Crank Bolt, It’s Not M8 x1.25 Options

[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]

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. (IMG:style_emoticons/default/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. (IMG:style_emoticons/default/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]

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

(IMG:style_emoticons/default/agree.gif)

[Mark Henry]

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. (IMG:style_emoticons/default/shades.gif)

[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.

Type 4 stock rocker shaft mount studs and nuts. (IMG:style_emoticons/default/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]

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. (IMG:style_emoticons/default/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



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.

[Mblizzard]

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]

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

(IMG:style_emoticons/default/agree.gif)

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

[mepstein]

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

(IMG:style_emoticons/default/agree.gif)

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

(IMG:style_emoticons/default/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.