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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: TommyT on April 27, 2023, 08:36:27 AM
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I have an Progressive rear shock that came off a 90 K75RT. I got bike running recently (runs well) and the rear shock looks impressive but in need of work. Before I start calling around for quotes, (1 old post here said they spent over $800 for rebuild of a rear shock like this) I like anyone here tell me if the ride different between this shock and a stock 75 rear shock (i have a spare one on it now) would be worth the $$$ to rebuild it?
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90% of the rebuild is replacing the shaft seal and the bladder in the reservoir(if it is bad). A spring compressor, the fluid and the parts(seal and bladder) is about all you need. When you're done take it to any shop that caters to motocrossers to have it charged. You should be able to do it for under a hundred bucks.
I've done several and it's not rocket surgery. The trickiest part is filling with the shock fluid so there are no bubbles.
$800 is ridiculous, that's more than the replacement cost of the shock!
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I do have a spring compressor! Parts are available to order for the shock by DIYers? I am not finding anything online for Progressive parts except new springs.
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I just did a search and couldn't find any specific parts for your shock. It is probably obsolete and not supported anymore. However...
If you disassemble it and take the old seal to an industrial bearing and seal shop they will be able to match it. The important stuff is the shaft size, O.D., height, and the number of lips.
To see if you need to address the reservoir, after you disassemble the shock, apply 60 to 75 psi to the reservoir and see if any leaks into the shock body. Put the body in a vise and fill it with soapy water and let it sit for an hour or two to see if you get any bubbles.
The main thing is to make sure that when you take it apart, you record how everything is put together. Take lots of pictures and do one piece at a time. Clean it and then mark it with a Sharpie Pen as to which side faces the shaft. Your shock looks petty similar to my Works Performance, so I would refer to their rebuild instructions for filling instructions. If you can't find them, I have them here and can email them to you.
Safety is important, use a spring compressor, and discharge the nitrogen in the reservoir before you start to take anything apart. The nitrogen charge remaining in the reservoir will tell you whether or not you need to be thinking of replacing the bladder/piston o-ring. Any pressure remaining is a good indication that they are probably okay.
For lack of a specific requirement for shock fluid, you can get away with ATF or any decent SAE 5 shock oil. The damping adjustments will compensate for any variation in fluid viscosity.
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The difference between the OEM shock and that Progressive adaptive shock—if both of them were in usable condition—is that the OEM shock is only adjustable for preload weight.
The Progressive pictured looks like it not only had preload adjustment plus hydraulic damping but also air damping, and maybe even had automatic variable adjustment capability responding to road conditions. It possibly could have an air bladder but maybe only an air chamber sealed by o-rings.
The Progressive is decrepit at this stage and not worth an $800 rebuild when there are good quality new shocks capable of treating back- and ass-parts relative as tenderly as the Progressive could in its youth, considering those parts are being conveyed on top of a motorcycle seat. TFX shocks available at Ted Porter's come to mind but I bought a less costly YSS emulsion shock from Porter because I'm not always thrashing the moto on tracks, highways and backroads; I'm just pounding it along the pothole and gully fields that comprise the town-maintained roads where I live. 177381
If you want a mechanical challenges rather that more time in riding pleasure, attempt a rebuild. Gryph is an excellent advisor. 112350
The most startling modification improving my motorcycle riding pleasure was simply using space-age honeycomb padding to replace the OEM seat stuffing. The YSS helped too, without a doubt. Gryph recommends cartridge emulators in the front suspension but that's a fork bridge too far for me. :laughing4-giggles:
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Thank you the detail instructions and tips. I will update the effort here in case others want to try this in the future. Probably will be a day or so before I can start, I have a number of more pressing tasks I need to do first.
Again thanks a lot.
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Good luck!
The job shouldn't cost more than $50. That would be $12-15 for the seal, $8 for a quart of ATF(my favorite damping fluid for forks and shocks), $2 for some kerosene, and $25 to do the nitrogen charge.
Get an egg carton to keep the piston parts in the proper order as you take things apart. There will be some valving that has to go back in a specific order to work properly.
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The difference between the OEM shock and that Progressive adaptive shock—if both of them were in usable condition—is that the OEM shock is only adjustable for preload weight.
The Progressive pictured looks like it not only had preload adjustment plus hydraulic damping but also air damping, and maybe even had automatic variable adjustment capability responding to road conditions. It possibly could have an air bladder but maybe only an air chamber sealed by o-rings.
The Progressive is decrepit at this stage and not worth an $800 rebuild when there are good quality new shocks capable of treating back- and ass-parts relative as tenderly as the Progressive could in its youth, considering those parts are being conveyed on top of a motorcycle seat. TFX shocks available at Ted Porter's come to mind but I bought a less costly YSS emulsion shock from Porter because I'm not always thrashing the moto on tracks, highways and backroads; I'm just pounding it along the pothole and gully fields that comprise the town-maintained roads where I live. 177381
If you want a mechanical challenges rather that more time in riding pleasure, attempt a rebuild. Gryph is an excellent advisor. 112350
The most startling modification improving my motorcycle riding pleasure was simply using space-age honeycomb padding to replace the OEM seat stuffing. The YSS helped too, without a doubt. Gryph recommends cartridge emulators in the front suspension but that's a fork bridge too far for me. :laughing4-giggles:
Thanks. all you say makes good sense. I do like working in the garage, I am retired, so I will at least take it part to see what it might need, but I don't want to invest too much time and money when like you say there are pretty of good replacements that will improve the ride over a stock shock.
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https://www.firefoxracing.uk/bmw-k100-rs-gt-lt-rs-sport-yss-shock-absorber-mz366-350trl-01-11122-p.asp
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Take your time and enjoy the job. The money you save will buy enough fuel to ride to Florida and back.