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TOPIC: Monocross shock... Do I have to rebuild?

Monocross shock... Do I have to rebuild? 14 Mar 2019 18:43 #1

  • Luke
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79 dt250. Took this out to inspect and it looked surprisingly good. With the spring removed, it definitely still has a nitrogen charge, requiring about 50 lbs to start compressing it. No visible oil leaks. However, it doesn't seem to have much damping effect. The book gives a vague value like "significant resistance" on the rebound stroke. To me it feels like it's springing back out as fast as I take the pressure off.

I read the excellent rebuild article by MarkT several times, and I'm sure I can dive into this thing (not really looking forward to it). Just wondering if compressing by hand and releasing is a thorough test, or if I'm expecting too much and heading towards a pointless rebuild.

Any other ways to measure the damping? Or is the feel test about it? Any black magic tricks to restore the internals by shaking three times and spinning under the full moon?
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Monocross shock... Do I have to rebuild? 15 Mar 2019 03:16 #2

  • RT325
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Stand over it on the floor on a block of wood so it won't slip with threaded end into the wood & bottom it out & let it go quickly. Gas pressure will send it back fairly quickly but not with a bang & no damping at all. You could remove the spacer on the shaft & realy bottom it out to get a good indication of damping because it'll have more travel to send back so a fraction more time to think about it. If it's definitely Pogo Stick then the gas will have got past the fat Oring on the Gas Separating piston so a new story unfolds. By memory the troubles didn't start till 1980 with Tin Cup gas pistons picking up & scoring the body, hence the gas getting past the then scored Oring & mixing with the oil--then Pogo Stick. Prior to the trouble starting they had cast steel or iron gas piston & never gave any problems. First TT250's were the start of the problems with tin cup gas piston. By memory your model was never very strong on damping, but definitely did damp reasonable. It's possible the damper piston ring is worn of course, &/or the bore of the shock body but unlikely.

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Last Edit: by RT325.

Monocross shock... Do I have to rebuild? 15 Mar 2019 03:24 #3

  • MarkT
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There is a couple of hundred psi of pressure in the shock that will push the shaft right back out... completely normal.

Unless you are saying the shaft pops out "violently" after you compress the shock? As in extremely fast? That's not normal. It does return fairly quick though. They don't have a ton of damping, not needed as it's a fairly long travel shock compared to the wheel travel.

With everything assembled on the bike, it shouldn't feel too "bouncy".

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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

Monocross shock... Do I have to rebuild? 16 Mar 2019 17:29 #4

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It definitely returns as fast as I can take pressure off, popping the end of stroke pretty solidly. I just don't detect any damping at all. Pogo stick is a good description. I've got the rear wheel torn apart right now too, but I'll reassemble one more time just to check it out again before launching into a rebuild.

Should I be looking for original piston seals, or some aftermarket improvements?

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Monocross shock... Do I have to rebuild? 16 Mar 2019 17:52 #5

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I don't know of any source for OEM parts except buying a shock on ebay and hoping it's better. Of course you could change the oil weight too.

I believe RaceTech makes a custom valving piston kit... not sure if they have it listed or if you have to call.

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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

Monocross shock... Do I have to rebuild? 16 Mar 2019 18:09 #6

  • Biknflyfisher
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If you are looking to have the shock rebuilt (by someone else), PM Greg French here on the forum -GFrench.
He has a guy near him in Colorado that rebuilds vintage monoshocks. He did some shock work for Greg when he went through his IT490 a ways back.
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1983 IT490K; 2007 Yamaha FJR 1300,
2003 KTM 450exc
Too many fly rods,,,

