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Mblizzard
This not a stock crank. The stock is M8 x1.25. I am sure this is some variation of the AA 78 mm crank but the bolt into nose of crank is clearly not stock.

As near as I can tell it is a M7 1.25. 1.0 is too coarse to start. Anyone know for sure what the threads are on these cranks and where to get the bolt?

Type 4 store online has the stock bolt.
Mark Henry
It should be M8, I'd try chasing it with a tap or a stiff wire galley brush. Likely the tolerances tightened up during heat treating, dirt, or congealed storage coating.

Message the seller, if it's LN they'll likely check one to confirm.
Mblizzard
QUOTE(Mark Henry @ Aug 18 2019, 06:21 AM) *

It should be M8, I'd try chasing it with a tap or a stiff wire galley brush. Likely the tolerances tightened up during heat treating, dirt, or congealed storage coating.

Message the seller, if it's LN they'll likely check one to confirm.


Tired all that. M8 tap will not start. LN sent new bolts as they think it is M8. But I pulled my old broken crank from scrap pile and it is not M8 either. Yea should have kept that bolt but I always use new crank and fly wheel bolts. headbang.gif

Have 1.8 crank that M8 starts fine and even after inspecting the new crank with borescope I can’t see anything that would prevent bolt from starting.

Will check with drill bit and tap in one combo to see just how far off it is.
Superhawk996
Useful trivia on ISO metric fasteners and trying to figure out what you might have if unknown.

M6 x 1.00 fastener will be drilled with a 5mm drill then tapped.
M8 x 1.25 fastener will be drilled with a 6.75mm drill then tapped
M8 x 1.5 fastener will be drilled with a 6.5mm drill then tapped
M10 x 1.5 fastener will be drilled with a 8.5mm drill then tapped.

See where this is going?

You can gauge a metric hole (approximately) by the size of drill rod that will just barely fit into the tapped hole. From there you can roughly infer the bolt size and pitch. At which point I'd find that tap and see if it runs in easily. If not, don't force the tap unless you're sure you have the right one or you'll have a complete mess on your hands.

Highly doubtful you have a M7 x any pitch fastener. That would be a very uncommon size for automotive use. If you really think that is the case, check with the crank supplier.

Don't get me started on British machinery and Whitworth fasteners. chair.gif

Digital calipers and a thread pitch gauge are a must for working on old stuff in my opinion.
mepstein
My Moto Morini motorcycle tank is held on with M7 bolts but I figured it was an Italian thing. I’ve never seen them in a German car.
HAM Inc
Any chance it was improperly tapped to a U.S. size?
draganc
QUOTE(HAM Inc @ Aug 18 2019, 03:57 PM) *

Any chance it was improperly tapped to a U.S. size?


agree.gif
Mark Henry
QUOTE(mepstein @ Aug 18 2019, 05:36 PM) *

My Moto Morini motorcycle tank is held on with M7 bolts but I figured it was an Italian thing. I’ve never seen them in a German car.


Type 4 stock rocker shaft mount studs and nuts. shades.gif
mepstein
QUOTE(Mark Henry @ Aug 19 2019, 09:44 AM) *

QUOTE(mepstein @ Aug 18 2019, 05:36 PM) *

My Moto Morini motorcycle tank is held on with M7 bolts but I figured it was an Italian thing. I’ve never seen them in a German car.


Type 4 stock rocker shaft mount studs and nuts. shades.gif

I only know 1% of this stuff so I’m not surprised I missed that.
I guess that’s why those nuts are hard to find.

Mike, I hope you eventually catch a break with this build.
Mblizzard
QUOTE(Superhawk996 @ Aug 18 2019, 11:02 AM) *

Useful trivia on ISO metric fasteners and trying to figure out what you might have if unknown.

M6 x 1.00 fastener will be drilled with a 5mm drill then tapped.
M8 x 1.25 fastener will be drilled with a 6.75mm drill then tapped
M8 x 1.5 fastener will be drilled with a 6.5mm drill then tapped
M10 x 1.5 fastener will be drilled with a 8.5mm drill then tapped.

See where this is going?

You can gauge a metric hole (approximately) by the size of drill rod that will just barely fit into the tapped hole. From there you can roughly infer the bolt size and pitch. At which point I'd find that tap and see if it runs in easily. If not, don't force the tap unless you're sure you have the right one or you'll have a complete mess on your hands.

