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Hi,
On flicking through the latest edition of Roadholder I came across an article about a trip from Spain to England on a Mk3.
It mentioned 'a Ken Merrick re-worked sprag'
I did a bit of Googl'ing and found the info below, has anybody on this forum performed this mod?:
(Article written by Ken Merrick)
There is, however, a further weakness in the starter system and that is the spragg clutch. This clever little device is a form of “free wheel” which engages the main shaft via a reduction gear drive from the starter motor. The sprag clutch transmits the drive to the main shaft when the starter button is pressed then free wheels once the engine fires up. Early sprag clutches had 14 sprags or teeth but later ones were increased to 18 to improve performance and reliability. In free wheel the sprags lean over to allow free rotation. When the starter is engaged, the sprags come up close to the vertical and carry the main shaft with them giving the required drive to the main shaft for starting. With use, the engine sprocket bore wears by engaging the sprags as does the gear which contacts the other end of the spragg allowing the sprags to go over centre and jamming the whole assembly, resulting in an expensive replacement sprag, sprocket and gear costing £200.00. If this happens, it’s no use replacing just the sprag, it’s necessary to replace all three items.
2
This happened on my bike and I was unable to turn the engine by any means. I managed to free it by dismantling the whole of the primary side and, upon examination, the I.D. of the engine sprocket and
the O.D. of the gear, engaging the sprag were damaged with grooves where the sprags had “dug in” whilst going over centre. The sprag of course was completely wrecked. To avoid the expense of a new sprocket and gear I internally ground the sprocket diameter until the grooves had been removed, taking out 0.010” on diameter.
The gear was not so easy as it required an increase in diameter. I overcame the problem by selecting the outer hardened sleeve of a needle roller bearing measuring 42mm O.D. and 35mm I.D. from my local bearing supplier for £5.00 and by reducing the gear diameter fixed the sleeve with LOCTITE RETAINER. This involved reducing the hardened gear diameter by rough turning the diameter to give a clearance of 0.004” on diameter below the sleeve I.D. of 35mm. Rough turning is required to give the maximum “key” and was easily achieved with a carbide tipped tool on the lathe.
The sleeve was grooved internally with a number of spiral grooves ground freehand with a “DREMAL” hand grinder to give a similar “key”. The O.D. of the sleeve was then externally ground so that the ground diameter just entered the sprag and was free to rotate backwards but when rotated in the opposite direction, the assembly locked and drive to the main shaft would result.
Because the clearance between the sprocket and gear dia was reduced, the sprag angle was increased making it impossible for the sprag to travel over centre thus overcoming the previous disastrous jam up.
I have run this set up for the last 4 years and 5000 miles without a problem and although it all sounds a little complicated, it can easily be achieved with a little help from your local engineer and will save you a lot of money purchasing new.
On flicking through the latest edition of Roadholder I came across an article about a trip from Spain to England on a Mk3.
It mentioned 'a Ken Merrick re-worked sprag'
I did a bit of Googl'ing and found the info below, has anybody on this forum performed this mod?:
(Article written by Ken Merrick)
There is, however, a further weakness in the starter system and that is the spragg clutch. This clever little device is a form of “free wheel” which engages the main shaft via a reduction gear drive from the starter motor. The sprag clutch transmits the drive to the main shaft when the starter button is pressed then free wheels once the engine fires up. Early sprag clutches had 14 sprags or teeth but later ones were increased to 18 to improve performance and reliability. In free wheel the sprags lean over to allow free rotation. When the starter is engaged, the sprags come up close to the vertical and carry the main shaft with them giving the required drive to the main shaft for starting. With use, the engine sprocket bore wears by engaging the sprags as does the gear which contacts the other end of the spragg allowing the sprags to go over centre and jamming the whole assembly, resulting in an expensive replacement sprag, sprocket and gear costing £200.00. If this happens, it’s no use replacing just the sprag, it’s necessary to replace all three items.
2
This happened on my bike and I was unable to turn the engine by any means. I managed to free it by dismantling the whole of the primary side and, upon examination, the I.D. of the engine sprocket and
the O.D. of the gear, engaging the sprag were damaged with grooves where the sprags had “dug in” whilst going over centre. The sprag of course was completely wrecked. To avoid the expense of a new sprocket and gear I internally ground the sprocket diameter until the grooves had been removed, taking out 0.010” on diameter.
The gear was not so easy as it required an increase in diameter. I overcame the problem by selecting the outer hardened sleeve of a needle roller bearing measuring 42mm O.D. and 35mm I.D. from my local bearing supplier for £5.00 and by reducing the gear diameter fixed the sleeve with LOCTITE RETAINER. This involved reducing the hardened gear diameter by rough turning the diameter to give a clearance of 0.004” on diameter below the sleeve I.D. of 35mm. Rough turning is required to give the maximum “key” and was easily achieved with a carbide tipped tool on the lathe.
The sleeve was grooved internally with a number of spiral grooves ground freehand with a “DREMAL” hand grinder to give a similar “key”. The O.D. of the sleeve was then externally ground so that the ground diameter just entered the sprag and was free to rotate backwards but when rotated in the opposite direction, the assembly locked and drive to the main shaft would result.
Because the clearance between the sprocket and gear dia was reduced, the sprag angle was increased making it impossible for the sprag to travel over centre thus overcoming the previous disastrous jam up.
I have run this set up for the last 4 years and 5000 miles without a problem and although it all sounds a little complicated, it can easily be achieved with a little help from your local engineer and will save you a lot of money purchasing new.
