Crankcase thread blunder - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 14 (permalink) Old 07-30-2016, 02:11 PM Thread Starter
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Crankcase thread blunder

I've just made a big error. I'm starting the engine rebuild on my basket case 1968 T120. The threaded holes in the crankcases for the primary cover, timing cover and gearbox outer cover screws were full of hard rubbish that wouldn't come out with vapour blasting. So I thought the best thing to do was run a tap down each one to clean them out properly. I picked up my 1/4 inch tap and ran it down each hole. I then tried one of the cover screws in each one - disaster - the screws threaded in ok but they wobbled a bit. So I checked the tap and discovered I'd accidentally put a Cycle thread 1/4 inch tap into my BSF tap set. So now I have Cycle thread holes and BSF screws. When I put the timing cover onto the crankcase and tighten down the screws, they tighten fully and hold tightly without any wobbling and the cover is secure. So do you think it will be ok? I was think that if I put some medium-strength threadlock on each screw I might get away with it. Any opinions and advice gratefully accepted.
Thanks
Tony
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post #2 of 14 (permalink) Old 07-30-2016, 02:45 PM
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Next time you clean out screw threads you WILL use a brush used for cleaning the barrels of firearms to clean threads and NOT a tap because taps are way too aggressive on threads. And all of us here at this forum were beginners at one time or another. 8)
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post #3 of 14 (permalink) Old 07-30-2016, 03:21 PM
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Hi Tony, It's easy to get the wrong tap. Just happens.

Cleaning the threads with a tap is often the only way to get them clean as the gunk inside can harden so much.

Now that the threads are damaged you have to do some repair. Don't try to use as is, in the long run they will not hold.

I've repaired hundreds of threads. It's easy with some practice. If this is your first attempt I recommend you practice on a spare piece if at all possible. In the USA helicoil is the repair kit of choice. Google it. Look at YouTube for tutorials. Do not use a timesert type sleeve. Helicoil type repair is the very best in this situation. There are other brands of "Helicoil" that work fine, I just like the genuine brand best.

Maybe the hardest part is drilling the old threads out. We want the new threads to be straight so the screw doesn't go in at an angle. The easiest way to insure that is to remove the old threads by stripping them out. Use a bolt with a hex head the original size. Stack a pile of flat washers under head such that the bolt grabs only 3-4 threads. Tighten bolt & strip out those threads. Then add more washers & strip more out until they are all gone. Be sure to blow the chips out as you go.

Finally drill the holes with the drill size specified with the repair kit. Since you removed the old threads the drill will follow the hole very nicely. If you leave the old threads sometimes the drill will want to follow the side of the old thread & the hole will be in a different place so to speak, or crooked.

Lube the special tap with oil & try to start straight as you can. Back tap out & clean chips as you go. Clean hole well before installing coil.

The coils come in different lengths. Look at how deep the new threads are & see what you need. Probably the short or next longer is good. Now this is important... The end of the helicoil must be at least 1 full thread below the gasket surface. 2 threads is fine also. The new coil will be substantially stronger that the original thread so the coil doesn't need to be the full depth of hole.

Expect to spend several hours. This is a lot of work. That's ok since were not punching a time clock.

Don
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post #4 of 14 (permalink) Old 07-30-2016, 03:23 PM
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If they hold I might be tempted to leave be. However, to get rid of that niggling voice in my head I would Helicoil them. For what it's worth I nearly always clean threads with the correct Tap.

Rod
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post #5 of 14 (permalink) Old 07-30-2016, 05:17 PM
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What if you change your bolts to 1/4" Cycle?
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post #6 of 14 (permalink) Old 07-30-2016, 06:01 PM
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Quote:
Originally Posted by tjkoko View Post
Next time you clean out screw threads you WILL use a brush used for cleaning the barrels of firearms to clean threads and NOT a tap because taps are way too aggressive on threads. And all of us here at this forum were beginners at one time or another. 8)
No sir. the correct pitch and diameter tap is not too aggressive. A rifle brush may get most of the threads clean but not as well as chasing the threads with a thread chaser or correct tap. It will loosen built up dirt or sludge in the hole, and also restore the pitch if it has been pulled, or galled in any way.

This beginner has been in the industry as a mechanic since 1977. Use the correct pitch and diameter of tap. IF the threads are too loose or unserviceable, using the correct tap did not bring it to that state.

