Cat V60B forklift

My dad has a Cat forklift, that If my memory serves me is a V60B. It's a 6 cylinder motor.
He told me that today he had to change 4 plugs before it would start...they werent all that old, nor was it that cold (Phoenix Arizona). I'm not positive, but I think they were Champion D15Y, 515. Probably should say that the rings may be poor, when it runs it smokes, but memory a few from a few years back tells me it still had fair compression.

Today he asked two questions of me and so far I haven't found the answer on line.
1) How many different engines did it use?
2) What plugs should he be running in it(

There's a site called, "First Class Downloads" which has all the info you want. You have to pay, but I did find engine specs on the V60B. Have your S/N handy in case there's a break or a option where they changed engines.

Can't remember the exact model number but Dad's V60B has a Continental in it. I have no way of checking right now but I seem to remember it was a Y112, but I could be wrong. He has a second one that we bought with a worn out engine and it also has the same engine. That said the EXACT plug recommended isn't that big of a deal with those old things, as long as the thread size, seal type, and reach/lenght are right. Beyond that if you run it in short spells, or the engine has a good amount of ring wear, it's better to run a hotter plug than recommended to so the plug will retain enough heat to self clean. Usually if you'll find one about mid range and then go up a heat range or two you'll be OK. The main thing is don't run an extreemly cold plug or it will tend to carbon up and foul out. On another note one of the biggest problems I see in the situation your describing is the use of Champion plugs. Granted Champion is a big name when it comes to plugs, and in years past had a really good product. While I know areguments can always be made, being a mechanic for a living, I have seen far more problems stemming from the use of Champion plugs than I have any other brand. These problems range from the plug 'going bad' because the machine set for awhile, to being bad straight out of the box. Personally I only use Champion if it's an odd plug that only they make, otherwise I typically find a compariable from NGK, Autolite, or Motorcraft, with the NGK's always seeming to come out on top for some reason.