[hotbrat]

I have a couple head bolt holes that will not accept bolts

I can tap them easily with a taper tap or bottom tap with ease

when I tried to run a head bolt on it would not thread

i went 2 the store and got another new bolt and tried that today. It would not thread on.

Then I tried a chaser bolt but that would not thread on either

Does anyone have any ideas on how to get a bolt to thread on if it does not thread? It seemed to thread on the first try but it jumped off the threads

[dgcantrellsr]

what are you working on? Standard or metric? A little more info might help.

Originally Posted by hotbrat

I have a couple head bolt holes that will not accept bolts

I can tap them easily with a taper tap or bottom tap with ease

when I tried to run a head bolt on it would not thread

i went 2 the store and got another new bolt and tried that today. It would not thread on.

Then I tried a chaser bolt but that would not thread on either

Does anyone have any ideas on how to get a bolt to thread on if it does not thread? It seemed to thread on the first try but it jumped off the threads

[hotbrat]

metric

size m12 x 1.75 thread

[dgcantrellsr]

Are the taps a good brand or maybe a cheap set from Harbor Freight?
If its a good brand tap and the bolt has the correct thread I see noway you should be having a problem.

Originally Posted by hotbrat

metric

size m12 x 1.75 thread

[302 Z28]

What are you working on. I know that some English cars had a weird thread on them, it wasn't true metric and it wasn't US.

Vince

[MARTINSR]

Yeah, I think it was called "witworth"?

Brian

[302 Z28]

Originally Posted by MARTINSR

Yeah, I think it was called "witworth"?

Brian

That's it, thanks Brian

Vince

[hotbrat]

Originally Posted by dgcantrellsr

Are the taps a good brand or maybe a cheap set from Harbor Freight?
If its a good brand tap and the bolt has the correct thread I see noway you should be having a problem.

Tap is high grade tap. not from HF; Tap is a bottoming tap marked JAPAN on it. I got it from a bolt specialty shop

[heyjude076]

First off - take the bolt and lay it along the tap. Do the threads match? If not there is the answer.

Second - you said it tried for a few turns but then popped out. This sounds like it tried to cross thread. Take a sacrificial bolt and taper the end on a grinder. Clean up the threads with a thread file and then try it. If that works you may have to chamfer the very top threads of the hole or bolt.

Third - you say a head bolt. Are you trying to screw the bolt into the bare block? If you are trying with the head and gasket on maybe the holes are not lining up.

[pepi]

You might try using a nut to get an line on the bolt size and verify that tap is correct... could it be the bolt is to short ...?

[bobjob]

it sounds like you are trying to go thru head,,gasket,, then into block,, like other posts said.... shank of bolt is same size of outer dim. of thread,, tap has a lead in on front,, and shank is smaller than min. size of thread... so it would fit even if you are a little off....

[oldred]

Originally Posted by heyjude076

Clean up the threads with a thread file and then try it.

A much better and easier way to clean up threads from tapering a bolt, of if you have to shorten a bolt by sawing or grinding, is to thread a nut (all the way to the shank) onto the bolt first. Then after grinding the threads, either to taper the bolt or to just taper the first thread or two after sawing it off, remove the nut and it will clean up the threads as it comes off. Doing this is much easier than using a thread file and leaves the threads perfectly aligned and clean so it will start straight every time.

[hotbrat]

Originally Posted by heyjude076

First off - take the bolt and lay it along the tap. Do the threads match? If not there is the answer.

Second - you said it tried for a few turns but then popped out. This sounds like it tried to cross thread. Take a sacrificial bolt and taper the end on a grinder. Clean up the threads with a thread file and then try it. If that works you may have to chamfer the very top threads of the hole or bolt.

Third - you say a head bolt. Are you trying to screw the bolt into the bare block? If you are trying with the head and gasket on maybe the holes are not lining up.

lined up 3 different bolts with the TAP, they line up perfectly
the tap runs through the threads with ease

I took a bolt and grinded 3 sides making a thread chaser, it does not thread on period.

The other 2 bolts do not thread on. All 3 bolts are the same size and thread pitch.

both the bottoming tap and plug tape or taper tap thread on perfectly

I am going to try again with a tapered bolt according to the last response. I do not think its going to work.

I almost think the first thread is not large enough to allow the bolt to thread in

[heyjude076]

Originally Posted by hotbrat

lined up 3 different bolts with the TAP, they line up perfectly
the tap runs through the threads with ease

I took a bolt and grinded 3 sides making a thread chaser, it does not thread on period.

The other 2 bolts do not thread on. All 3 bolts are the same size and thread pitch.

both the bottoming tap and plug tape or taper tap thread on perfectly

Not being a smart ***** but this not make sense. Are you sure the tap and bolts are both M12X1.75? I don't have my book here but it may be possible that the bolts may be M13 or M14 with a 1.75 pitch. I suggest you get a nut that is M12X1.75. Try the bolts first and make sure the nut threads on. If it does then try the tap. It should go through the nut with minimal resistance. A lot of taps may cut a little material. That is why thread chasers are used rather taps when cleaning bolt holes.

Originally Posted by hotbrat

I am going to try again with a tapered bolt according to the last response. I do not think its going to work.

I almost think the first thread is not large enough to allow the bolt to thread in

If the above suggestion with the nut verifies that both the bolts and tap are M12X1.75 you may have to chamfer the first few threads of the bolt hole. A lot of times if the surface is flat it is very difficult to catch the first few threads of the hole.

[hotbrat]

Originally Posted by heyjude076

If the above suggestion with the nut verifies that both the bolts and tap are M12X1.75 you may have to chamfer the first few threads of the bolt hole. A lot of times if the surface is flat it is very difficult to catch the first few threads of the hole.

I was thinking about this today. If I hammer on a tapered bolt would that work to chamfer up the first threads