• Achtung, achtung!  :)

    During July 27th 2024 (or 28th, depending on your time zone), hosting server maintenance is planned and the forum will not work normally (we'll set it to read-only mode at best, though it will be completely offline for a while). Hopefully, it will all go well and we will be back on line within less than 8 hours.

    Thank you in advance for your patience.

    Note, this has nothing to do with the planned forum upgrade to version 2.3, which will most probably happen at a later date (probably in Autumn).


Shimano vintage XT-M739 compatibility problem


Hub Hero
20 years ago I've bought a second hand bike with XT components: ST-M739, FC-M739, FD-M739, RD-M739. I have no idea what chain and cassette was installed on the bike at that time. Since then I was facing shifting problems on rear derailleur. The chain was randomly dropped from 5th to 6th cog when putting more force on pedals. The bike has been tuned in several specialized shops without success. The bike made less than 5k km.
After 10 years of resting I've revived the bike by myself.
I've bought a new 8s cassette CS-HG51 and have also tried with some similar ones. If the rear derailleur was aligned on 8th cog then the chain was rubbing in 2nd and 3rd cog and randomly dropped from 1st to 2nd cog. So I've aligned the rear derailleur to 4th cog but the chain was still rubbing on 1st and 8th cog. I've also replaced a current RD with Alivio 8s RD but no improvements. H & T limit screws were set properly. I've also replaced an old chain with a regular 8s chain (HG-71) but that chain was too wide for RD's cage so the chain was rubbing on the cage.
Now I have installed 9s HG53 chain which seems to be OK with that RD's cage. At this point I became suspicious about shifter. So I've replaced 8s cassette with an old 9s Shimano cassette and it woks well so far with 8s shifter, but without using 1st cog.
I have wondering if Shimano ST-M739 shifter have a different pull cable ratio than other Shimano 8s series and which 8s cassette would be compatible with that shifter?
Would RD-M739 derailleur be able to pull a chain to a cog with more than 32 teeth? (Nowadays I can get a cassette with 36T or 42T on 1st cog so why not ;) )
Thanks in advance, M.
The problem you've described sounds like it was caused by one of these (one not excluding the others):
Generally, chains for more speeds will work fine on cassettes with fewer speeds (but not vice-versa).

If an 8-speed chain rubs on an 8-speed RD cage, maybe the wrong pulleys (with too short axle-spacer) were installed to make the cage effectivelly narrower.

The fact that it works fine on a 9-speed cassette, with 8-speed RD and shifter, leads me to believe there is something wrong with the hanger, or with the shifter (worn, pulling and releasing less cable per click). But, it could also be because the shifter cable is not properly pinched (wrong side of the pinch bolt, thus changing the geometry and the RD movement per 1 mm of cable travel).

I often see bikes that have been to several shops, and weren't fixed. That doesn't mean the bike is problematic. I discussed the problem of non-standardised bike mechanic training with Danielle (that part starts at 16:42). Having said that, it is very difficult to trouble shoot over the Net, and looking at the bike often helps a lot - in this case that burden is on you. I made two videos that could help (simple, step-by-step cover of every catch and caveat):

If all else fails (perhaps better to start with that right away):

A step-by-step derailleur tuning tutorial (with a pretentious-sounding title :) ):

If memory serves me, RD-M739 came with long cage (SGS) and medium cage (GS) versions (not sure about the short SS version). That will affect how big a cassette it will take. If you have a 32 T cassette, you could try screwing in the B-screw all the way in (make sure to use a good cross-tipped screwdriver and unload the RD spring while doing it, by twisting the RD cage backwards and up). Then, see how much room there is from the RD's top pulley to the bottom of the 32T sprocket when you shift to the smallest front and the largest rear chainring/sprocket. One can often "get away" with a couple of teeth more than the factory says. Having said that, you could get some 22-32-44 triple cranks and have very low gearing even with a 32T cassette.

Regarding the shifter compatibility, I don't recall the ST-M739 having a different pull - but it could be broken or worn.

Hope this helps at least a bit. Let me know if you have any other questions.

Thank you Relja for your quick and detailed answer. Yes, I believe it is hard to find dysfunctionalities without seeing and testing.

The bike is Lee Cougan like this one . I cannot find the manufacture date of this series. I bought it in year 2000, but Shimano launched M739 series in 1997 (by the 4 arm crankset) so it could be possible that I bought a bike with already worn components since I had the problems from the start, even if the bike should be max 2 years old when I bought it. It could be also possible that the bike was ridden aggressively for 2 years before I bought it

About FD: when the chain sit on the middle front chainring cog then the front derailleur has no affect (no rubbing) to the chain when shifting from 1st to 8th speed so there is enough clearance with FD.

About RD: it is a SGS and it has no hanger - it is directly mounted on the frame. I've also checked to make sure that the mounting point and RD is not bended.
I suspect that the cage is narrower than normal 8s RD because I've tried to replace a pulley from another Alivio 8s RD but it didn't fit because the pulley was too wide for the M739 pulley screw. In other words, the M739's pulley screws are shorter than Alivio 8s pulley screws and that determine the width of the cage in my case. But I didn't check the axle spacers from both RD's.

About cables and housings: they were replaced several times without any improvements to the rear shifting. The cable for the RD leads along top tube and not beneath BB, so the cable is in the housing wherever it changes direction.

So there is so many details that I had no other idea than pointed me to a different shift ratio, but I might be wrong.

Thank you for your suggestions, M.


Support BikeGremlin

Help BikeGremlin stay online with a Patreon donation:

Top Bottom