Honda CR250 Engine Build part 8

[dogger315]

This segment will cover piston, rings and cylinder installation and power valve
adjustment.

Tools required: Torque adaptor, torque wrench, standard assortment of
metric tools.

Most people have rebuilt the top end of their ride so a lot of this information
is old news. I am presenting the techniques that have worked best for me
over the years. Hopefully you will see something that might be helpful the
next time you tackle this job.

First order of business is protecting the crankwell from debris. Stuff a shop
cloth in and around the connecting rod.


Apply oil to the cylinder studs and install with the rounded portion of the stud
facing down. Use either a stud installer or a couple of nuts tightened
together and torque to 9 lb.ft.


Coat the small end bearing with two stroke oil and install it into the rod.


Install a circlip into position on the piston. Coat the wristpin with two stroke
oil. Place the piston over the connecting rod orienting it with the skirt cutout
facing aft (If you are using an OE piston, orient the piston with "IN" facing
aft.


Install the other circlip insuring both are correctly seated in their grooves.
Also, never re-use circlips, you're just asking for trouble for a dollar's worth
of parts.

Apply two stroke oil to the piston rings and ring landings. Orient the rings
with the mark (next to the gap) facing up and install with the ring gap
straddling the pin in the ring groove.

Apply oil to the two dowel pins and install.


Install a new base gasket.


Apply grease to the pinion shaft and make sure it is oriented correctly.


Liberally apply two stroke oil to the cylinder wall. Place the cylinder over the
studs with one hand while compressing the top ring with the other.
Carefully lower the cylinder over the top ring then compress the second ring
and repeat. Once the cylinder is past the rings, be careful not to turn the
cylinder anymore than absolutely needed to orient the cylinder over the
studs - It is very easy to snag a ring on one of the ports and break it.

As you carefully lower the cylinder, take a look under the right front corner
and "eyeball" the line up between the upper and lower power valve pinions,
you may have to rotate the power valve clockwise slightly to get a good
join up. The pin on the lower pinion goes in the gap in the upper pinion.
The cylinder should seat without any gaps. If you have a gap, chances
are the power valve pinions aren't joined correctly. Simply lift the cylinder
an inch or so and manipulate the power valve linkage as you lower the
cylinder again.

Once the cylinder is firmly seated with no gaps, give the power valve linkage
a tug to make sure it doesn't move. Apply oil to the cylinder studs, install
the four flange bolts and finger tighten. You will need the torque wrench
adaptor to tighten the front two nuts. Please do not use an open end or
even a box wrench and guestimate the torque. Unless you have been doing
this a long time and have a calibrated hand, you are setting yourself up for
a blown base gasket or possibly a warped cylinder base. Buy the adaptor
and do the job right.


Tighten the flangs bolts in a crisscross pattern in three steps.

Using the kickstarter, cycle the piston a few times. It should move freely
and quietly.

Next up is the power valve adjustment. Now that the power valve is
connected to the governor, the slack in the linkage can be adjusted out.
Using a 4mm hex bit on a socket wrench or a T-handle (something you can
generate some torque with), loosen the socket bolt on the pinion shaft
slightly. Turn the power valve fully counterclockwise and insure the flap
valve is fully closed (check the reference mark on the left side inspection
port points to "L"). With everything fully closed, rotate the pinion shaft
slightly more to the left with the 4mm hex bit and tighten and torque to
4 lb. ft. The power valve linkage is now slightly preloaded and their should
be no slack in the linkage. Honda recommends 0-0.5mm gap between the
pinion spring and pinion lever for this setting. I like to set mine right at
0.5mm.


Check that the power valve position indicator is still pointing to "L". Apply
two stroke oil to a new O-ring and install the left side cover and torque to
9 lb.ft.


I like to spray some Permatex gasket adhesive on gaskets that don't have
allignment dowels. Just a few quick sprays of Copper spray in this case.

