Tub Shower Stem Interchangeability

Our S/E Michigan home was built in 1976. Shower handles are leaking (again) and are ugly so we want to replace the hardware. I believe the existing stems are Sayco, but not absolutely sure.

The new handles we purchased are Price Pfister. The gear at the handle end of the current stems is a finer cut than the new faucet handles. I went to the hardware to get the same length 1/2" valve stems with the coarser PP gear cut and was told I have to change the entire manifold behind the wall. While the full explanation sounded believable, I am still having a hard time believing a 1/2" fitting isn't a 1/2" fitting.

Is there no conversion short of re-plumbing behind the wall?

Generally, no. Each manufacturer has its own internal design, stems, cartridges, handles, trim that fits their particular valve. There are some aftermarket companies that make repair parts for some of the more popular valves, but the quality is sometimes suspect. The part of a valve that is standard is where the water supply attaches, from that point on, it is all proprietary.

Plus, in the last 35-years, the technology and rules on how a shower valve operates have changed. The new ones require anti-scald technology, which yours may not have. So, it's probably time to bite the bullet and put in a new valve. If yo udon't have access from behind the wall, then there are various companies that make remodel plates that are a trim that goes behind the new valve's trim, that is big enough to cover the bigger hole you need to install the new valve from the front. I recently did this at my mother's house, and used Delta's remodel plate (which cost me about $25).

Would you go to the Chevy dealer to get parts for your Ford??/

There is absolutely NO interchangeability of parts between brands of faucets. Mostly, there is little interchangeability of parts between different MODELS of the SAME brand.

If you have a Sayco valve, you need Sayco stems and handles. Period.

A 1/2" fitting IS a 1/2" fitting. But, your Price Pfister stems are NOT Sayco stems. The two brands are as different as a Ford and a Honda. Both cars will get you to work and back, but you cannot use the same parts to fix them. Both faucets will give you a bath or a shower, but that is also the only similarity between them. Either install the Price Pfister, or any other brand, faucet inside the wall, OR buy Sayco stems to repair what you have.