Even if I haven’t updated this thread for a while, slow but even progress goes on and by now the engine is almost finished. Today I bring to you something quite interesting that, to my knowledge, has not been done or at least published before…
As I’m going to fit T1 camshafts on my 996 heads for my ST4S engine and I love the ‘vintage’ look of the original clocks, I decided to replicate this on my bike. Therefore, here is the modification process of the OA T1 camshaft to make it compatible with the mechanical tacho. The original 851 OA camshaft is somehow longer and protrudes out of the head to drive the gear connected to the tacho cable.
WARNING: This mod has been done on a professional engineering workshop. Although the process itself is quite simple and logical, the camshaft is hardened by cementation and therefore, very difficult to machine.
The real deal would be making a bore at the camshaft tip and then cutting a left hand thread –in order to avoid loosening– into the bore to fit a threaded insert. However thread cutting inside the hardened bore may be VERY difficult and time consuming. So we decided to take an easier approach and interference fit the new tacho insert. To do so…
We started boring the camshaft on the lathe with an 8 mm HSS drill…
Even with great care and a professional operator, the drill broke after some mm… So we had to try the heavy weapons way and this time we fitted a 10 mm titanium carbide drill… ‘But if it breaks, you pay pal and this drill alone costs about 50€’ ‘Gluuuuubssss!!!!’
But it worked VERY WELL. Hardened steel was drilled as easily as if it was soft aluminium!!
It's great to have a chuck big enough to place the camshaft inside...
This is the 10 mm bore in my T1 camshaft.
Then my friend made a roughly oversized steel insert as we already supposed that we wouldn’t get to insert it perfectly straight in the camshaft.
The original 851 OA camshaft above. My by now two part T1 OA camshaft below.
A critical part of the process starts now as we have to heat the camshaft to fit the insert. You need heat enough to expand the camshaft bore but overheating must be avoided at all cost to avoid losing the metallurgic hardening properties…
Careful heating…
And the invaluable help of a hammer got the insert in place!
And then, as important as even heating is careful and slow cooling, so we left the part to cool slowly for a while and went out for a couple of pints (Saturday 10:30 AM, Ouchs! 😎)
After checking on the lathe, we found out that the insert was quite straight. We took it to the right diameter and machined the groove for the seeger ring that secures the tacho gear in place.
Finishing the work on the lathe.
The original tacho gear is fitted via a woodruff key. As it’s necessary to have an specific cutter to make its seat that my friend doesn’t have, we went instead for an straight cut for a square key.
A CNC mill cuts the key seat.
And the camshaft mod finished. Interesting, isn’t it?
Afterword: Sorry for the pictures quality. I forgot my camera at home and had to take these with the cellular...
Acknowledgments: 'We' on the text refers to my very skilled friend Antxon who made the part while I took the pictures.
