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PRM 120D2 Confusion


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Hello all, 

 

So I've been having trouble with the gearbox on my Beta 38 and could use some experienced advice.

 

It was struggling to engage, and was getting increasingly worse so I pulled it off and took it apart with the assistance of my mechanic friend. We identified that the clutch cone and selector were pretty worn so I looked into spare parts and it appeared they have replaced the cone with a newer version, which is no longer compatible with the original gears. I bought the full "rework" kit MT8684 at an eye watering £450 only to find that the gears are helical and no longer fit with the straight cut gears on the rest of the gearbox! 

 

Now I'm in a bit of a pickle - I've spent £650 on new parts for an old gearbox and my boat is still grounded. I feel it was a little misleading to label the MT8684 as a rework kit when it isn't compatible with the old version and it seems like the rest of the parts to make it work are not easily available. Does anyone have any advice on the matter? 

 

Thanks, 

MixingWizard

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I haven't repaired many PRM120 boxes but I, from diminished memory, thought they were all helical gears, is yours very old?

 

Send the parts kit back for credit and swap the box in fo a new PRM125D2 is the best bet; then you have the guarantee as well. 

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18 minutes ago, Tracy D'arth said:

I haven't repaired many PRM120 boxes but I, from diminished memory, thought they were all helical gears, is yours very old?

 

Send the parts kit back for credit and swap the box in fo a new PRM125D2 is the best bet; then you have the guarantee as well. 

 

I thought they were as well, but is it possible the OP is confusing the actual gears with splines for the cone clutch? If any are spur gears it would be noisy in operation.

 

Just checked the 120 and 125 manuals. The drawings in the 120 manual seem to show spur gear, but I think that is a drawing convention for any gear, so not fact. The 125 drawing is the same, but another picture clearly shows helical gears. I find this interesting, so hopefully the OP will come back with photos and more info when he has talked to PRM.

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15 minutes ago, Tony Brooks said:

 

I thought they were as well, but is it possible the OP is confusing the actual gears with splines for the cone clutch? If any are spur gears it would be noisy in operation.

 

Just checked the 120 and 125 manuals. The drawings in the 120 manual seem to show spur gear, but I think that is a drawing convention for any gear, so not fact. The 125 drawing is the same, but another picture clearly shows helical gears. I find this interesting, so hopefully the OP will come back with photos and more info when he has talked to PRM.

I too would have thought the noise with straight cut gears would be unbearable.  I have looked at my service notes for PRM & older Newage gearboxes and they are all helical cut gears. 

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45 minutes ago, Tracy D'arth said:

I too would have thought the noise with straight cut gears would be unbearable.  I have looked at my service notes for PRM & older Newage gearboxes and they are all helical cut gears. 

 

I did similar for all the PRM mechanical gearbox manual and found the same. I await photos in case it's not a PRM, but can't see any maker not using helical gears in this type of box. I think the reverse planet gears in the BMC B type and the little Watermota boxes may have been spur gears, hence the whine in reverse.

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It wasn't the quietest of things!

IMG-20240525-WA0007.jpg

2 hours ago, Tracy D'arth said:

I haven't repaired many PRM120 boxes but I, from diminished memory, thought they were all helical gears, is yours very old?

 

Send the parts kit back for credit and swap the box in fo a new PRM125D2 is the best bet; then you have the guarantee as well. 

 

I think this might be the end solution. Would you say it's worth the extra money to go hydraulic with the 160?

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Thank you, very odd. Have you actually talked to PRM about this?

 

As for the 160, if it is an easy swap then I would always go PRM hydraulic, but their mechanical boxes don't seem to give the problem Hurth seem to.

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4 hours ago, MixingWizzard said:

It wasn't the quietest of things!

IMG-20240525-WA0007.jpg

 

I think this might be the end solution. Would you say it's worth the extra money to go hydraulic with the 160?

 

If you plan on keeping the boat for a while then it’s definitely worth going hydraulic (PRM 150). I did look at doing this on my old boat with a PRM 90 installed and unfortunately it’s not exactly a drop in replacement. The output shaft position is different so the engine will need significant realignment, and you’ll need to fit an oil cooler for the gearbox, all of which ramps the cost up.  

