Here are four things I learned using a vernier timing gear on the MGB camshaft, while replacing the stock single row gear on my 1977 18V engine.
Here's how it looks out of the box:
I got mine from Basil Adams, but also available from Moss, VB, and NB Northwest.
Here are the Four Things I Learned...
The dimension "a" in the below picture was too long. It resulted in out-of-spec endfloat (camshaft would be able to move laterally in block a bit). I took it to a machine shop, and they took "a" down by 0.006", and then endfloat was in spec. Cost $65. Here's a Forum thread on the topic. At least one other person had this same issue with this gear.
2. Securing camshaft locating plate
In my case, the vernier gear was on the camshaft so tightly that it was much easier to put the vernier gear on the camshaft before inserting into the block. When doing it that way, one bolts down the camshaft locating plates through the holes bored in the face of the timing gear.
What I found was that these holes are smaller on the vernier gear than on the original single-row gear I was replacing. They were just small enough that I could not fit a socket around the heads of the hex-bolts. I used the equivalent bolt (1/4-28 x 3/4" - finally located at OSH) in allen head. Using an allen wrench, there is plenty of clearance to turn the bolt.
3. Timing chain clearance over tensioner mounting plate
An AE timing chain came with my Vernier kit. This specific chain has one extra thick link on the bottom
(sort of gold color in this blurry picture):
I found that this got caught on the small plate that the tensioner sits on. I ended up cutting a corner off the tensioner plate so there was clearance for this link to pass by without getting stuck.
4. Ease of use
With the vernier gear giving you ability to change the cam timing by just over 20 degrees (relative to TDC), you can adjust within the entire range (20 degrees) of a full tooth. It was very easy to adjust to within half a degree of where I wanted to be. I can't imagine attempting to do this with offset keys. I read an article from Moss saying that there is a tolerance problem around timing keys specifications, and some will be very tight in the keyway. That was the case with mine. Once my key was in, I couldn't easily remove it. No need with the vernier gear!