I've read with keen interest over the years the posts from Frank R. and others regarding the necessity of cleaning out the head bolt holes in the block with a tap to ensure proper clamping pressure. I'm definitely in the "I'm going to make sure I do this too" camp <img src="/forums/images/graemlins/cyclops.gif" alt="" /> I bought myself a M12 X 1.25 bottoming tap based on what I read in other posts to do the job with.

With the heads off I thought I'd just give it a try with simple finger pressure. I put the tap in place and turned it with just my fingers -- after a turn or two it seizes up. It doesn't seem to matter which bolt hole I try it in. The head bolts thread just fine with finger pressure, so I'm confused...

Did I buy the wrong size tap? Will it require putting a handle on the tap? I really, really, really don't want to damage any of the threads in the head <img src="/forums/images/graemlins/scared.gif" alt="" />

Help me out here guys -- is there a technique I should use? Do I need to oil the tap? What to do, what to do?

Eric W.

I just did this on mine a few months ago. I thought the thread was m10 x 1.25, but I am not sure. Check by matching your tap with your bolts you removed. You can stack your tap on top of your bolt so the thread are touching each other and if they fit togethor like a zipper then you have the right pitch and thread count.

I sprayed some lube on each hole as I was running the tap to limit the friction, and blow the hole out once I was done, to get all the trash out.

Just checking you did get new bolts?

A threading tap often takes a cut of the existing threads. Not a deal breaker, just make sure you have the right tap.

A thread cleaning tap is slightly undersized and should be just large enough to take any dirt out.

If your deepening the threads, you need the bottoming tap. Often times the bottoming threads just need cleaning.

Kevin

Hmmm. I bought a bottoming tap on the advice of Frank R. I'm not sure what the difference would be between this and a thread-cleaning tap?

From what I can see the thread pitch is the same as the head bolt threads. I will check it again this evening. Maybe the issue is the M10 vs. M12? Anyone know definitively what the correct one is?

Yes, I have all new bolts and washers for installation.

Eric W.

The bottoming tap is to make sure you get all the crud cleaned off the bottoms of the holes. By doing so, you ensure you can run the bolt ALL the way down, thus creating the clamping pressure necessary to maintain head gasket integrity.

The main objective is to get that hole deep enough so that you're not bottoming out.

If the hole's not deep enough, you could actually torque down the bolts, but apply very little clamping pressure to the head.

Yup, makes perfect sense. Now I just need to know if I have the M12 x 1.25 is the correct tap for the job or not <img src="/forums/images/graemlins/cyclops.gif" alt="" />

Eric W.

Yes, that is the correct size. M10 would be very apparent to be incorrect, as it would be too small and would simply fall into the hole.

Yes, the tap will need to be wrenched in and out. And yes, some light oil on the tap wouldn't hurt. I use a shop vac, compressed air and some cloths to clean out the threads.

John B.

I got lazy and dug thru the odd socket box and found one that "fit" the sq. drive on the tap, tight enough to turn the tap against resistance, and drove it with a 18v drill on low speed, using a dewalt 1/4hex x 3/8sq socket driver.

After running down the tap into each hole a couple of times, I even used some cotton swabs on long wooden sticks to make sure I cleaned out all the oil and crud from the bottoms of the holes.

It took sssssseveral to do the job. <img src="/forums/images/graemlins/smile.gif" alt="" />

You want the hole pretty dry and pretty clean. You can hydrolock the head bolt on a little bit of oil/water/thread lube in the bottom of the hole. And I lube the threads and washers and underside of the bolts. <img src="/forums/images/graemlins/evil.gif" alt="" />

To be really safe, measure the height of the head, add the thickness of the washer plus the head gasket compressed thickness (I just use .040 to .045 and call it good for the gasket. Record the total (this is A). Now run the bolt into the head until it bottoms, and back it up a quarter turn, then measure the distance from block deck to underside of bolt head, bolt pulled up away from bleck tight. Record this measurement and name it B. A must be greater than B or the bolt bottoms before it hits torque.