Highly doubtful you have a M7 x any pitch fastener. That would be a very uncommon size for automotive use. If you really think that is the case, check with the crank supplier.

Don't get me started on British machinery and Whitworth fasteners. chair.gif

Digital calipers and a thread pitch gauge are a must for working on old stuff in my opinion.


OK so did a lot of checking. 6.5mm drill will fit into current hole with a little movement. Not much. OD of M8 bolt 7.90mm tap 7.99mm as best I can measure.

There is a clear point (slightly shinny where bolt has rubbed) where a M8 bolt hits a lip in the crank. See below

Click to view attachment

Hard to see but there is a clear difference in the size of an M8 compared to the threads in the crank. The lines exaggerate that difference somewhat but it can seen with the eye.

Best I can tell the crank has a number 780B 180435 that brings up no results when searched.

Click to view attachment
Mblizzard
QUOTE(HAM Inc @ Aug 18 2019, 02:57 PM) *

Any chance it was improperly tapped to a U.S. size?


It almost seems that way.
mb911
Maybe a 5/16 sae bolt as they are Very similar to 8mm in size.
Superhawk996
If you can just barely get a 6.5mm drill in there I’d suspect m8x1.5 pitch. Try that tap and if it doesn’t just glide right on in, I think the others may be right check for SAE threads.
Mblizzard
Went to 2 bolt specialty stores today. Neither one had any metric or SAE bolt that would start in the hole.

I think I need a new crank. What a Mongolian cluster f#$k this has been.
mb911
That is terrible.. How is that even possible??
Mark Henry
Take it to a machine shop they can tell you what the size is.
Superhawk996
QUOTE(Mark Henry @ Aug 19 2019, 07:56 PM) *

Take it to a machine shop they can tell you what the size is.


agree.gif

Machine shop could also heli-coil it to whatever you want. That crank nose fastener is not highly stressed.
mepstein
QUOTE(Superhawk996 @ Aug 19 2019, 08:36 PM) *

QUOTE(Mark Henry @ Aug 19 2019, 07:56 PM) *

Take it to a machine shop they can tell you what the size is.


agree.gif

Machine shop could also heli-coil it to whatever you want. That crank nose fastener is not highly stressed.

agree.gif any machine shop could make it perfect for you with the right type of insert
HAM Inc
Drill and tap it for a M10 Bolt.
porschetub
I might be a bit thick but you are talking about the fan hub bolt right?? if so just step up drill size to get to 8mm root diameter of 6.75 and tap to 8 x 1.25 no issue then.
Sharp drills and good taps are the key points here.
Mike read my new post on AA performance parts,good luck mate.
Superhawk996
QUOTE(porschetub @ Aug 20 2019, 02:53 AM) *

I might be a bit thick but you are talking about the fan hub bolt right?? if so just step up drill size to get to 8mm root diameter of 6.75 and tap to 8 x 1.25 no issue then.
Sharp drills and good taps are the key points here.
Mike read my new post on AA performance parts,good luck mate.


You're not thick but the details are in the math related to the thread forms.

The step up in drill size between 6.5mm and 6.75mm is only 0.25mm which is approximately 0.010" (i.e. not much). The previous threads would have been approximately 0.75mm from thread root to top of thread. So ideally you would need to step up to a fastener with a thread root of at least 8mm to clear the previous threads (i.e. M10 x 2.0, M10 x. 1.5, or M10x1.0 would all fit the bill).

The risk is that if the fastener was M8x1.5 the step up in root size to 6.75mm won't clear all the previous M8 threads. There would then be potential for intersections in thread forms between the 1.25 pitch and the 1.5 pitch. It would look nasty, be prone to cross threading and would be lot weaker than a proper heli-coil repair or tapping fresh to M10.

All theoretical since we don't actually know what this was drilled for.

I'm with Len, have the machine shop drill and tap it for M10.
Mblizzard
With lots of good advice from everyone and working with the Type 4 Store (they have been awesome) we decided to do a new crank just to be sure.
Mark Henry
agree.gif Since it's new from T4store it's best to send it back and let them sort it.
Mblizzard
This is what it is suppose to do.

Click to view attachment
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