The model number will be 28?707. In most cases the missing number will not make any difference. Note that the early version of this engine did use a different rocker arm system but that should be reflected in the Type Number. It should be easy to differentiate between the two by looking at yours.

Also note that the 28 series of model numbers also include flat head engines.

Walt Conner

Missing the design series digit shouldn't be a deal breaker as Walt said. Common were 287707 , 28n707 in OHV. You didn't say what this was on. If you reference the equipment model #- you may find the exact model in their literature.

That's a big help. I looked at numerous manuals for 28* engines. There are a bunch of them! The head and gaskets are the same on the OHV models, so I will assume that I can order parts safely using a model number like 28N707. Where can I find the meaning of the type numbers? I know they are paint colors, etc., but haven't seen what each number stands for.
The engine came from an unknown riding mower.
I have more questions. Should I start another thread?

I don't know if there is an actual listing or detailed explanation of the "TYPE NUMBERS" that would reveal anything.
My impression of the Briggs "type number" is that it is more of a "production run" identification for Briggs to refer to for production records, and later for the consumer to refer to for replacement parts.
The type number positively identifies a specific production run or mass build configuration so that Briggs has a record of what that "build" included.
You can start a new thread if you like (under a new title) or you can continue to use this one since you are the original poster.
The image below does explain the model number nomenclature system.

{{gwi:317491}}

Not only will the 28 series head gaskets fit but the 31 series without the letter in the model number will fit, MAYBE all 31s, not sure about the ones with letter in model number.

Walt Conner

If you are having difficulty yet, post the type # as well and I'll look for the correct combo.

Type number: 1255-E1

The only match was 287707-1255-E1. Get out the engraver!