Do-It-Yourself Honda V4 Oil System Modification

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As of late 2005, Phil Rastocny may be reached at

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                    Copyright (c) 1993, Phil Rastocny

No portions of this article may be reproduced in any form without the
express written permission of the author. Contributed portions of this
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This is the second part of a two-part article that describes one
way to slow down a known problem with upper cylinder lubrication on
Honda V-4s.

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CAVEAT:  The author assumes no responsibility for any damages or
personal injury that may result from performing the following
modifications. If you feel uncomfortable about performing these
modifications, have an certified mechanic do them for you.
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In 1988 (and some '87 models), Honda changed the oil line to a larger
diameter tubing in an attempt to improve oil flow/pressure to the upper
cylinder area. This new oil line (Honda p/n 12240-MN0-000) retails for
about $75 U.S. and appears to be exactly the same tee-construction as
older lines with the exception of the use of slightly (almost
insignificantly) larger diameter tubing (block diagram shown below).

            --- Upper  ---
            |  Cylinder  |
            v  Couplings v
------------              ------------
| Front    |O============O| Rear     |
| Cylinder |      ||      | Cylinder |
| Bank     |      ||      | Bank     |
------------      ||      ------------
                  ========================O <--- Transmisison Coupling

I recommend replacing this oil line with 5/16" brake line and
matching steel braided flex lines (as described below). When changing
this line, I also recommend that you replace all six crush washers
(Honda p/n 90443-MB0-000, about $12 U.S.).

>From Maximum Auto Parts, 6321 W. Alameda Ave., Lakewood, CO 80226,
phone (303) 234-9223, I purchased the following:

     Qty  Description           Amount
     6    1004 M Clamps          2.70
     2    847555 Brake Lines     9.20
     2.5' 420006 Braided Tubing 12.55
     1    512ST T-fitting        1.55

You will also need a can of WD-40 to flush out any stray metal filings
from the newly-fabricated oil line and some black plastic electical
tape to cover the frayed ends of the braided tubing.

WARNING! Remember that in this part of the modification, you will
expose the otherwise sealed portion of the oil supply system. Use
common sense and keep everything as clean as if you were performing
open-heart surgery!                 ^^^^^
^^^^^^^^^^ ^^^^^^^

0.  This procedure re-uses as much of the original parts as possible.
    If you'd rather not chop up the old parts, and don't mind spending
    some more money, then buy new banjo couplings of appropriate size
    instead of doing steps 2 through 4.

1.  Removed the old oil line. It's alright to bend it in the process
    since you only use the old banjo coupling and toss the metal tubing.

    NOTE:  One banjo coupling has a slightly larger internal ring groove
    than the others.  Remember their positions!

2.  Cut off the existing tubing from each banjo coupling with a hacksaw
    flush with the end of the coupling. Clean off the burrs and filings.

       Banjo Coupling
       (this is round,
       not square)
       /------\         existing oil line
       |       ---|
       |   o      |=============================================
       |       ---|
       \------/   ^
		  cut off flush to here on each coupling

3.  Cut off three 6" sections from the two 5/16" brake lines.  (The
    flared ends are used here and will extend out of the banjo couplings
    when finally assembled.) Clean off filings and burrs. Sand the outer
    section of the unflared end.

4.  Drill out all 3 banjo couplings to accept the 5/16" brake line. Clamp
    the banjo couplings in a vise in between two small pieces of wood so
    as to prevent damaging the surface. Clean off the filings and burrs.

       Banjo Coupling
       |       ---|
       |   o      | <---- drill out to accept the 5/16" line
       |       ---|

5.  Slide the 5/16" brake line 6" sections into each of the three
    banjo couplings. Make sure that the ends of the brake line do not
    protrude into the larger hole of the banjo coupling. Solder each
    line into place. Make sure that you sweat each joint well so that
    it will not leak. I used a propane torch and rosin core solder.

       Banjo Coupling
       /------\             new 6" brake line           flared end
       |       ---|
       |   o      |=====================================<
       |       ---|^
       \------/    |
		  Solder here

6.  Flush each part of the new line with WD-40 and make sure that
    *NO* particles remain inside the final line.

    WARNING!!!  Skip this step and you'll trash the engine!

7.  Rebolt the banjo couplings into place on the bike (do not torque
    bolts at this time). You will have to put gentle bends into these
    lines (easily done with your thumbs) to clear obstacles and allow
    access to the carburator synchronization adjustment screws and
    transmission case hardware. Make sure that the transmission case
    banjo bolt is drilled out as described earlier in this posting.

8.  Cut one 4", one 6", and one 12" section of the braided tubing.
    These pieces will connect the flared ends of the three banjo
    couplings to the tee fitting. The 4" section goes to the rear
    bank, the 6" to the front bank, and the 12" to the transmission
    section.  I used a hack saw and vise to cut through the stainless
    steel braid here.

    CAUTION:  The braided tubing will have some sharp edges so wear
    gloves from here on to keep from tearing up your fingers.

9.  Push the braided tubing pieces over the flared ends of the brake
    lines and then push the tee in place (do not install clamps at this
    time) creating a temporary oil line assembly. If the braided tubing
    is too long, remove it and trim it off so that the new oil line
    routes smootly around existing mechanical assemblies.

10. Once the new oil line assembly fits nicely, remove the banjo bolts
    and remove the new oil line assembly.

11. Remove the tee fitting from the assembly.

12. Wrap all six ends of the braided tubing with 2-3 layers of
    electrical tape. (Make it pretty :^)

13. Slide the six clamps to clamp the end of the braided tubing to the
    lines and then flush out the assembly with WD-40 and make sure that
    *NO* particles remain inside the final line.

    WARNING!!!  Skip this step and you'll trash the engine!

7.  Rebolt the banjo couplings into place on the bike (do not torque

14. Reinstall the tee fitting.

15. Reinstall the banjo bolts and the new oil line assembly.

16. Tighten these 6 clamps down tight since there's over 70 psi on them
    at speed.

17. Start the bike and check for leaks.

18. Retorque all banjo bolts after a few hours of operation.

And Now:

You should notice an immediate improvement in throttle response even when
the bike is cold.    ^^^^^^^^^

The machined oil channels in the head are still the weakest link in the
oil-supply chain. A friend of mine also used some sort of telescoping tool
that followed the oil channels and increased their diameters, but he
borrowed this from NASA and the results are still out as if this really
helped anything.

Some folks have reported that the valves needed readjustment after this
mod. Since there is now more oil in the upper cylinder area, this makes
sense. Althea's and my scoots didn't need adjustment.

Oh, BTW, both our highway mileages increased about 18% right off the bat.
In town, mileage was a little less than before. (I guess that's directly
proportional to the angular displacement of the wrist twist :^).

Enjoy!  Let me hear from you!

    Phil Rastocny
    Now Deceased V-45 Magna (Thunder)
    DOD 4444