About two years ago, the Triumph parts pipeline was flooded with tappets of substandard manufacture, both four and six cylinder type, and we began a rather sordid period in which these tappets were taking out camshafts in ridiculously short lengths of time. After about six months of this nightmare, we felt we had solved the problem with the use of nitrated stock tappets along with a good supply of chilled iron tappets.
Now this plague starts all over again with the markeet being flooded with a poor quality chilled iron tappet, compounded by a general shortage of nitrated ones. Most responsible suppliers have pulled these new chilled iron tappets off their shelves, but a few have persisted in selling them.
We thought we had a long term solution last year when we found a major US supplier that supposedly had TR6 tappets in hardenable iron (a very hard and dense material). When we went to buy them, however, we found that they had never been put into production because the demand had been so low!
Some experimentation proved that we could take a slightly larger-diameter "bucket tappet" used in American cars and machine it down to the correct size for the six-cylinder. The cost of the tappets plus the machining make them an expensive, but very good solution. These tappets have a .100" radius on the bottom face to ensure good rotation, and are about .090" longer than the original part. It would have been too expensive to shorten them to the original length, so they were left a little long, which is fortunately not a problem. That takes care of the six.
The four cylinder, however, is not as easy. There is not a bucket tappet large enough to do a similar modification. That leaves one other option: the tappet from a 235 Chevy six-cylinder can be modified to work. It is a hardenable iron type which has a top seat. The big complication with this one is that it requires a pushrod about 2" shorter than the stock TR unit. Obviously, this will be a more expensive solution because, A: it will be necessary to buy the Chevrolet tappet and machine it to the correct size, and B: make up shorter pushrods to go with it.
As you can see, these represent fairly expensive solutions to the bad lifter problem, but until and unless a reliable product is again manufactured, the ideas described here will remain an available long term option.