Vintage Dirt Bike Q & A

All,

I drained my forks of the 34 year old oil that was still there. Can you say sludge!! I rinsed it our thouroughly with a kerosene while puping the forks on the stand. I drained the kerosene thouroughly and then followed up with two good rinses of Dextron transmission fluid. I would say about 1/2 quart of Dexron is about all it took for both forks. NOTE: all was done with the springs removed!

The manual says refill with Dexron automatic transmision fluid to 160CC. I just happened to find this link.....[url]http://motorbikearchives.com/Bike-Tests ... R175-1975/[/url. In which the authors stated they ended the stiff ride by dropping the dampening oil down to 140cc (1/2 cup each) instead of the recomended 160cc.

It was very good advise! and I cant say enough about the handling now.

I opened it up and was doing 45mph across the soybean rows afterwards and it was a good ride with handling improving at least 75%. The forks did their job and were bouncing like crazy but the the handlebars were moslty level.

After 5-6 miles of running "Old Greenie" through as many bumps as I could find with the forks 20cc low, I did not notice any bottoming out. That includes me bringing the front end off the ground 100% after I hopped a couple of small hills.

Hope this advise helps some of you with 1974 -1977 125's and 175's (same forks)


Larry

Larry,

The Honda MR175 shop manual (purple/white cover) shows 145cc for the refill amount (155-160cc for disassembly/dry fill) , and my CR125M owner's manual indicates 135cc for refill - so it looks like you've got the right amount in there. I used 145cc in mine. Neither mentions Dexron, so I just used non-Dexron ATF. I really don't know the specs for Dexron and other ATF types, although from a quick Web search it appears there are real differences between them, for whatever that's worth. It would be nice to know the equivalent viscosity, if nothing else.

I've gotta do my shocks next... I imagine the benefit will be about the same. :>)

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Hello Ray,

Since most front forks are required to work at temps from -10 to +110 Degrees F any of the Dexrons’ ATF will work fine. Since shocks need good laminar flow with low turbulence at low temps the original Poise number of the Dexrons’ will remain consistent.

Now if the forks had to work at temps ranges from 165 to 225 Degrees F like in an automatic transmission then I would be concerned about viscosity breakdowns but without that heat the ATF flow will be uniform.

Larry

Hi Larry,

Wow, you really know your ATF! Is there a standard viscosity for ATF, or is it just whatever it ends up being after all the more important characteristics are finalized? The synthetic stuff I have here feels very thin, maybe 5W or even less.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray,

Forget somewhat about viscosity and look to the viscosity index or the VI. Viscosity index (ASTM D2270)

Troughout my engineering carrer I have learned to use the following characteristics: viscosity (ASTM D445-97), viscosity index (ASTM D2270) mostly.......since we are not worried about oxidation stability and wear resistance well throw them out for our hydraulics.

Instead of me talking on and on........here

http://www.machinerylubrication.com/article_detail.asp?articleid=884

Larry