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Stripped water pump threads


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Long story of how one little job snowballs into a nightmare.

Ran the car on the Dyno last weekend. (136 HP, 215 ft-lb Torque). Got home and noticed the radiator was low on fluid. Started looking and have a pin hole in the radiator. No problem, I already traded earlier this year for a 3 row from Motorsport, I will just swap, and while I am there, I will go ahead and replace the thermostat and water pump, then the car will be cooling just fine for a long time.

I am in the middle of replacing my water pump on the 78Z. There are 6 bolts holding this pump onto the front of the timing cover. 2 with 12mm heads and 4 with 10mm heads.

Apparently at some point in the past when I did the job before, I did not get a good seal between the pump and the bolt holes. The coolant has worked its way down around the long 10mm bolt (at the 11:00 position when looking from the front of the car).

There are literally no threads left in the block.

Long story to get to the Question:

Is there enough wall behind there to drill and tap the next largest size?

My plan was to attempt to do this with the timing cover still on, but this is a long bolt and I do not know if I can get a tap that length (I am sure they have to be available, but it is not a "jobber's" length, which is what everyone sells around here over the counter.

I would like to avoid removing the timing cover if possible, but it may be a good time to replace the chain, sprockets and guide while the radiator is out. My wife is going to kill me when she finds out how much all this will cost.

Thanks for any help and advise you can give.


78 280Z

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That #10 11o'clock is usually the once that snaps off when removing or tightening most z's are missing that one. I drove one for lotsa years without it. Try going to napa or such and buying a helicoil that locks in the hole and supplies threads. It looks like a spring. Any auto place knows about them. Oh! the thermo housing bolt closest to the valve cover is the next snap area. Try tighten a little loosen a little more all the way out. That back and forth clears/cleans the threads and can stop the snap.

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I was scared to think about a helicoil because it takes a large hole to put in.

Is there enough "meat" in the block in that area to hold a helicoil?

I did find some long taps (expensive), in the MSC catalog. My current plan is to open the hole up with a drill and re-tap to the next thread size (Standard or metric, I don't care). I have not had the time to research the thread sizes yet.

Thanks for the advise.


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  • 18 years later...

18 years!  (Is that "awhile?)  Re-tap  to 1 size bigger if possible (metric prefered as most is metric) or recoil it.  Or if there is enough dept, make the hole deeper and extent the thread and use a longer bolt! 🙂

If the bolt is M5 you use a drill 0,8 times 5 = 4,0 mm    ( M6 is 6 x 0,8 =4,8 or 4,5 if you don't have 4,8mm) 

You could weld it shut and re-drill/tap but that's a specialist's work..  I would use a good recoil.

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