What do I have?

[jimchappelear]

Hi. I have a 400? stamped YH204588 with 4X heads. Can anyone tell me what this engine is and what year? Thanks, Jim.

[Larry]

If you check you will find that the "YH" can mean a number of things. The "Y" generally was for automatics, but you will find "YH" used for 1967-'69 428s and for 1970 455s (ful-size Pontiacs). No where did I find the "YH" in reference to a 400. No help there.
Check the side of your bolck; if it is a 1969 and later block, it "should" have the displacement cast into it. According to Pete McCarthy's "Pontiac Musclecar Performance 1955-1979" book "350, 400, and 455 vertically, and 428 laterally between the front freeze plugs." If that doesn't help you are going to have to start counting motor mount holes and freeze plugs to zero in on the engine application.
You didn't mention where the numbers you listed were found on the block.
Since I cannot find any numbers matching those you show after the "YH" (204588) is it possibly some of the VIN? Are these stamped or cast numbers?
Any other ideas out there?

[Stu Norman]

A YH code block could be any one of the following:

1965 421 automatic 338 HP
1966 421 automatic 338 HP
1967 428 automatic 360 HP
1968 428 automatic 375 HP
1969 428 automatic 360 HP
1969 428 automatic 360 HP
1970 455 automatic 360 HP
1972 455 automatic 200 HP
1974 400 automatic 190 HP
1975 400 automatic 400 HP
1977 301 automatic 135 HP

4X heads came on on the 1974 190 HP 400. If you find a date code you'll know for sure.

[Stu Norman]

You know, I goofed on that list.

A 1975 YH is not 400 HP!

It's 170 HP. Sorry about that.