Crank Bolt, It’s Not M8 x1.25 |
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Crank Bolt, It’s Not M8 x1.25 |
Mblizzard |
Aug 18 2019, 06:28 AM
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#1
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Advanced Member Group: Members Posts: 3,033 Joined: 28-January 13 From: Knoxville Tn Member No.: 15,438 Region Association: South East States |
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 |
Aug 18 2019, 08:21 AM
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#2
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that's what I do! Group: Members Posts: 20,065 Joined: 27-December 02 From: Port Hope, Ontario Member No.: 26 Region Association: Canada |
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 |
Aug 18 2019, 09:25 AM
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#3
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Advanced Member Group: Members Posts: 3,033 Joined: 28-January 13 From: Knoxville Tn Member No.: 15,438 Region Association: South East States |
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 |
Aug 18 2019, 01:02 PM
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#4
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914 Guru Group: Members Posts: 5,903 Joined: 25-August 18 From: Woods of N. Idaho Member No.: 22,428 Region Association: Galt's Gulch |
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 |
Aug 18 2019, 03:36 PM
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#5
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914-6 GT in waiting Group: Members Posts: 19,315 Joined: 19-September 09 From: Landenberg, PA/Wilmington, DE Member No.: 10,825 Region Association: MidAtlantic Region |
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.
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HAM Inc |
Aug 18 2019, 04:57 PM
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#6
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Senior Member Group: Members Posts: 846 Joined: 24-July 06 From: Watkinsville,GA Member No.: 6,499 Region Association: None |
Any chance it was improperly tapped to a U.S. size?
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draganc |
Aug 18 2019, 10:51 PM
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#7
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Senior Member Group: Members Posts: 725 Joined: 2-November 09 From: central new jersey Member No.: 11,000 Region Association: North East States |
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Mark Henry |
Aug 19 2019, 07:44 AM
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#8
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that's what I do! Group: Members Posts: 20,065 Joined: 27-December 02 From: Port Hope, Ontario Member No.: 26 Region Association: Canada |
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 |
Aug 19 2019, 08:08 AM
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#9
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914-6 GT in waiting Group: Members Posts: 19,315 Joined: 19-September 09 From: Landenberg, PA/Wilmington, DE Member No.: 10,825 Region Association: MidAtlantic Region |
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 |
Aug 19 2019, 09:27 AM
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#10
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Advanced Member Group: Members Posts: 3,033 Joined: 28-January 13 From: Knoxville Tn Member No.: 15,438 Region Association: South East States |
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 |
Aug 19 2019, 09:30 AM
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#11
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Advanced Member Group: Members Posts: 3,033 Joined: 28-January 13 From: Knoxville Tn Member No.: 15,438 Region Association: South East States |
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mb911 |
Aug 19 2019, 10:25 AM
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#12
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914 Guru Group: Members Posts: 6,883 Joined: 2-January 09 From: Burlington wi Member No.: 9,892 Region Association: Upper MidWest |
Maybe a 5/16 sae bolt as they are Very similar to 8mm in size.
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Superhawk996 |
Aug 19 2019, 12:58 PM
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#13
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914 Guru Group: Members Posts: 5,903 Joined: 25-August 18 From: Woods of N. Idaho Member No.: 22,428 Region Association: Galt's Gulch |
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.
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Mblizzard |
Aug 19 2019, 04:20 PM
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#14
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Advanced Member Group: Members Posts: 3,033 Joined: 28-January 13 From: Knoxville Tn Member No.: 15,438 Region Association: South East States |
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 |
Aug 19 2019, 04:57 PM
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#15
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914 Guru Group: Members Posts: 6,883 Joined: 2-January 09 From: Burlington wi Member No.: 9,892 Region Association: Upper MidWest |
That is terrible.. How is that even possible??
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Mark Henry |
Aug 19 2019, 05:56 PM
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#16
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that's what I do! Group: Members Posts: 20,065 Joined: 27-December 02 From: Port Hope, Ontario Member No.: 26 Region Association: Canada |
Take it to a machine shop they can tell you what the size is.
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Superhawk996 |
Aug 19 2019, 06:36 PM
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#17
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914 Guru Group: Members Posts: 5,903 Joined: 25-August 18 From: Woods of N. Idaho Member No.: 22,428 Region Association: Galt's Gulch |
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 |
Aug 19 2019, 07:26 PM
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#18
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914-6 GT in waiting Group: Members Posts: 19,315 Joined: 19-September 09 From: Landenberg, PA/Wilmington, DE Member No.: 10,825 Region Association: MidAtlantic Region |
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 |
Aug 19 2019, 08:03 PM
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#19
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Senior Member Group: Members Posts: 846 Joined: 24-July 06 From: Watkinsville,GA Member No.: 6,499 Region Association: None |
Drill and tap it for a M10 Bolt.
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porschetub |
Aug 20 2019, 12:53 AM
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#20
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Advanced Member Group: Members Posts: 4,706 Joined: 25-July 15 From: New Zealand Member No.: 18,995 Region Association: None |
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. |
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