Fitting a CX650 gearbox into a 500.

One of the most frequently asked questions on CX forums is "is there any way to reduce the engine revs on these bikes,it seems to need a 6th gear at motorway speed?"
The CX range of V-twins are of typical Japanese design in having an oversquare engine, and are high revvers, and although many owners (and the bikes) are quite happy to sit at 6/7000rpm for miles on end i find it`s not condusive to relaxed riding. Maybe i`m riding the wrong bikes... Any engine once past the 5000rpm mark tends to `shout` and for me is the point at which the motor starts to feel a bit stressy and busy..
The text and pics here relate to the 500 engines where there are a few options to achieve the goal of lower revs.
There are a couple of more straight-forward ways of making the bikes higher geared, such as swapping the primary drive (crank sprocket and clutch basket) or fitting a different rear wheel/larger profile tyre (custom/GL models only which have a 16" wheel as std *see bottom of page for details on that modification on a 650), although this will only provide a subtle change in gearing - i wanted something which would make a greater difference.
The way i have done it is the most complex and involved - to fit the gearbox from a CX650ed which has higher gear ratios.
This procedure would probably not be worth doing for most owners though, the main reason i was able to do it was because i already had the new-to-me GL500 stripped down for inspection whilst seeking out an odd noise (turned out to be a worn gudgeon pin/s), i had a donor 650 engine i could harvest for spares, and i could get all the engineering work done for free.
I wanted to do it `just to see if it could be done`, plus i cannot stop myself messing around with these bikes..
These pages are not `how-to` type instructions, i have made them purely to give other CX owners who are interested in these bikes an insight into a modification that they might find of interest. Remember, if anyone decides to have a go at this themselves this is a custom-fit modification and your experience may vary!
Here`s the two gearbox`s - the 500`s is on the left. They both look similar, but there are subtle differences - if it`s too big,it won`t go in.
I had to perform modifications to both the crankcase and some gearbox components first.
The crankcase had to have a small `scallop` machined out of the lower cylinder web to allow room for the larger 5th gear pinnion. Top gear pinion on these gearbox`s is the second from the back - see pic.
The two rearmost pinnions on the main and layshaft (2nd gear) had to be made slightly narrower; the 650 has wider pinions, presumably to cope with the higher output of the larger engine, plus a slightly longer gearset, and the input shaft had to be made slightly shorter to allow it to fit inside the 500`s case: even when clearance for the gears was made the gearbox would not sit fully home, the front bearing housing being proud of the main crankcase by approx 2.5mm.
Once those parts had been modified the gearbox would fit into the engine case, but the clutch also had to receive some attention; i had to reduce in size the spacer/bearing that the clutch basket runs on and also rework slightly the oil feed drilling on it, and remove some depth of the clutch basket boss to suit(this would be unecessary on Euro models with the separate, longer spacer behind the clutch (also with the spring type shock absorbers inbuilt into the clutch basket, similar to 650 ones) The GL i was working on was fitted with the old-style clutch with the rubber-ring type dampers as fitted to the original twin-shock models. Whether this was an original GL clutch or not i do not know, it could well of been swapped over from an early model CX sometime during its 25 year life, i thought all later Prolink models had the spring-type clutches.
This is the result of the milling; a small amount of material has been removed to provide clearance for the 5th gear pinion.
The shaft diameters on both 500 and 650 are the same, so they will fit straight in the bearings.
The thickness of the gears were reduced by approx 0.060" on a flat-bed grinder, a special gear-holder was made to attatch it securely to the bed during machining:
The layshaft 2nd gear pinion fouled the crankcase because of its `recessed` design.The 650 engine has a clearance cast into the crankcase to allow for this. I reduced the width of the gear to provide the clearance. Without this machining when the 650 `box was placed inside the 500`s crankcase the pinion came into contact with the engine case.The thickness of the gear after machining was still greater than that of a std 500 gear.
Here`s a couple of (clickable) images of the difference in crankcases, and the before and after pics of pinion machining;
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The input shaft after having been reduced in length by approx 0.100" to allow the front gearshaft bearing housing to fit flush against the main engine case.
The splined section as well as the total length was reduced in length.
After the modifications were made,the end-float on the gearbox shafts was checked,and suitable thrust-washers were installed - i found that the oil filter spring washer had the correct internal diameter and being only about .010" thick was ideal for shimming up the end-float by using one or more until it was set.
Heres the clutches.
The GL (and ZAB type?) is on the left.
The central boss - the round alluminium part in the centre (not on the later Eurosport-type, right - they have a seperate spacer), needed to be reduced by approx 7mm to enable the primary gears to line-up and mesh correctly - the front mainshaft bearing housing protrudes further out on the 650 gearbox,see the pic below right, and thus so would the 500 clutch basket. To get the primary gears to line up this difference had to be removed .The bearing the clutch runs on was also adapted to suit by being reduced in length and the oil feed drilling modified to enable lubrication continuity.
The Euro clutch would probably be easier to modify as it has a long spacer to replace this cast-in boss, just the spacer would need to be reworked to provide the correct primary drive gear mesh.

