- Joined
- Jun 25, 2004
- Messages
- 929
It all started on a semi-lazy Sunday, when I was out for a ride on my 1970 Commando, nicknamed “The Purple People Eater”. I was coming back home from the Aylmer area, and decided to detour to East London to check out the Sunday Night Car Show in the Argyle Canadian Tire parking lot. As I pulled in and idled around the lot, I noticed an unusual metallic “clank” emanating from the engine. This motor is typically very quiet, and this noise was disconcerting. It only did it at idle - increasing the RPMs made it go away.
I decided to cut my visit short and eased my way home. The noise came and went, and the engine never missed a beat on the 10 minute ride home. Pulling into the driveway, there was no sign of the noise at idle, but within about 20 seconds, it was back. It seemed like it was emanating from the timing chest.
The bench was empty, so up she went, and I proceeded to pull the timing cover, spark plugs, and the valve covers. Rolling the motor over by hand, I noticed that the clank occurred every time a valve reached its apogee and began to close. I could see the end of the cam snap forward when the sound occurred. Checking the nut, it was not very tight. I tightened it a bit, and the noise seemed to go away. Maybe I got lucky – but I was a bit concerned that there was this much play in the keyway; that just didn’t seem right.
So I took the joint apart and measured the key versus other new Woodruff keys I had, and it measured good. I decided to put it back together with Loctite and tighten the nut well to clamp up the joint. It is a ¾” fine thread and should be capable of taking a bit of torque (the manual does not specify a torque for this nut and just tells you to tighten it well). In the process of tightening it, the end of the cam shaft snapped off!! OMG!! “SH*T, Wilson – you’ve done it again!!”
A sinking feeling passed over me – what had I just done?? But then my engineering senses kicked in – I looked at the fracture zone – it didn’t look right. It had a very coarse uneven grain structure – not consistent with any sort of typical steel tensile failure. It looked like cast iron or maybe a bad heat treat. I am leaning towards the latter. And there is no way that this shaft should have failed like this – the thread should have failed first, and I was not applying enough torque for that to happen.
But the repair path was clear – the motor had to come out and be torn apart to replace the cam. I went into the house and started putting together a plan and a list of parts. I decided to replace everything in the keyed joint connection at the cam. I also formulated a plan to minimize the amount of disturbance to the engine, as it had been fully rebuilt in 2005 and not done too many miles since. Thankfully, Walridge Motors had everything in stock, and a quick ride on my ’72 Combat netted me all of my provisions.
I would remove the engine, and lay it forward on the bench. Since it is a 750, the barrels can be removed from the crankcase without disturbing the head. The crankshaft would be rotated to TDC as the barrels lifted clear, so that the pistons stayed in their bores and the rings were left undisturbed. Then the drive side of the crank case could be removed, for access to the cam shaft.
The cam that was in it was a 2S grind (replaced as part of the 2005 rebuild), and the replacement is a 3S grind. The new cam is reportedly made from EN40 steel, which has a very similar composition and mechanical properties to 4340 steel. I was pretty happy about that. According to the shop manual, original cams are made from EN32, which is very similar to 1021 mild steel. It is case hardenable, but has a soft inner core. I am pretty confident that the 2S cam was not EN32 – the fracture surface is totally inconsistent with this type of steel.
The dismantling technique worked out very well. All mating surfaces were cleaned, and a fresh coat of Permatex MotoSeal 1 was applied. The base gasket had to be cut in order to fit around the connecting rods; the cut was made at the front center stud. In order to create a labyrinth path, cuts were made at right angles – first left to right on the inner side, then straight ahead for the second cut. The whole area was coated in Motoseal to make the cut path leak tight.
With it back in one piece, I put the engine on the stand so that I could time it and put the timing cover back on. I notice that the new key and sprocket went together on the camshaft with no slop at all, which I was also quite pleased with. I began to think that the keyway was possibly yielding as well, creating the play that I found.
With the timing cover back on, the engine could go back in the bike. I ran out of brake cleaner as I was getting to the primary drive; it was now about 9:30 in the evening, so I figured I would call it a night. I was pretty pleased with my progress. Engine out, repaired, and back in the bike in roughly 4.5 hours.
Day Two started with fetching some brake cleaner and ATF from the local hardware store. (I buy brake cleaner by the case – lol).
