and torque
head to spec.
Top-end is now finished so next article will be cam installation and preliminary tuning.
Top-end and valve train