So,what does all this work result in?
These mods were done to a GL500, the CX500 custom has the same gearbox ratios and rear-wheel size combination so will have the same results.

Speed figures for this bike: std; at 70mph in top gear, the engine is doing 6300rpm (11.1 mph/1000rpm)

With the 650 gearbox fitted; 70mph in top gear i expect 5700rpm - a drop of 600rpm (12.4mph/1000rpm)

the Honda GL500 Ratwing is now complete,on the road and i have put a thousand or so miles on it with the modified gearbox.
Unfortunatly i did not have this bike running before i did the gearbox transplant(it was a new-to-me bike) so i cannot do a `before and after` type comparison, only compare it to my CX500e Euratsport which has the same engine but is a completely different bike...
The GL`s speed/rpm figures are as expected - a drop of 600 rpm at 70mph has been achieved. Cruising at 70/75 is more relaxed than my other 500.

It was a worthwhile mod to carry out, and interesting to do, but probably not one which is worth taking the engine out especially for; the primary drive swap is easier and if you are able to fit a larger rear wheel and tyre as well (Custom/GL models), these two changes would give similar results(although your GL may look weird with that 18" wheel and there could be clearance issues with respect to suspension travel).
Roughly; primary drive swap = 300rpm difference, gearbox swap = 600rpm.
A GL500 fitted with a STD 130/90-16 rear tyre @6000rpm;
6000 ÷ 2.242 (primary reduction) = 2676rpm
2676 ÷ 0.931 (5th gear ratio) = 2877rpm
2877 ÷ 3.09 (final drive ratio) = 931rpm
931rpm x 60 (minutes) = 55876
55876 x 6.36` (circumference of rear tyre) = 355371`
355371` ÷ 5280 (ft/mile) = 67
So,6000rpm = 67mph,or 11.1mph/1000rpm.
GL500 with 650 gearbox;
6000 ÷ 2.242 (primary reduction) = 2676rpm
2676 ÷ 0.839 (5th gear ratio) = 3189rpm
3189 ÷ 3.09 (final drive ratio) = 1032rpm
1032 rpm x 60 (minutes) = 61936
61936 x 6.36` (circumference of rear tyre) = 393915`
393915` ÷ 5280 (ft/mile) = 74
So,6000rpm = 74mph,or 12.4mph/1000rpm - thats 7mph more at 6K rpm or approx 650rpm less @ 70mph - 10% difference over STD.
And just for reference,the ratios that you`d get with a 650 primary drive swap (with std g`box);
6000 ÷ 2.114 (primary reduction) = 2837rpm
2837 ÷ 0.931 (5th gear ratio) = 3047rpm
3047 ÷ 3.09 (final drive ratio) = 986rpm
986rpm x 60 (minutes) = 59160
59160 x 6.36` (circumference of rear tyre) = 376257`
376257` ÷ 5280 (ft/mile) =71
So,6000rpm = 71mph,or 11.8mph/1000rpm - thats 4 mph more at 6K rpm or approx 350 rpm less @ 70mph - 6% difference.
The highest-geared version of the (non-turbo) CX/GL`s is the CX650 Eurosport, which does 14mph/1000rpm (i use a 120/90-18 tyre which is slightly higher profile than the STD 120/80), so at 6000rpm would  do 84mph.
The GL650 would be doing approx 300rpm  more at the same speed due to the smaller circumference of the rear wheel/tyre.
This modified GL500 engine is now fitted to the CX-Periment. **as of Spring `11 it was replaced with a 650 engine after doing approx 13/14,000 miles**
 My Ratwing was originally a 500. Then i put a 650 engine in it. Much better, but not quite the same engine to road speed of a 650 Euro due to that oddly sized rear wheel/tyre, and the difference could definately be felt on the road - those extra few rpm`s on the motorway nudging the GL into the `shouty` zone @5500 rpm at cruising speeds.  The 650`s are not quite as smooth once past 5K as the 500`s - they thrive on revs, and a little bit of extra vibes were adding to the GL`s feeling that it wasn`t quite as comfortable as i would like.
The GL could also pull away from standstill quite easily in 2nd gear - it was too low geared and needed a little bit of a tweak.
I obtained an 18" reverse Comstar rear wheel from a STD CX and fitted it with a 120/90-18 tyre. I`m using an Avon Roadrider, and they recommend a Mt2.5 - MT3.0 rim. The one off the Std CX is an MT2.15. It`s a bit skinnier than recommended but should be fine..
I noticed a difference as soon as i got the bike off the centre stand after doing the swap and put it on the side-stand - the bike leant over more than it did before! Not a dangerous angle of dangle but i may have to modify the already-modified side stand to correct it. GL`s side stands seem to suffer from wear which causes them to lean over a bit too far and many an owner has had to lengthen them or add something underneath the foot to compensate, and the larger wheel had exacerbated the already too-short original stand.
