Midshaft Talk - Building, Rebuilding, SN vs. RN, etc.

[JetManiac]

There seems to be alot of confusion among many freeriders about midshafts aka intermediate shafts.

Midshafts:
All superjets and blasters have the same length midshaft. This midshaft is unique in length and cannot be found in any other yamaha ski or couch. Older SN's have a smaller threading on the coupler and a thinner and weaker design and should be avoided for builds. Also, you will not be able to swap out their oem couplers for a/m ones like Cold Fusion or ADA.

Midshafts have internal splines in the rear which mate with the splines on the driveshaft. There are 2 orings which seal (supposed to anyway) water from getting to the splines. Splines should be greased during installation because they are inaccessible after pump and midshaft are installed.

Midshaft Housings:
Yamaha midshaft housings come in 2 sizes. The original smaller sized housing was only used with 650 SN's from '90 thru '93. The Waveblaster was introduced in 1993 with the 701 61x motor and it has the larger sized housing. The '94-95 SN and all RN '96-present all have 701's and the same larger sized midshaft housing.

Midshaft housings are made from an aluminum outer housing which is separated from the aluminum inner bearing core by rubber. The inner and outer housings are vulcanized to the rubber. The rubber acts as a dampener which allows the inner bearing core to flex with driveline movement. The rubber of the SN housing is relatively thin and can separate just from vibration. The larger redesigned 701 housing has a much larger rubber area and separation of rubber from aluminum generally only occurs from corrosion.

Housings which separate cannot be rebuilt and must be replaced.



Midshaft housings have a rubber lip to seal the housing to the bulkhead where they are mounted. This seal is easily damaged and can lead to bilge leaks and cavitation issues. It is important that your midshaft seal well to the bulkhead. I prefer to add a thin bead of silicone just to the outside of the rubber lip as a secondary seal. It is extra work to remove and reinstall midshaft but your seal will be assured.

You can see where corrosion is compromising the seal on this salty dog. Corrosion can also pit the inner ring where the grease seal goes causing leaks there as well.

Midshaft housings have a single front grease seal followed by a sealed bearing. On the back side, there is another grease seal(same as in the front), followed by a double lipped, reinforced grease seal in the rear. It is important to note that all of the area to the inside of the rear rubber lip is underwater when your ski is floating.

Midshaft housings which are in good condition can be rebuilt to replace the bearing and seals and function like new again.

There is a hose with a zerk fitting on newer SN and all larger housings. This is used to replace grease which is washed out over time. The grease goes between the 2 rear grease seals only. It doesn't grease the midshaft/driveshaft splines. Caution must be used when adding grease because excessive grease added will either blow out rear seal or the rubber of the housing.

[JetManiac]

Midshaft press-through is critical because this helps to determine the amount of spline overlap. The factory specified amount is 19.5-20.5mm as measured between the rear of the housing (not the rear of the seal) and the rear of the shaft.



When completing a build it is important to check spline overlap, especially with a/m hulls, custom builds, and setback pumps with a/m shafts.

Degrease both driveshaft and midshaft splines first. Install and align pump. Then put a small amount of grease only at beginning of the driveshaft splines and then install midshaft. Carefully remove midshaft and check how far the grease was pushed. Sometimes you need to repeat several times but this is the only way I know to accurately check spline overlap.

If you need to press shaft farther in to correct for a spline underlap, you can add an additional washer behind midshaft coupler to maintain alignment.

[McDog]

If 19.5-20.5mm is how far the midshaft should protrude from the back of the midshaft housing(on a OEM hull), what is the distance of spline overlap needed? This seems like a better measure for us aftermarket hull folks.

[JetManiac]

If 19.5-20.5mm is how far the midshaft should protrude from the back of the midshaft housing(on a OEM hull), what is the distance of spline overlap needed? This seems like a better measure for us aftermarket hull folks.

I don't know if there is a spec for this, but you want full engagement minus about 3/16-1/4"

[Motorheads5]

Sweet post
Thank's for the info.

[FearMyWrX]

Nice!

[Freakie]

tks Chris...one of my first jobs on a long todo list now....I cant wait...

...FYI my SF crate and BB 781 has landed in Cape Town !!!!!...got confirmation it was cleared by customs this morning....so actual collection by me will be Tuesday 3rd.......a very big ThANK YOU to you, Blue and Berky for an incredibly smooth process to get my project this far....this forum has really made a massive difference to me and is why i could drop more than 10K into a build that i have never done before, with people I have yet to meet and shake hands with....big thanks to all the X members who give their input and guidance so openly.

[BruceSki]

Good info here. This post should be stickies or added to the tech faq

[JetManiac]

Good info here. This post should be stickies or added to the tech faq

Thanks.

[extreemthrottle]

great thread chris
hey chris, got a question. you are saying the blaster and the sj use the same intermediate shafts?
i ask because the part numbers are different and i thought the length was different
also the 94/95 vxr 650 has the same part number as the blaster, and i have used one from a vxr650 in my current setup in my blaster its working great.
just curious, and trying to see if there are other skis that run the same shafts.

[JetManiac]

great thread chris
hey chris, got a question. you are saying the blaster and the sj use the same intermediate shafts?
i ask because the part numbers are different and i thought the length was different
also the 94/95 vxr 650 has the same part number as the blaster, and i have used one from a vxr650 in my current setup in my blaster its working great.
just curious, and trying to see if there are other skis that run the same shafts.

The B1, SJ, and VXR650 all have the same length midshafts. These are the only yamahas which use this length midshaft.

[Listy]

Thanks for the info Chris.
Quick question, avoided the weaker midshaft and ordred the part. Came with a diameter of 23.75mm which I think is 5/16. The old one is 21.8 with a 1.5 pitch metric thread. Imperial diameter and metric thread (interesting). New coupler required. Is new the coupler thread the same for Cold Fusion and ADA, and does a new ADA or CF coupler going to fit on the crank shaft thread, being about 26.0mm??

[JetManiac]

Thanks for the info Chris.
Quick question, avoided the weaker midshaft and ordred the part. Came with a diameter of 23.75mm which I think is 5/16. The old one is 21.8 with a 1.5 pitch metric thread. Imperial diameter and metric thread (interesting). New coupler required. Is new the coupler thread the same for Cold Fusion and ADA, and does a new ADA or CF coupler going to fit on the crank shaft thread, being about 26.0mm??

The ADA or CF coupler set each has 2 different sized coupler halves. One is larger to fit your crank and one is smaller to fit the RN midshaft.

[raytherace]

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