I have just bought a used MTD tractor with a 18.5 hp single cylinder B&S engine (the more recent one with the remote filter). On starting, when the cylinder first fires, it emits one strong metallic "thwack" and then runs normally. I does this even after it has just run, so I am sure that the lube system is not dry. Question is, is this normal for these engines. My earlier 14.5 hp (splash lubed) briggs didn't do this. Note also that the previous owner has put a multigrade synthetic oil in it, and I was concerned that this may be part of the problem, although the manual says this is OK of the engine.

My concern is that this could be a rod or main bearing about to go and I have bought somebody else's troubles.

Not normal (have the same engine, all it has is the normal rocker arm tap) Its certainly not the oil... It doesn't sound like the rod(as a rod would keep on knocking) could be a loose bearing clearance or maybe the avs weights.

Could it be that the compression release isn't (releasing enough that is) so that the first hit of the engine is much stronger than it might normally be? Also, what is the normal valve lash? If the valves are too loose the compresson won't be properly relived and I was thinking that this might be part of the problem.

Ok, here's the scoop.

Went out and fired it up and closely watched the engine. Tried to start it with low throttle and a bit of choke, thinking that would provide lower starting cylinder pressures... it fired and I found that it "clanked" at very low speeds, but as you ran the speed up it smoothed out. Appeared that the engine was "moving" a tiny bit relative to the frame. Tightened the mounting bolts. None were really loose, but all four could be tightened a bit more. That appears to have fixed it. Had me more than a bit worried, but the briggsdude was right, if it was a bearing or something like that it would have gotten worse with speed, not gone a way... Fixed, but lesson not forgotten..

Ahhh, good thinking... (those bolts must be down really tight, the torque will actually make it move if they are loose)... glad to be of any help...
If they loosen up again, take them out and use some threadlocker on them buggers.