Once sat on the bike i felt i was sat slightly higher than before and pitched forward a tad more. After adjusting the back-rest it felt normal again.
Looking at the figures on paper the reduction in revs compared to how fast you are travelling at does not look like it would make a worthwhile difference or a noticeable improvement.
When both wheels were off the bikes i measured the circumference of the tyres and worked out what the ratios`/road speeds should be;
Std GL650: 130/90-16" rear tyre has a circumference of 1.99M / 6.52`.                           GL650 with 120/90-18" tyre: circumference 2.08M / 6.82`
                      70mph = 5220 r/min                                                                                   70mph = 5000rpm (same as a 650 Euro)
                    13.4mph per 1000 r/min                                                                                          14mph per 1000 r/min
So, on the face of it a drop of 220rpm (or 3mph more at about 70mph) seems hardly worth the effort, but on the test ride the small drop in revs made a noticeable improvement and i found i could ride just out of the vibe zone whilst riding at a reasonable minimum (for me) motorway speed. The difference on the road is more than it appears to be on paper. I`m glad i did it.
Note: the 130/90-16 tyre was relatively new. The 120/90-18 was nearing the end of it`s usable life, so the difference in circumference between the two (and thus the rpm drop/speed gain) was not as much as it probably should have been if both tyres were at similar wear rates. I reckon the real difference if i used a new 120/90 to be a 4mph gain in speed and a 350 rpm drop. This is still only a subtle change, but tips the balance towards whether it`s worth doing or not. This is similar to a primary drive swap on a 500.
Note that a 650 will be doing xxmph at 1000rpm less than a 500.
I usually run the Ratwing with 0 air pressure in the rear shock, and this particular shock is quite softly damped. During the test ride it bottomed out a couple of times on fast `whoopy` road undulations. When i got home i checked the underguard and yes, the tyre had kissed it so i`ll have to watch that for clearance, especially when loaded up on my travels. My Euro`s do the same with this size tyre fitted but as i`ve been running them for about 25 years i`m not worried it`ll ever be a problem - it`s never going to wear through the mudguard or anything like that.
Handling-wise the new wheel didn`t make for any added unpleasantness, and in fact may even of improved on the Silverwings feel a tad. It felt slightly different than before but quite secure, smooth and stable in turns. It`ll do - it`s a tourer, not a knee-scratcher.
When doing the calculations i may round up/down 50rpm or so for the final figures. The ratios of the gears are non-variable, but the sizes of the tyres are, which may also provide a small variable in my figures. I have found that one tyre manufacturers tyre size and physical dimensions may not be exactly the same as anothers, and then of course there is the difference between a new tyre versus a part worn one - the `new` tyre i put on this rim was nearing the end of it`s life, so all the figures above may vary slightly by a hundred rpm or a mile an hour or so compared to a new tyre - but the percentage difference obtained from before and after the wheel swap (or primary swap) will be the same in each particular case. Add on what you see via the inaccuracies of the Mickey Mouse speedo and tacho and the result is that `seat of the pants` feedback when riding is more of a guide to what effects the changes have made after mods like this.
Over the next few rides i`ll get the Satnav out to check the accuracy of the speedo anyway and to see what (if any) effect this mod had had on fuel mileage. The Ratwing was getting an average 58mpg before, and i doubt it will effect that to any significant degree. Here`s a few pics;
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