The line boring of motors is becoming increasing necessary as engines age and as the demands on them continue to rise. And while this process does provide a lot of needed benefits, the only downside is that it moves the crank closer to the camshaft which means that even a new sets of gears and chain will not be tight enough to accurately control camshaft timing. These special gear kits are designed with very slightly oversized gears to compensate for this distortion. Available in different increments to accommodate various line bores.
If you are working on your engine and you want to do it right then you should consider how you are going to drive the camshaft. People often spend a lot of time picking the camshaft and then ignore the exact reason they spent so much time picking the cam. Driving the cam and controlling the timing relationships between the cam and the crank is what camshaft drive systems are all about. It makes no difference what the cam does if it does not do it in sync with the crank and pistons. The stock system on many 848, 948, 998 and 1098cc engines is a single chain drive between the crank and cam. The 'S' and some 1275 have a much better "duplex" chain. But even those do wear out! The best judge is to check it while you are taking it apart. With the timing cover off the engine rotate the crank a slight amount back and forth. The camshaft must move the exact distance at the same instance!
The least expensive but better than an old one is a stock cast gear and chain set, This budget standard cast duplex gear and chain set is the next step up, for road use only.
Fitment of an uprated cam drive system is essential when building a performance engine. Timing scatter induced by the standard set up can reach up to 15 degrees once the single row chain has stretched, which it does after only a few miles. This scatter not only affects the cam timing, but also the ignition, the distributor being driven by the camshaft. Power loss suffered by this phenomenon is substantial. Replacing the standard single row (simplex) system with a dual row (duplex) system greatly reduces the problem; use of a toothed belt system all but eliminates it. The belt system vastly reduces valve train noise and also helps damp out some of the odd harmonics generated by the 3 main bearing 'A' series engine. It is also extremely important to time any cam in to its required setting to obtain maximum performance, especially with performance cams. The 'dot to dot' method can, because of manufacturing tolerances, be out by as much as 15 degrees. Anything above 2 degrees out and power suffers -- most noticeably in small bore engines. In race engines you probably lose 1 hp for every degree the cam is out, more when over 6 degrees!