92-01 CR250 Engine build

[dogger315]

I am scratch (build with new parts) building a 1993 CR250 HPP valve
engine for a restoration I'm doing. I always photographically document
my work and thought this information would be usefull to those out there
that are running this engine and are contemplating some internal work
of their own.

I will submit the information in multiple installments over the course of the
build.

Round one includes the center cases and right side case cover.

There are many ways to install bearings. These bearings were installed by
heating the cases to 225 degrees in an oven (do not use a torch) and
placing the bearings in a freezer and cooling them to 0 degrees. The heat
expands the case and the cold contracts the bearings. The cold bearings
drop right in the hot cases then develop an interference fit when both reach
room temperature. I recommend this procedure because it has the least
potential for damage. All retainer bolts should be coated with a locking agent
(imagine the carnage a loose 6mm bolt could do inside your transmission).
And finally, clean is the word. Do not do engine work on a dirty engine and
do not do engine work on a dirty work bench. Don't sabatoge all your effort
by introducing contaminents into your engine internals.

Engine work is straight forward and relatively simple. Jump in and save
yourself some money.

If you have any questions or comments, please let me know.

Round two will cover crankshaft, transmission and shift drum installation.

dogger


Clutch side of the right side center case with bearings and shift lever return
pin (line) installed


Crankshaft side of the right side center case. Note rubber vent baffle
(line). A potential air leak if not installed.


Crankshaft side of left side center case with bearings and retainer installed


Right side case cover at water pump housing (crankshaft side) showing water
pump impeller oil seal installed


Same section with water pump impeller shaft bearing installed. It is safe to
heat this casting to 225 degrees even with the seal installed to facilitate
bearing installation.


Same section again with water pump impeller drive gear installed


Clutch side of right side case cover with water pump water seal installed.
This seal is prone to damage when installing. Use of a specialized seal driver
or silicon lube and extra care is recommended


Water pump impeller installed and torqued to 9 lb.ft. Use extra care when
tightening the impeller nut. You will need to hold the plastic drive gear. I
recommend a gear jamming device available from Motion Pro and others.
Use of pliers or screwdrivers may damage the plastic gear.

[MXtras]

Excellent so far - keep it coming!

Scott

[json]

I'm running that engine. I'll definitely be following. Thanks!

[mrcribbs]

Wow...that's one clean engine case!

[Woody_393]

somehow I missed this. I am trying hard to follow all of these since I'll openly admit the low end on any bike for some reason scares me. This is helping me 1) Understand how it all goes together, and 2) remove the fear so one day I can tackle this same task on any bike.

The same rules apply it just might look different.

Great Job dogger315!!

[dogger315]

1) Understand how it all goes together, and 2) remove the fear so one day I can tackle this same task on any bike.

Woody,

You hit the nail on the head. That's exactly why I'm doing it.

dogger

[dbroms]

Just finished bottom end rebuild on 1992 cr250. All bearings etc replaced. Now ive just put the two cases together and the crank spins ok but im not sure if i have a problem with the gearbox. If i hold onto the shaft that the sprocket goes onto and i rotate back and forward what i think is called primary drive shaft ( clutch goes on ) theres loads of freeplay or backlash. i can rotate the gear selector drum ok and it seems to change gear but i dont want to proceed any further in case i have to strip again.
not sure if ive explained well enough but hopefully someone can ease my fears .

[dogger315]

If i hold onto the shaft that the sprocket goes onto and i rotate back and forward what i think is called primary drive shaft ( clutch goes on ) theres loads of freeplay

You are talking about the countershaft (the shaft the sprocket mounts to)
and the mainshaft (the shaft the clutch mounts to). There is supposed to be
some backlash. Remember, at operating temperature, these gears will expand
slightly and some of that "slop" will be gone. Honda doesn't specify limits for
backlash but does specify service limits for each gear and shaft. If there is
more than 2 or 3 degrees of rotational play in either direction, the gears and
the shafts should be checked with a micrometer and a digital caliper for
excessive wear. The service limits can be found in the Honda Service manual.

dogger

[dbroms]

i definately have more than 2 or 3 deg, looks like im gonna have to strip it down again, not happy now but thanks for the help mate.

[dogger315]

looks like im gonna have to strip it down again

Before you split the cases again, just how much backlash are
you talking about? 5 degrees, 10? Did you check the tolerances
when you inspected the gears and shafts when you had it apart
before?

The reason I am asking, if the bike ran and shifted OK before the
teardown and you just wanted to tighten up the bottom end with
bearings and seals, don't worry too much about the backlash. Well
worn components will obviously have more slop than new. As long
as the parts are not worn beyond limits, your engine will perform
fine and you WILL notice a nice improvement with the new bearings
and seals.

It's your call.

dogger

[dbroms]

well the slop is a good 30 deg but i need to understand the transmission/ . not sure whats wrong but i will be checking in the morning. as far as i can see it seems to be that there are gears with raised items that are giving me the problem. i will post a pic in the morning

[dbroms]

well just an update, ive just split everything down again and checked the gearshafts, everything is fine according to the schematic diagrams in the manual. The slop is coming from when the gears engage there are dogs on them, when these go into the slots of the other gears theres plenty of space for them to move. i just cant see where its wrong or is it just me not understanding it.