The inner primary required a bit of cleaning around the transmission sprocket – road grime mixed with chain lube, and looking at it closely, the top visible surface was kind of ugly… Starting with a flap wheel, then moving to the palm sander with 100, then 320 grit paper, all of the scratches and rough surfaces were massaged out. And over to the buffing wheel…. In about 20 minutes, total transformation. The outer primary cover had a good sized blemish in it too, so it got a similar treatment. And since I was set up, the valve covers and ignition cover got a quick polish up. Much better!
While the primary was off and the bike was supported by the frame stand, the isolastics got a quick adjustment. The bike didn’t handle badly, but it was just not as smooth as I knew it could be. It has Mk 3 adjusters, so this was accomplished quite readily.
Everything went back together quite routinely. A nuisance oil leak was dealt with – believed to be the oil filter, it turned out to be a crappy clamp on the blanked-off chain oiler spigot, leaking down the oil lines to the filter.
The engine started on the third kick, and a timely visit of my CVMG buddy Andrew made timing the ignition a breeze. A vintage Suzuki guy, Andrew marveled at the simple elegance of the Commando, and how easily accessible everything is.
After about 15 minutes at 2500 RPM to help break the new camshaft in, it was time to take it for a good 30 minute ride to finish breaking in the cam. I “nut and bolted” all the key fasteners and the engine exhaust nuts during this time.
A quick costume change into some riding gear and I was off. I immediately noticed that the isolastic adjustment was successful – smooth everywhere now, even down to 2500 RPM. About mid-way through the ride, I wicked it up into the 5000-6000 RPM range. The top end performance with the 3S cam is impressive, and the 34mm Mikuni does not seem to hamper performance at all. The mid-range is very stout, pulling from 4000 RPM in fourth gear with authority. Opening it up on a downhill run heading into an up-hill, it maintained 6000 RPM in fourth while going up a reasonably long 5%-ish grade – it wouldn’t do that before. While the 2S cam tends to give the exhaust note a distinct staccato, the 3S changes it yet again – I don’t know how to describe it, but it sounds really good.
Back in the drive way, she was parked on a clean section of concrete, and after 15 minutes, not a mark on the ground – Yay!
Although this was definitely not a planned task, and was pretty frightening when it unfolded, it came together beautifully and I am very pleased with the results. It is still a work in progress, but this bike is responding to therapy very well.
I decided to cut my visit short and eased my way home. The noise came and went, and the engine never missed a beat on the 10 minute ride home. Pulling into the driveway, there was no sign of the noise at idle, but within about 20 seconds, it was back. It seemed like it was emanating from the timing chest.
The bench was empty, so up she went, and I proceeded to pull the timing cover, spark plugs, and the valve covers. Rolling the motor over by hand, I noticed that the clank occurred every time a valve reached its apogee and began to close. I could see the end of the cam snap forward when the sound occurred. Checking the nut, it was not very tight. I tightened it a bit, and the noise seemed to go away. Maybe I got lucky – but I was a bit concerned that there was this much play in the keyway; that just didn’t seem right.
So I took the joint apart and measured the key versus other new Woodruff keys I had, and it measured good. I decided to put it back together with Loctite and tighten the nut well to clamp up the joint. It is a ¾” fine thread and should be capable of taking a bit of torque (the manual does not specify a torque for this nut and just tells you to tighten it well). In the process of tightening it, the end of the cam shaft snapped off!! OMG!! “SH*T, Wilson – you’ve done it again!!”
A sinking feeling passed over me – what had I just done?? But then my engineering senses kicked in – I looked at the fracture zone – it didn’t look right. It had a very coarse uneven grain structure – not consistent with any sort of typical steel tensile failure. It looked like cast iron or maybe a bad heat treat. I am leaning towards the latter. And there is no way that this shaft should have failed like this – the thread should have failed first, and I was not applying enough torque for that to happen.
But the repair path was clear – the motor had to come out and be torn apart to replace the cam. I went into the house and started putting together a plan and a list of parts. I decided to replace everything in the keyed joint connection at the cam. I also formulated a plan to minimize the amount of disturbance to the engine, as it had been fully rebuilt in 2005 and not done too many miles since. Thankfully, Walridge Motors had everything in stock, and a quick ride on my ’72 Combat netted me all of my provisions.