Monocross shock... Do I have to rebuild? 17 Mar 2019 02:30 #7

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Launch is a good word & if the gas has got past the piston & into the oil you'll get launched if you don't take great care so 'please do'. You'll have to drill on an angle in the base to relieve the gas pressure But If the gas has got up top then you'll have to drill a small hole in the gas piston also because if leaking past then the gas piston will be right there at the bottom. I've done lots with the tin cup piston as i said up top previously but don't think i ever needed to drill a cast piston as never picked up & scored the body internally. As a test, if you stand the shock on end but with the shaft vertical, then if the gas piston has been forced to the bottom, then you'll have a cavity of gas at the top because there's now not enough oil to fill the body above gas piston. So you'll have a nothing area for an inch or two of movement before the Damper Piston strikes the oil. Pretty unusual situation if this is the case so for your sake i hope its 'not that'
Where i said hole at an angle its because you'll need to drill & tap it eventually to take a valve & you need a good angle to clear the shock pivot part where the cross pivot pin or bolt goes through. Sorry i've lost where you said what model--thought there was a pic of the shock so maybe i'm on the wrong planet, was thinking late "70's DT250 for some reason. Ignore That-- pic now shows now i've posted, must be me lol.
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Last Edit: by RT325.

Monocross shock... Do I have to rebuild? 17 Mar 2019 08:12 #8

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Yes, what RT325 says is very important! Leave the shock standing shaft up for a while to let things settle. Then try compressing the shaft a few inches. When you release, does the shaft come back a little slow and then "jump" and slam the rest of the way? That would mean the gas got into the oil section and the piston would be moving through gas the last couple of inches of travel which would mean no damping at all.

Please note that if you pump the shock a few times, the gas and oil will tend to "emulsify" and while damping will be weaker than it should, it may not "jump" at the end of travel like it would after it sits for a while.

All the shocks I've had apart have the cast piston with an o-ring to keep the oil and gas separate. I've only seen very minor seepage of oil past that piston on the Enduro monoshocks. I think they went to the cheap tin piston on later models.

But if that piston o-ring completely failed, driliing the bottom of the shock to relieve the gas may not relieve the pressure in the shock! That would be bad if you tried to take it apart without the pressure released. So after drilling the base, make sure the shaft is no longer under gas pressure before trying to diassemble the shock. Just leaving the shock alone for a while with the base drilled might let the pressure bleed past the bad separator piston. If that doesn't work, I would either find another shock or carefully drill an extremely tiny hole in the "seal pack" next to the shaft. Carefully because oil and gas under high pressure is likely to escape!

Thanks Morley! I need to update my instructions.
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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

Monocross shock... Do I have to rebuild? 17 Mar 2019 14:43 #9

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Yep bit of dangerous game. On an XT200 a couple of years ago i rebuilt one where the weld on the end had rubbed through by constant rubbing of an inverted plastic shield that stops dirt getting up on top of the shock. Weld must've been porous just below surface. Left the gas piston out & filled it with oil but left room for shaft displacement. Glued up the leak--think i soldered it actually--& worked great. If you were picky you could feel a fraction of open or squelchy movement if you lifted the bike off the ground at the back but only an inch or so. For a farm bike it was good, & riding it the damper piston was always in the oil--just became an ordinary old shock before all this gas stuff lol. Anyway--woffling on about nothing except if you're 'short of funds' & shock is bad as in scored in the body then you 'could' turn it into a straight oil shock.

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Monocross shock... Do I have to rebuild? 18 Mar 2019 08:26 #10

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Thanks for all the info, guys! I can't detect any changes in springiness or dampiness (??) either by letting it settle in the vertical position, pumping it, etc. Even tried shaking to see if I could detect oil sloshing in the gas cavity. If I had to guess, I think the nitrogen seals might be fine but the main piston is what is blowing by. Understood about the dangers of disassembly while any portion of the innards remains under pressure!

I'm going to take a chance and have ordered a used shock, so best case it works much better and worst case I'll have two assemblies for parts/comparison, or can keep running one while collecting some rebuild parts for the other.

I fat fingered the original post as a 1979, but it's actually a 1977...not that it makes any difference, but you have mentioned several times that the pistons got worse on the later models. I'll post an update when I get the critter all torn apart.
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