I have a build that i started from scratch. My plan was to use a midlength magpul moe handguard and a yankee hill gas block. I thought when i had the two in my hands it would make sense how the yankee hill gas block would fit into the handguard but it looks like i still need an A2 style cap for the handguard to lock into. Can anyone verify this?

Here is the gas block I have. YHM-9836A
https://www.midwayusa.com/product/1003281669/yankee-hill-machine-gas-block-single-picatinny-rail-with-bayonet-lug-and-sling-swivel-ar-15-standard-barrel-750-inside-diameter-steel-matte

Here is the handguard I have. Magpul MOE
https://www.primaryarms.com/magpul-moe-m-lok-handguard-mid-length-flat-dark-earth-mag426-fde

I dont have a barrel yet but it will be a midlength barrel.

Thanks for any input.

Yes, you need a cap behind the gas block. Those just replace the standard handguards.

Your "conventional" A2 Front Sight Base had a forward handguard retainer (Cap) sandwiched between the barrel gas mount shoulder and the back of the FSB.

If you go to the YHM gas block, you will still need a forward handguard retainer (Cap) to use the MOE (or any other "conventional / standard / drop in" handguards).

Note the forward handguard retainers come in either triangle LINK - Triangle

or

round.  Link - Round

Also, you if you plan to run back up sights, you will need a Gas Block Height Front BUS that is metal (as poly / plastic will melt from the temperature)