!BORING TECH RABBIT HOLE ALERT!
Marshall,
I'm glad you have somebody you can trust-- but I'd recommend following the advice of RR, "Trust, but verify". Knowing a few things as you enter into dealing with the mechanic means you aren't flying blind. Forewarned is forearmed.
Difficult to find and fix oil leaks almost always come from the crankcase being pressurized, whether it's and oil mist blowing out behind the pulley, or pushrod tubes, etc. There is pressurized oil in the galleys, and there will sometimes be leaks at threaded joints, seals in the oil cooler, etc.-- but most of the hard to correct leaks are from crankcase pressure.
Cylinder piston ring seal is what keeps the pressure in the upper half of the cylinder where you want it, and out of the bottom (which is the crankcase) where you don't. If the valves leak by, it will pressurize the intake or exhaust, but not the crankcase. If you have a pressurized crankcase, the pressure is coming from the pressure in the cylinder leaking by the rings. It could be worn out rings, but in hot-rodded Type 1 engines, cylinder distortion is a real problem. As the cylinders heat up, they expand. The thinner the cylinder wall, the more they distort as they expand (making a good ring seal very difficult). This is an instance where more material is always better.
A stock 1600 Type 1 cylinder measures 85.5 mm and is 4.15 mm thick at the top of the cylinder. These cylinders are great, but it's hard to make power without displacement, and the fastest way to increase displacement is with a bigger bore size.
A 94 mm cylinders used in 1915s, 2276s, and 2332s are 3.55 mm thick and often have distortion issues in engines running higher compression or higher RPM, unless a guy is very, very careful with ring selection and cylinder prep (most people are not ). There is a "thick-wall" 94 mm cylinder, but it's very rare, and not very effective, because the outside of the cylinder needs to be grooved for the head-studs to fit, and this creates thin spots. JPM in Sweden makes a 94 mm cylinder out of some super-special iron, and makes them as thick as possible (to be used with 8 mm head studs, etc), while holding a constant cylinder wall thickness. They cost about $1000/set of 4, without pistons or shipping.
The Mahle 90.5 mm cylinders everybody uses in 1776s and 2110s are 3.75 mm thick, which is why everybody uses them for "happy" engines that don't spit a lot of oil. AA makes a "thick-wall" 92 mm cylinder that is 4.55 mm thick, which is the king of the hill in terms of wall thickness, but the pistons in most of their sets are not forged. Now, if JPM made a thick-wall 92 with the super-special iron, and sold them with a set of JE pistons running Deeves rings and a Total-Seal second, we'd probably never throw a drop of oil.
If your engine really is a 1641, then it is a stock stroke engine with 87 mm "slip in" cylinders. The stock 1600 cylinder bore is 85.5 mm. The only way for the cylinder with a bigger bore to fit in the stock case register is for the cylinder walls to get thinner (a lot thinner). 87 mm "slip in" cylinders are 3.4 mm thick-- thinner than a 94, thinner than anything commonly used except an old Mahle 92 mm cylinder (which measures 3.0 mm and is not recommended by anybody).
The 88 mm "slip in" were the thinnest of any cylinders ever used. They are only 2.9 mm thick. For reference, that's only .11". I think they were a thing for about 10 minutes in the 60s and nobody's used them since.