I would remove the engine, and lay it forward on the bench. Since it is a 750, the barrels can be removed from the crankcase without disturbing the head. The crankshaft would be rotated to TDC as the barrels lifted clear, so that the pistons stayed in their bores and the rings were left undisturbed. Then the drive side of the crank case could be removed, for access to the cam shaft.
The cam that was in it was a 2S grind (replaced as part of the 2005 rebuild), and the replacement is a 3S grind. The new cam is reportedly made from EN40 steel, which has a very similar composition and mechanical properties to 4340 steel. I was pretty happy about that. According to the shop manual, original cams are made from EN32, which is very similar to 1021 mild steel. It is case hardenable, but has a soft inner core. I am pretty confident that the 2S cam was not EN32 – the fracture surface is totally inconsistent with this type of steel.
The dismantling technique worked out very well. All mating surfaces were cleaned, and a fresh coat of Permatex MotoSeal 1 was applied. The base gasket had to be cut in order to fit around the connecting rods; the cut was made at the front center stud. In order to create a labyrinth path, cuts were made at right angles – first left to right on the inner side, then straight ahead for the second cut. The whole area was coated in Motoseal to make the cut path leak tight.
With it back in one piece, I put the engine on the stand so that I could time it and put the timing cover back on. I notice that the new key and sprocket went together on the camshaft with no slop at all, which I was also quite pleased with. I began to think that the keyway was possibly yielding as well, creating the play that I found.
With the timing cover back on, the engine could go back in the bike. I ran out of brake cleaner as I was getting to the primary drive; it was now about 9:30 in the evening, so I figured I would call it a night. I was pretty pleased with my progress. Engine out, repaired, and back in the bike in roughly 4.5 hours.
Day Two started with fetching some brake cleaner and ATF from the local hardware store. (I buy brake cleaner by the case – lol).
The inner primary required a bit of cleaning around the transmission sprocket – road grime mixed with chain lube, and looking at it closely, the top visible surface was kind of ugly… Starting with a flap wheel, then moving to the palm sander with 100, then 320 grit paper, all of the scratches and rough surfaces were massaged out. And over to the buffing wheel…. In about 20 minutes, total transformation. The outer primary cover had a good sized blemish in it too, so it got a similar treatment. And since I was set up, the valve covers and ignition cover got a quick polish up. Much better!
While the primary was off and the bike was supported by the frame stand, the isolastics got a quick adjustment. The bike didn’t handle badly, but it was just not as smooth as I knew it could be. It has Mk 3 adjusters, so this was accomplished quite readily.
Everything went back together quite routinely. A nuisance oil leak was dealt with – believed to be the oil filter, it turned out to be a crappy clamp on the blanked-off chain oiler spigot, leaking down the oil lines to the filter.
The engine started on the third kick, and a timely visit of my CVMG buddy Andrew made timing the ignition a breeze. A vintage Suzuki guy, Andrew marveled at the simple elegance of the Commando, and how easily accessible everything is.
After about 15 minutes at 2500 RPM to help break the new camshaft in, it was time to take it for a good 30 minute ride to finish breaking in the cam. I “nut and bolted” all the key fasteners and the engine exhaust nuts during this time.
A quick costume change into some riding gear and I was off. I immediately noticed that the isolastic adjustment was successful – smooth everywhere now, even down to 2500 RPM. About mid-way through the ride, I wicked it up into the 5000-6000 RPM range. The top end performance with the 3S cam is impressive, and the 34mm Mikuni does not seem to hamper performance at all. The mid-range is very stout, pulling from 4000 RPM in fourth gear with authority. Opening it up on a downhill run heading into an up-hill, it maintained 6000 RPM in fourth while going up a reasonably long 5%-ish grade – it wouldn’t do that before. While the 2S cam tends to give the exhaust note a distinct staccato, the 3S changes it yet again – I don’t know how to describe it, but it sounds really good.
Back in the drive way, she was parked on a clean section of concrete, and after 15 minutes, not a mark on the ground – Yay!
Although this was definitely not a planned task, and was pretty frightening when it unfolded, it came together beautifully and I am very pleased with the results. It is still a work in progress, but this bike is responding to therapy very well.
Last edited:
