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 have an oversquare engine design,and are revvers,and although many owners (and the bikes) are quite happy to sit at 7000rpm for miles on end,i find it`s not condusive to relaxed riding.
The text\pics here relates 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) and fitting a different wheel/larger profile tyre (custom/GL models only?),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 ratio`s.
This procedure would not be worth doing for most owners though,the main reason i was able to do it was because i already had the 500 stripped down for inspection,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 these two pages purely to give other CX owners who are interested in these bikes an insight into a modification that they might find of interest.
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.
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 on these box`s is the second from the back of the gearbox shafts - see pic.
The two rearmost pinnions on the mainshaft and layshaft had to be made slightly thinner;the 650 has wider pinnions,presumably to cope with the higher output of the larger engine,and the input shaft had to be made slightly shorter: even when clearance for the gears was made the gearbox would not sit fully home in the crankcase,the front bearing housing being proud by approx 2.5mm...
Once those parts had been modified,the gearbox would fit into the engine case,but the clutch also received some attention;i had to reduce in size the spacer/bearing that the clutch basket runs on and rework slightly the oil feed drilling,and remove some depth of the clutch basket boss to suit(this may be unnessacery on Euro models with the seperate,longer spacer behind the clutch,the GL i was working on had the old-style clutch).
This is the result;a small ammount of material has been removed to provide clearance for the 5th gear pinnion.
The shaft diameters on both 500 and 650 are the same,so will fit straight in.
Next see for what i did on the pinnion gears and mainshaft that had to be reworked.
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:
The layshaft 2nd gear pinnion fouled the crankcase because of its `recessed` design - see pic below,left.The 650 engine has a clearance cast into the case to allow for this,i reduced the width of the gear to provide clearance.The thickness of the gear after machining was still greater than that of a std 500 gear.
The two rearmost gearbox pinnions (2nd gear) needed to be made thinner,here`s the before and after pics;
The input shaft after having been reduced in length by approx 0.100"
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.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 be easier to modify as it has a long spacer/bearing to replace this cast-in boss,just the spacer/bearing 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,i think the CX500 custom has the same gearbox ratio`s/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)
*update*
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 75/80 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.
Ratios.
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 ratio`s 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.
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