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14 hours ago, MixingWizzard said:

It wasn't the quietest of things!

IMG-20240525-WA0007.jpg

 

I think this might be the end solution. Would you say it's worth the extra money to go hydraulic with the 160?

I am shocked, never seen that before. It must have been very noisy.  Going hydraulic is an option but not a simple swap. Dimensions are different, an oil cooler plumbed into the cooling system and hoses all add up to more expense. 

PRM150 or PRM 280, much bigger,  are the choices. Better with a PRM 125, current replacement for the 120.

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4 hours ago, Peugeot 106 said:

It might be worth having a chat with Richard at Primrose Engineering. There was a thread on here where he took a 120 box apart and explained the workings

Excellent idea, that video was very helpful when taking it apart!

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28 minutes ago, MixingWizzard said:

Excellent idea, that video was very helpful when taking it apart!

Richard has been a help to me in the past. I didn’t see the video before I’ve made a note of it just in case…..

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  • 3 weeks later...

Just an update on this for anyone having a similar issue in the future - it turns out that the pre-2001 PRM120 does in fact have straight cut gears and is not supported by the update kit they released. We got it working for now by roughing up the surface of the clutch cone but it will be a new gearbox in the future. It's a shame but I guess 20 years is a long time in gearbox world!

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8 hours ago, MixingWizzard said:

Just an update on this for anyone having a similar issue in the future - it turns out that the pre-2001 PRM120 does in fact have straight cut gears and is not supported by the update kit they released. We got it working for now by roughing up the surface of the clutch cone but it will be a new gearbox in the future. It's a shame but I guess 20 years is a long time in gearbox world!

 

Thanks from me also. PRM engineering and design has gone down in my mind now. No doubt a bright accountant's input to design.

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As helical gears are more expensive to make than straight cut spur gears I suspect it was  not the Department of Enumerated Beans idea to change, but rather a real product improvement.

 

Is the cone clutch metal to metal, or might it be possible to get a reline?  There are plenty of clutch relining companies about.  The old motorcycle mags should give you a start, or google Bremskerl.

 

N

 

 

 

 

 

 

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1 hour ago, BEngo said:

As helical gears are more expensive to make than straight cut spur gears I suspect it was  not the Department of Enumerated Beans idea to change, but rather a real product improvement.

 

I don't buy that. It has always been known that spur gears are noisier than helical gears, helical gears can also transmit more torque than the same sized spur gear, so why would a design engineer who knows that choose to use a known inferior design. It must be bean counting, pure and simple.

 

How can using a noisier and weaker design be a product improvement other than as a cost saving measure.

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1 hour ago, Chris G said:

Surely the "improvement" was to change from straight cut to helical. So it was a real improvement.

 

Yes, I agree, but the original design to use spur gears seem appalling to me. Spur gears in marine gearboxes wend out with the mechanical sun and planet gearboxes, so say, about 1970. Even then, those boxes used helical or double-helical gears in their reduction boxes.

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5 hours ago, Tony Brooks said:

How can using a noisier and weaker design be a product improvement other than as a cost saving measure.

 

At least PRM realised the error of their ways in 2001 and went to helical gears. 

 

6 hours ago, BEngo said:

Is the cone clutch metal to metal, or might it be possible to get a reline?  There are plenty of clutch relining companies about.

 

It would be good if that was an option, however not sure it's an option with the design of clutch. (Pictured below)

 

image.png.61dc08c7609c6b95af97913ded61d087.png

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45 minutes ago, booke23 said:

 

At least PRM realised the error of their ways in 2001 and went to helical gears. 

 

 

It would be good if that was an option, however not sure it's an option with the design of clutch. (Pictured below)

 

image.png.61dc08c7609c6b95af97913ded61d087.png

 

They look like ether sintered metal or modern brake lining material, I would think the latter. So it all depends upon fining a company to do it. This link may be worth exploring.  https://www.industrialfriction.com/friction/brake-relining-services/clutches/cone-clutches/

 

I wonder if this problem is related to the change from engine oil to ATF Dexron that PRM brought in. What oil is the OP using?

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