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Cylinder head nut torque


FredG

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Below are three separate cylinder head torque recommendations from Rotax, with the identification of the source document and the document date.

 

From ROTAX SI-012-019-R1 (October 3, 2007): “torque collar cap nuts and hex. nuts gradually in a cross torque pattern to 22 Nm (195in.lb) and remove 
cylinder aligning tool”

 

From ROTAX Heavy Maintenance Revision Pages (October 01, 2010, Rev 1, Ed. 3) “Screw on collar cap nut and flanged nut (11) with washer (12) and tighten cylinder heads in the order shown in Fig.72-108. Tighten to 10 Nm (90 in.lb) and then in addition tighten further by applying a 180° rotation. Remove 4 allen screws (9) and cylinder aligning tool.”

 

From ROTAX ASB-912-062UL R1 (March 5, 2013): “At tightening the cylinder head with 2 collar cap nuts M8 and 2 hex. nuts M8 pay attention to the changed tightening torque. Tighten to 10 Nm (90 in.lb) and then in addition tighten further by applying a 120° rotation.”

 

Note that the March 5, 2013, ASB states "pay attention to the changed tightening torque".

 

So, is the 2013 torque the right one? Any thoughts about why there are three distinct torque recommendations for the 912 engine?

 

Thanks.

Fred

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Here is an interesting article on the benefits of Torque Turn to Tighten as a way to ensure uniform clamping pressure in critical joints.

http://www.enginebuildermag.com/Item/2343/threaded_fasteners_torquetoyield_and_torquetoangle%20.aspx

Here is a discussion of various ways to achieve precision in bolt tightening.

http://www.boltscience.com/pages/tighten.htm

After reading these, one may be more inclined to use new bolts in critical assemblies where one does not want to take any chances that the elasticity is already gone in a used bolt. The manufacturer should know and tell us if reusing critical bolts is OK.

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Jim, the first article is very illuminating. As you know, many engine builders use new con-rod bots with every rebuild, presumably because of changes in the length-tension relationship resulting from repeated tension-relaxation (assembly-disassembly) cycles. It would be nice to know if new 912 head studs are desirable after a certain number of assessembly-disassembly cycles.

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Fred,

 

From near the end of the first reference. No idea if this is definitive.

 

"A well-respected OE engineer specializing in engines tells me that critical fasteners have about six rundowns in their useful life. They use four of those at the OE manufacturing operations, leaving rebuilders just two. One rundown for checking sizes puts us on the last rundown during final assembly. My thinking is: why take the chance? Replace the fasteners! The relative cost compared to the total engine job is small and the peace of mind is high."

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At my IA renewal meeting this year one of the speakers was from an engine shop. He said Continental started requiring that all fasteners and studs be replaced at overhaul. He also said Lycoming does not require this. He couldn't understand why they were going this far as they didn't think it was needed. That was until the had a couple major failures on Lycoming engines. They started sending stuff to labs to get things checked out. They found the old hardware was not allowing for the part to be torqued to the correct omount. In some cases it was 25% to 33% less than what it should be. Now they replace all the hardware on both brands of engines.

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