Hi Mark,
I think it is a really interesting idea with little potential downside.
The only downside is that some individuals won't buy chapters that they're not interested in. When you sell a whole book, of course they buy all chapters -- interesting and non-interesting ones to them. It seems to me that this market share would be offset by individuals who are only interested in a specific chapter and wouldn't buy the whole book just for those specific chapters. Engine teardown would be a good example of this.
I'm not sure what you're thinking of for sell price, but If you would say sell the entire book at $30 witih 5 sections, or $7.00 per section, you would find a lot of buyers.
You may want to offer a purchase in advance option. It will require more record keeping on your part, but if you allow people to purchase your entire book for $30 in advance and they could receive all complete chapters and you could send them links to chapters as they become available. This would help to fund your enterprise and extend the entire book discount to early purchasers.
My numbers are for illustrative purposes only, I'm not trying to influence your sell price.
Sign me up either way. I have a Haynes, Clymer, and Honda Shop manual. They all have their own defficiencies. I'm looking forward to how your work would fill in the many, many gaps in my knowledge.
If you need any PhotoShop or Illustrator help, let me know.