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Old 11-16-2015, 04:40 PM
Captain Avgas Captain Avgas is offline
 
Join Date: Oct 2005
Location: Melbourne, Australia
Posts: 1,959
Default Stick with 8.8

Quote:
Originally Posted by dbhill916 View Post
Hi all,


Is it safe to use bolts that are stronger than specified by Rotax, or will there be an unintended consequence (e.g., damage to the internal threads, etc.)?

I over torqued the metric bolts on my water pump housing and will be pulling the engine to remove & replace the affected bolts. Unfortunately, LEAF is quoting 6 weeks to get replacement M6x90 bolts. I have located a domestic source of M6x90 bolts, but they appear to be considerably stronger as they are Class 12.9, vs Class 8.8 as found on one of the ruined bolts in my possession.

A quick internet search shows that Class 8.8 has a yield strength of 660 MPa while Class 12.9's yield strength is 1100 MPa. Is it possible to do damage using bolts that are too strong? Will the torque specifications be the same? If not, is there a known conversion? If not, then I will not use them and will just wait for Christmas.

thanks in advance,
-dbh
The long answer to this requires a thesis on bolts. The short answer is to replace the 8.8 bolts with the same grade as specified by the engine manufacturer. This is not a moment for backyard intuitive engineering on your engine which is, after all, your life support system.

12.9 bolts have a high ultimate tensile strength but a low ductility (they're more brittle). If they're plated they're also much more prone to hydrogen embrittlement. That could be crucial in this instance. They also need to be torqued up to a much higher proof load than 8.8s.

In engineering, more is sometimes less. Play it safe and stick with 8.8.
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