Let it tack up a couple of minutes and position the gasket right where you
want it. This spray is available in Copper (hot areas) and silicone and is
also good insurance for areas like center case gaskets.


Install the right side power valve cover and torque the bolts to 9 lb.ft.


Starting to look like a motor now.


dogger

[Woody_393]

Very nice! And yes, it's starting to look a lot like a motor

[Tonk]

don you are amazing seriously! i am going to attempt this for the first time in a week and a bit i have no idea what im doing and some of the name parts just flew over my head? maby you could post some part pictures or something if u have the parts or time =]

thanks for this any ways

Ian

[dogger315]

some of the name parts just flew over my head

Ian,

Let me know which parts you are talking about, and I will be glad to post
pictures of them.

dogger

[widegliderich]

Sir. you articals on cr250 rebuild are great. i am a micro sprint racer. i would like to do my own engines for the 270 class. could you tell me were to get all the information on rebuilding, or can you send me all the info i need.

have you done any micro engines? i think you would be a great help to me. please see if yo can help me out.

[dogger315]

First of all, welcome to ATM.

If you are interested in doing your own rebuilds, I recommend you
purchase the factory service manual and maybe a Clymers manual
as well. These will be your maintenance and building "bibles". You
can pick up the factory service manual for specific years from this
site: [Only registered and activated users can see links. ] You can get the Clymer manual from
any bookstore.

The series of post on the engine build will walk you through the
steps and hopefully, help clear up some of the confusion people
get from the manuals as well as cover some "gotchas".

Once you start your rebuild, I will be happy to answer any specific
questions you have, just post them here. If I can't help, there are
other on this forum with more knowledge and experience than me,
that can.

dogger

[Faded]

Originally Posted by dogger315

Install a new base gasket.

I've seen too many leak @ the base gasket where the two case halves meet so I apply a small bead of RTV on both the front and rear seams before installing the new base gasket.

Originally Posted by dogger315

As you carefully lower the cylinder, take a look under the right front corner
and "eyeball" the line up between the upper and lower power valve pinions,
you may have to rotate the power valve clockwise slightly to get a good
join up. The pin on the lower pinion goes in the gap in the upper pinion.
The cylinder should seat without any gaps. If you have a gap, chances
are the power valve pinions aren't joined correctly. Simply lift the cylinder
an inch or so and manipulate the power valve linkage as you lower the
cylinder again.

One thing I do to make installation of the cylinder and the powervalve actuator arm easier is to remove (or leave out) the cotter pin on the top of the crow's foot actuator arm and let the PV linkage assembly barely capture the top of the crow's foot acutator arm. Gently tighten the allen-head pinch bolt just enough to keep it from falling out of the cylinder leaving the crow's foot portion hanging below the base of the cylinder. It's hard enough to try and see the engagement of the two and many people end up smashing the crow’s foot causing the exhaust valve to bind and be inoperable. At least this way you can see what you're doing.

[FFRacing11x]

Originally Posted by Faded

One thing I do to make installation of the cylinder and the powervalve actuator arm easier is to remove (or leave out) the cotter pin on the top of the crow's foot actuator arm and let the PV linkage assembly barely capture the top of the crow's foot acutator arm. Gently tighten the allen-head pinch bolt just enough to keep it from falling out of the cylinder leaving the crow's foot portion hanging below the base of the cylinder. It's hard enough to try and see the engagement of the two and many people end up smashing the crow’s foot causing the exhaust valve to bind and be inoperable. At least this way you can see what you're doing.

Sounds like a pretty neat little trickTdub

[Faded]

Originally Posted by FFRacing11x

Sounds like a pretty neat little trick

This trick is a Tdub/FFRacing exclusive. One of the many that has been told to me over the years and one that I've always used since learning it. Tdub's probably forgot more than I'll even know about dirt bikes.

[dogger315]

Gently tighten the allen-head pinch bolt just enough to keep it from falling out of the cylinder leaving the crow's foot portion hanging below the base of the cylinder.

Great tip! Thanks for sharing.

dogger