Chasing Threads and Re-Threading

Greetings. I've been looking online but havent figured out what size tap i need for a thread in my engine block where a 17mm metric sized bot goes in to botl on the tensioner. I appreciate any help! 
I think I will have to rethread it. It would be my first time.   

???

to my eyeballs that looks like either a 10mm x 1.25 pitch or a 12mm x 1.25 pitch bolt, both are very common on Toyota engines. Measure the major diameter of the threads. post that up and measure from peak to peak across 2 threads. Push come to shove most hardware stores that have a nice nut bolt isle should have a thread gauge, also Tractor Supply has thread gauges as well. Looking at the condition of the threads, I would get a thread chaser tap, they don't cut they just reform thread. If you can't get that to start, you have 2 options.

1. helicoil or timecert repair

2. tap and drill to the next size

if you end up doing a thread repair or oversize drilling, piece of advice, don't use a fine thread, use a course thread pitchs like 1.5 or 1.75, I have no freaking idea why the hell toyota, nissan and honda have to be so stupid about thread pitch in cast aluminum, but here we are 60 yrs later with the same stupid problems.

Metric threads are pretty simple. My guess it would be a metric fine thread 17mm x 1.5mm pitch
So if you don't have a thread gauge, measure 10 threads and see if it is 15mm long.
Then you just need  to buy a 17x1.5mm tap.
to size the drill for a metric tap, just deduct the pitch from the diameter. In this case, 17-1.5 = 15.5mm

Never came across an M17 bolt (but M17x1 and M17x1.5 taps are available)
Is the hex of the head 17 mm?
In that case it would probably be an M10 bolt as smc.collings already guessed.
M10x1 is common on rear axles for bicycle hubs, so that would be easy to confirm, by screwing the bolt into a nut from a bicycle.
Taps for it are "findable" e.g. Yamawa, Gühring, SUS, Corai, Regal come to mind.


Should it be M10x1 I would probably just make two slots into a hardenend bicycle axle with the angle grinder, then carefully screw it in with some cutting oil to clean up the thread.