R25 Manual Gearbox Swap
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  1. #1
    Tadpole
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    Default R25 Manual Gearbox Swap

    Our 1986 R25 GTX is starting to give ominous noises from its manual gearbox. In at least 3/4/5 there's a whirring sound on load that could be a chipped gear or dying bearing. The onset of this was fairly sudden. The rest of the car isn't too bad - the electrics are fine and even the air works passably.

    The car is a B29B with (I understand) the NG3 manual gearbox, which I've heard is not as good as the earlier UN1 box. So one approach is to try to locate an R25 UN1 box (hen's teeth?) and transplant. At the least this may involve swapping to a hydraulic clutch, too, (which may be not such a bad move giving the harsh feel of the cable clutch. A while back I had an R20 with the UN1 and the clutch was superb).

    Has anyone done this particular swap? Any clues / suggestions / advice would be much appreciated.

    Thanks,

    Greg

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  2. #2
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    How many k's has the gearbox done? Whilst I've heard of some problems with NG3's, on the whole they seem fairly reliable, especially the later Fuego ones with improved synchros. If the 'box has lived to a ripe old age it may be an idea to fix it before it is a terminal problem that destroys the gearbox. The UN1 on the 25 also seems to have its share of problems too, I've heard of at least two with popped diffs.

    The problem with retrofitting a UN1 to a 25 is that you will also need to find driveshafts for a UN1 25, as the NG3 gearbox is narrower. If you do decide to go that route though, there is also the 369 which was the forerunner to the UN1 that was used in the late R20's, however these have a cable speedo as opposed to the electronic speedo of the UN1.

  3. #3
    1000+ Posts HONG KONG PUGGY's Avatar
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    Quote Originally Posted by gregx
    Our 1986 R25 GTX is starting to give ominous noises from its manual gearbox. In at least 3/4/5 there's a whirring sound on load that could be a chipped gear or dying bearing. The onset of this was fairly sudden. The rest of the car isn't too bad - the electrics are fine and even the air works passably.

    The car is a B29B with (I understand) the NG3 manual gearbox, which I've heard is not as good as the earlier UN1 box. So one approach is to try to locate an R25 UN1 box (hen's teeth?) and transplant. At the least this may involve swapping to a hydraulic clutch, too, (which may be not such a bad move giving the harsh feel of the cable clutch. A while back I had an R20 with the UN1 and the clutch was superb).

    Has anyone done this particular swap? Any clues / suggestions / advice would be much appreciated.

    Thanks,

    Greg
    Greg,
    Have you changed the gearbox fluid recently? If so, what type did you replace it with.? I ask because a reliable Renault mechanic once told me Reno transaxles only drink straight 80w(or heavier single grade) box fluid, and not the multi-grade stuff, which is recommended more often. I had experience with this in my R17 years ago also, I was sold a bottle of Castrol 80/90 and after about a week it started to whine on over-run, changed back to single grade 80 and all fixed.
    If you haven't changed the fluid recently, may-be do a change and use a single grade oil first and see if it improves.
    Chris
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  4. #4
    Banned Haakon's Avatar
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    easiest and cheapest way out here is to buy a good late Fuego box (pretty cheap), dismantle both and use the Fuego bits to rebuild. Ratios are all pretty similar - just need to use the R25 case and diff crownwheel for the speedo pick up.

  5. #5
    Simon's Avatar
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    I wouldn't mix the crownwheel and pinion separately between different gearboxes, besides the speedo sensor ring is a push fit on the diff housing, so pull it off the 25 diff and press it onto the Fuego diff, or just swap the CW&P's so they are a matching set. Also the Fuego case has a casting where the speedo sensor goes so it can be drilled accordingly to fit the speedo sensor.

  6. #6
    Tadpole
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    Quote Originally Posted by Simon
    How many k's has the gearbox done? Whilst I've heard of some problems with NG3's, on the whole they seem fairly reliable, especially the later Fuego ones with improved synchros. If the 'box has lived to a ripe old age it may be an idea to fix it before it is a terminal problem that destroys the gearbox. The UN1 on the 25 also seems to have its share of problems too, I've heard of at least two with popped diffs.

    The problem with retrofitting a UN1 to a 25 is that you will also need to find driveshafts for a UN1 25, as the NG3 gearbox is narrower. If you do decide to go that route though, there is also the 369 which was the forerunner to the UN1 that was used in the late R20's, however these have a cable speedo as opposed to the electronic speedo of the UN1.
    Simon - the car has done 211k. Thanks for the cautions about the UN1. Old driveshafts can be a minefield too.

    I agree that the more straightforward idea is doing up the NG3, but I wonder about availability of parts beyond easy bits like bearings and seals.

    Greg
    Last edited by gregx; 14th April 2005 at 01:55 PM.

  7. #7
    Tadpole
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    Quote Originally Posted by HONG KONG PUGGY
    Greg,
    Have you changed the gearbox fluid recently? If so, what type did you replace it with.? I ask because a reliable Renault mechanic once told me Reno transaxles only drink straight 80w(or heavier single grade) box fluid, and not the multi-grade stuff, which is recommended more often. I had experience with this in my R17 years ago also, I was sold a bottle of Castrol 80/90 and after about a week it started to whine on over-run, changed back to single grade 80 and all fixed.
    If you haven't changed the fluid recently, may-be do a change and use a single grade oil first and see if it improves.
    Chris
    Chris - thanks for that. Tomorrow we're going to have the oil changed for starters (cheaper than the whole box, initially!), just to be sure.

    However, by the nature of the noise (more of a repetitive whirr than a while) I'm not too optimistic. The magnetic drain plug may provide some clues, too.

    Greg

  8. #8
    Tadpole
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    Quote Originally Posted by Simon
    I wouldn't mix the crownwheel and pinion separately between different gearboxes, besides the speedo sensor ring is a push fit on the diff housing, so pull it off the 25 diff and press it onto the Fuego diff, or just swap the CW&P's so they are a matching set. Also the Fuego case has a casting where the speedo sensor goes so it can be drilled accordingly to fit the speedo sensor.
    Thanks, Simon. So do you mean that apart from the changes for the electronic speedo, a Fuego NG3 would go straight in?

    I had initially discounted this because of the different ratios (Fuego has smaller tyres?), but perhaps I should crunch the numbers properly.

    Greg

  9. #9
    Banned Haakon's Avatar
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    Fuego had different clutch release lever setup, so a bell housing swap would be in order, but other wise its basically the same (speedo pickups notwithstanding)

    Ratios are fairly simliar - the Fuego has smaller wheels, but revs more in each gear relating to road speed.

  10. #10
    1000+ Posts Europa's Avatar
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    Quote Originally Posted by Simon
    I wouldn't mix the crownwheel and pinion separately between different gearboxes, besides the speedo sensor ring is a push fit on the diff housing, so pull it off the 25 diff and press it onto the Fuego diff, or just swap the CW&P's so they are a matching set. Also the Fuego case has a casting where the speedo sensor goes so it can be drilled accordingly to fit the speedo sensor.
    It may not mean anything, but the Renault UN1 gearbox manual says that the speedo ring cannot be removed from the differential. That doesn't mean it can't be done though!
    '05 Pearl Black Mégane 5-Door LXR(Daily Driver), '75 Trak Yellow R16TSA (Parts Car), '74 Midnight Blue R17TS (Rebuilding), '73 457 Blue R17TL (Parts Car), '72 Alpine White R16TL (Retired), '69 Sunburst Brown R16TS (Awaiting Rebuild), '68 "Appliance White" Europa (Stored)

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