How to clear head bolt thread in block?

silent running

One of the threads for the head bolts in my block is very tight and I can't get more than 3 or 4 turns into it. Being as I'm guessing my ARP head studs are much tougher than the aluminium of the block, I don't want to wind it in by force.

Where can I get an M11x1.25 thread chaser/tap to clean the thread out, that is long enough to get all the way down there? Could I use an old head bolt to cut in there somehow? I threw mine all away only last week!

adzer

you not get one from the likes of halfrauds or somewhere similar?

NUTTYNICK

Are you sure they are the right thread pitch as 3 or 4 turns is about all you will get if they are out?

Not saying they are wrong but it isn't unheard of.

As for the tap, I guess a cheapish set will do and on the square drive, you can fit a small quarter inch bi hex socket (most good socket sets) with extension and ratchet to give extra length, worked for me before.

silent running

Yes, the other 15 studs go in fine, the whole way down, smooth as butter. This one last hole won't have any of it. I'll have a hunt around tomorrow and see if I can turn anything up.

NUTTYNICK

Ah good, just thought I'd suggest it

I expect you've tried swapping this stud with one that deffo screws in to ensure it's not a rogue.

As you are simply wanting to clear a thread out, I reckon the cheapest tap set you can find will do with some lubrication.

Hope all works out

silent running

Yes, it's definitely the thread in the block that's at fault. Problem is the size is really unusual and M11x1.25 doesn't even come in most of the decent quality sets let alone the cheapies. I have a cheap set that has M10 and M12 but not M11. I've tried the M10x1.25 and the pitch is right but obviously it's not a close enough fit to do much good.

I think what I need is someone who's got an old head bolt knocking around that I could 'mod' to work as a tap. I just threw out dozens of the things that were lying around my garage!

cookstar

Im sure someone like API will send you one, must have millions of old ones, doesn't really help on a bank holiday weekend though.

__________________
Is very much looking forward of being on the right side of 400BHP once again!

172sport

I've ground a flat on both sides of a bolt to clear threads before with success, although nothing as potentially expensive......................!

NUTTYNICK

See what you mean, I should have read the thread properly, didn't realise you were after an M11.

They are available but not too cheap for the amount of times you will use it

Single Tap

This guy lists all sorts apart from the one you're after but may be able to source one if you're stuck

Ebay Link

silent running

Good link to that tap, that's the cheapest so far, but still £30 once VAT and delivery are on top... I just knew there was a good reason for keeping spare head bolts around!

The weird thing is, looking down the hole, I can't see any blockage at all. I don't know whether it had some water in there before and the threads got coated in that chalky hard white stuff?

silent running

Disaster has struck! Just to see if it might work, I sprayed some PlusGas in there to clear it out, then wound in the stud a few turns, as it had gone in 4-5 threads before. Within seconds it started to feel like it was tightening so I quickly started backing it out, but not quick enough! The thing seized rock solid, I've never known anything like it...at this point I was bricking it thinking I'd written off my block so using brute force it came out. I figured it was better to wreck my brand new 3/16" allen key and ARP head stud than to leave the block with this thing sticking out of it! Here are the results:



Here's the best photo I can do of the head stud hole, it looks the same as it always did, i.e. the same as all the rest but really tight for some reason, but now the first bit of the thread is chewed up, so I drilled a little of it out with a 10mm bit figuring there's no way the correct sized tap will run through it now anyway. That thing that seems to be there at the bottom doesn't stick out or anything, the length of the hole is the same as all the rest so it must be just a machining mark or something?



WHat the hell do I do next? A tap/thread chaser now seems out of the question. I've never helicoiled before, do I need to do that? Should I just carefully remove the first few chewed up threads with a drill the same size as the actual hole and then use the bottom half of the thread which I think might be OK - this is at least the 'no cost' option? Can I overbore it and retap for a bigger size stud? HELP!!!

Of course, with hindsight, what must have happened was that the moly lube was the only thing allowing the stud to go in by 4 or 5 threads at all. With the plusgas sloshing around in there, it washed away the lube as the stud went in and it locked in position. I still can't think what the original cause of the tightness was. It definitely didn't feel like a crossed or weak thread.

NUTTYNICK

The lube is there primarily for when you torque load the bolts, it reduces the shear stress between materials. For it to sieze so early on, the thread must be damaged or have swarf in there. You'll be suprised at how little you need for things to go wrong.

Avoid the drill for as long as you can, I've tried that and it still makes it very difficult to get the bolt to catch the initial thread right without crossing. You would have effectively removed the lead in thread.

The tap is still the best option you have and now potentially the cheapest. You must ensure that it is straight as you initially wind it in though.

A problem you have now, is that the tap itself might not catch the original thread correctly and try to cut a new thread, which means removing the old thread making the whole thing weak.

Helicoils might work, they are designed to have an existing thread there in the first place strong enough to use. Threadserts are what I have used before, it uses a solid insert and are ideal when you have totally lost the original thread. The hole is oversized, tapped and then the smaller insert screwed in restoring the original thread size. That would be an engineering shop job though as the kits are too expensive for one job.

As much as I hate to admit defeat, I think if the tap doesn't feel like it's going to work, then it's an engine builder/engineering shop visit as they will have the right kit. Not saying you can't do it, but the cost of the tools may be prohibitive.

NUTTYNICK

Just thought, the thread in the block, does it go right through so you can get to the other side? I don't think it does, just making sure.

You don't want any fluid in there at all, that plus gas stuff you've been using needs to come out. Only a small amount of lube is necessary.

Reason being, you don't want to create a hydraulic lock which in some cases, when the bolt is wound all the way down, it punches a hole through the other side especially on alloy blocks.

silent running

The thing is, I don't think there's any way now that the tap will catch the original thread and lead into it because the top part of the thread is scrap now. The threadsert thing you mention is I think what I've found - a kit for £60 (!) with the correct drill, helicoil insert, tools etc, so you drill down into it, tap it and insert the new coil, job done, restoring it back to the original thread size.

They are blind holes and when the other 15 studs went in, you could hear the air bubbles popping through the lube as they went down. So no way of getting through to the other side.

taffy with ppp

Buy a re-thread tool for the pitch of thread that you require, it is like a narrow hacksaw blade with grooves machined along it's length to match your pitch thread, put this down the bore so it alignes with the good thread and follow the thread backwards applying pressure to the damaged threads, this should roll your damaged threads back close to their correct position, you will be able to then see where the thread cuts the metal at the start (Top). Run a start tap down the bore, then a secondary and finally a plug tap, this has the largest diameter and should then match up with the good threads further down the bore.