How to Properly Torque Head Bolts with Engine in Bike?

[5-7HEAVEN]

From Quenton:

http://www.motoredbikes.com/showthread.php?t=9053&highlight=bolt+torque

Thanks go-rebels and Quenton.

 

[Quenton Guenther]

Hi Everyone,

Just thought I might add a few comments to aid in solving problems concerning stripped head bolt threads in the cylinder assembly.

There are 5 different possible reasons the head boltholes could lose the threads. Reason #1 is the material used in making the cylinder. #2 concerns the operating temperature. Reason #3 is defined by the metal curing process. Reason #4 could be the direct result of incorrect torque settings. #5 could be issue with bolt quality, correct washers, and bolt size.

Problem #1, #3, and #5 can vary from one production run to another. Problem #2 can be the result of head design, and cooling area. Problem #4 is almost always the fault of the owner/mechanic/dealer.

Since we can't control the metal used in production [problem #1], we can at least alter the outcome of problems #2, #3, #4, and #5.

Several steps can be taken to lower the operating temperature of the motor [problem #2]. Don’t let the motor idle for a long period of time while stopped. Installing a copper head gasket will also lower the temperature. Adding copper washers under the 10 MM spark plug on most NE motors will drop the temperature a little more. Painting the cylinder will also reduce the head temperature.

Problem #3 appears to be isolated to late 2007, and 2008 models. My research has shown the cylinders used on the later production NE/SE motors appear to have “brittle” threads, and can easily “shear”. After consulting several metal experts, they suggested the metal wasn’t cured correctly, and the threads “shear” because of hard aluminum. If the metal is soft the threads usually round off, but if hard “all” the threads are removed with the bolt, and the hole looks like it was “reamed”. So far the cure seems to be simple. Remove the cylinder; remove the valves, springs, keepers, etc. Bake the cylinder [now is a good time to paint it with black heat paint] @ 325 degrees for 2 hours, let it cool normally [approx. 2 more hours], and the metal is re-cured. The experts informed me, that it works better on motors with low time. The motors in question came with the aluminum intake manifold from the factory.

Problem #4 is caused by tightening the head bolts when the motor is “hot”, or by not using a torque wrench, or using the wrong torque settings. Never torque a “hot” motor, and always use the correct settings.

Problem #5 is repairable by removing the in-correct washers, and replacing any suspect bolts or washers. If the head is milled, make sure the bolts are shortened and don’t bottom out.

Hope this helps,

 

[go-rebels]

There is a 6th major reason and a 7th and 8th minor reason to consider:

Problem #6: Contamination in the threads, be it hard dirt (sand), burnt carbon, rust or metal shavings from either the male or female material can easily sieze the thread interface resulting in galling (microscopic welding) and subsequent thread strippage and failure. You MUST clean the threads before reassembly to minimize contamination. I take special care cleaning out the blind tapped holes.

Problem #7: A too tight of a thread fit resulting in a near interference fit can result in galling and thread strippage. I can't believe how tight the fit is between the 8mm hex bolts and the tapped holes in the aluminum cylinder. I can hardly hand turn any bolt in any hole to reach the bottom. I slightly damaged the threads of one 8mm hex bolt removing it, and when I chased the threads on the bolt with a new metric tap, I had a difficult time running the tap throughout the length of the thread. Maybe the bolts are a little oversize, but there is no engineering reason for this. Probably just poor quality control in the Chinese bolt factory.

Problem #8: Never, never substitute a common stainless hex bolt for a high grade steel bolt. I've seen this often as some people think that stainless is "better" as it's certainly more expensive and rust resistant. Some custom high strength stainless steel fasteners exist from the 17-4, 15-5 and 13-8 precipitation hardening stainless steels (and other propriatory metals) but these are significantly more difficult to obtain and have a significant major downside: they all have a high propensity to gall and sieze when mated with any other metal, including aluminum. A low-strength Home Depot variety bolt will significantly yield (stretch) when applying Grade-8 specified torques and consequently sieze.