I hesitate from posting much at this time, as I am here to learn about this engine platform and bike. But unless we are sure of our advice, we risk damage to other's machine's not just our own.
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Last edited by Dadrider; 07-30-2016 at 06:04 PM.
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post #7 of 14 (permalink) Old 07-31-2016, 12:57 AM
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If you used 1/4 cycle bolts you'd be in the same place. I expect they'd actually strip easier since the pitch is finer. A course thread tends to hold better in soft materials.

Regarding bent or damaged threads, there is a "thread chaser" tool. It's specifically made for "straightening" bent/damaged threads. It looks similar to a tap, but does not have sharp cutting edge like a tap. The idea is the shape of the tool's edges "pushes/forms" the metal back into position. It is not intended to remove metal but push the metal back to where it should be.

A chaser cannot put missing metal back if the thread is fractured or pieces of thread broken off/missing. We use chasers all the time at work. But the thread is not a strong or perfect as it once was. The more damage it had the weaker it will be.

Still for threads that are not too badly damaged chasers work quite well in steel or other metals that doesn't fracture too easy. However on aluminum, especially cast aluminum the metal fractures when you attempt to chase the threads. That leaves a very weak thread, which often will need repair with a helicoil or the like.

Remember a tap actually cuts the metal out leaving the thread form. What happens if you double cut a thread such as using the wrong tap is you end up with a bunch of "pointed" metal bits that resemble a thread, but are quite weak. If the screw is long enough it may hold, but I bet they would not properly hold the primary cover over time without stripping. Then they will be much harder to repair with motor in frame.
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post #8 of 14 (permalink) Old 07-31-2016, 04:54 AM
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Sometimes I despair of some contributors to this Forum. Guys, if you don't know, Look It Up. Certainly do not post such complete and unmitigated drivel.

Tony:-

You have not done any "damage" to your engine case threads - just at 1/4" major diameter, Cycle and BSF are exactly the same 26 tpi pitch. The only - tiny! - difference is each Cycle thread has a 60-degree included angle while BSF is 55 degrees.

In a brand-new, perfect thread cut with brand-new, perfect tools, you might've removed a microscopic sliver of metal from the bottom of each thread trough. In this real world, even if the threads started out brand-new perfect, they're now 48 years old, aluminium alloy is relatively soft so it wears faster than steel anyway, not to mention those threads have been subjected to 48 years of owners and mechanics (not to mention 'mekaniks') with vastly-different levels of ability and mechanical sympathy.

Quote:
Originally Posted by tjwood View Post
When I put the timing cover onto the crankcase and tighten down the screws, they tighten fully and hold tightly without any wobbling and the cover is secure.
You've done exactly what I did to my basket-case T100 engine case threads over a decade ago (although I knew I was using a Cycle tap), I secured it with Cycle-thread stainless Allen screws ('cos 1/4"BSF stainless Allen screws don't exist) and I haven't had any problems whatsoever.

One caveat: after forty-eight years, to paraphrase George Orwell, not all screw threads will be equal; I haven't found any need to use "thread locker", or any other gunge; in actual use, you might find one or more threads on your engine need help with gunge or, as Rod's suggested, a helicoil, but that'll be as a result of wear and/or lack of mechanical sympathy, not the use of a 1/4"Cycle tap in a 1/4"BSF thread.

One other tip, if buying new screws. Meriden never threaded blind holes to the bottom; using a hand-tap, you almost-certainly have; if you check, you will find that many of the holes will now accept the next-longer screw without bottoming in the hole; a longer screw will have more thread to secure it, and it'll use the newly-cut thread.

Hth.

Regards,

Stuart

Last edited by StuartMac; 07-31-2016 at 05:00 AM.
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post #9 of 14 (permalink) Old 08-01-2016, 01:39 AM
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Hi Stuart, Of course you are correct. Sorry for the post. I was thinking it had the later 20 tpi screws. Recently was involved with a 68 Bonnie & seem to recall it had 1/4-20 tpi primary cover screws. For sure the clutch hub was the later size as I had to use my later puller with the 1"-20 thread. Maybe it had a later case??
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post #10 of 14 (permalink) Old 08-01-2016, 10:34 AM
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Gentlemen. I am planning to buy a 73 Tiger this summer / fall. Does the Service manual list, or is there a guide that gives me a true inventory of bolt threads / sizes on the bike? What type bolt chaser / taps to I need to gather for taking care of the bike?
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