New machine? Chk with dealer.

Its off an old Sears garden tractor with an 8hp Briggs vertical. I put it on my Simplicity 728 because I like it better then just the plug. It doesnt smoke so or use any oil so I assume im not losing crankcase pressure. It'll screw down all the way but it screwed anymore it pops up

Uhhhhh, so you are saying that your Simplicity did not have a dipstick and you like the dipstick better than the plain plug your Simplicity had before?
If your Simplicity has just the plain oil level plug down low on the engine crankcase, you would be filling the crankcase until the oil level was even with the notch inside the oil plug hole of the crankcase. ???
So how is a dipstick which is calibrated to gauge the oil level by reaching down a sort of longish tube to get to the oil.............going to benefit your Simplicity in any way? Or am I missing something here??
The dipstick can't possibly give you the correct oil level reading in the different engine....so what is the attraction??

Both have the same engine 8hp Briggs vertical. The full mark on the dipstick is at the top of the filler plug like the oil level is suppose to be. Its the old style Briggs dipstick where it screws into the plug threads the then theres a clamp that screws in the side on the blower cover. Not the style with the rubber piece that that the dipstick sits in and the clamp on TOP of the cover pushes it down to seal it. and its the top cap im having troubles with. I perfer the dipstick over the plug because I find it easier to tell when I need to add oil if necesarry and makes for easier filling.

What happens if you tighten the dipstick just enough to seat it, without going that little bit further?
Now that I finally know the nature of your dipstick and the fill tube, I guess that the dipstick tube once had "stops" to tighten the dipstick cap against, and they have long since wore out or broke off.
If that turns out to be the case, and you like the convenience of the dipstick enough to buy a new one, they might be available from Briggs.
Your dilemma is certainly not the usual fair for this forum, just a bit rare (has this EVER been posted before?).
But, in the end, the dipstick & tube might have been "a Sears exclusive" that Sears specified for that genre of engines back then.

The cap needs to be in place and tight to assure the engine crankcase breathes through the OEM breather instead of to open atmosphere. The "breathing" of the engine is a series of alternating "inhale-exhale" events. That means if the "INHALE" portions of the process are through an unfiltered atmosperic source, dust can be drawn into the engine with every stroke of the piston (this would be the underside of the piston doing this in the crankcase, not the topside of the piston in the combustion chamber).
So, if the cap does not seal tightly, that is a source of dirt for the engine to eat. And then there is always the chance the cap could be vibrated off ande lost.

If I tighten it enough to seat it it does just fine. I believe the "stops" are wore out. And yes prolly is a Sears exclusive to that engine as I've only see one other which is on my Allis-Chalmers Homesteader 6hp which has a sticker on the on the cover that says "Made to Allis Chalmers specs by Briggs and Stratton Co" Except the one on the Allis doesnt have the bracket/clamp on the side and the top plug has three "claws" and it is pressed in and out instead of screwed on and off like the sears one.

a-c homesteader-Where in Wisconsin you at?

St Croix county. 25/30 miles from the Minnesota border