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General Category >> Rubber Side Down! >> Torque wrench adapter - 8mm nuts on cylinder head
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Message started by Dave on 08/14/25 at 12:41:40

Title: Torque wrench adapter - 8mm nuts on cylinder head
Post by Dave on 08/14/25 at 12:41:40

It is not possible to get a socket and torque wrench on the 8mm nuts on the front and back of the cylinder (nuts under exhaust and intake ports).

In the past I have just used a box end wrench and pulled at what I thought was an appropriate amount - and I have been lucky enough that it worked and I didn't have any leaks. However - that method is likely not very accurate.......and I currently have an oil weep at the head gasket on my most recent piston/cylinder/head change.

So I have built an adapter so I can do a better job getting the nuts at the proper torque.

I bought a 13mm Husky combination wrench at Home Depot - it is important that you get a 12 point box end to provide the needed range of motion. I cut the wrench off at 74mm and welded it to a spare 3/8" drive socket that I had. You need to grind off the chrome to get a good weld.

Title: Re: Torque wrench adapter - 8mm nuts on cylinder h
Post by Dave on 08/14/25 at 12:43:05

I welded the piece of the wrench to the socket with the box end parallel to the socket....so that the torque wrench will be inline with the new adapter.

Title: Re: Torque wrench adapter - 8mm nuts on cylinder h
Post by Dave on 08/14/25 at 12:51:18

This is what the adapter looks like connected to my torque wrench.

It is necessary to make a correction for the torque value - as the wrench is now longer than it believes it is (the torque value set will be larger than the setting as the lever is not longer than it used to be).

I measured the distance from the 3/8" square drive to the center of the handle length - for my wrench the distance is 315mm. When you add the 74mm length of the adapter - the total wrench length is 389mm.

Take the 315mm value and divide it by 389, and the result is 0.81. This is the correction value....you need to multiply the desired torque value by 0.81 to get the value to be set on the wrench.

For the 8mm cylinder nuts the torque value is 16.5 - 19.5 ft/lbs. If I take 19 ft/lbs and multiply it by 0.81 the result is 15.4 ft/lbs. (In my case the torque wrench reads in Newton/Meters so if I use 27 N/M, I set the wrench at 21.5 N/M).

Prior to using it on the bike - I did a test. I placed an 8mm barrel nut in the vice and turned an 8mm bolt into the nut. I set the torque wrench 20 N/M and tightened it. I then placed the adapter on, and reduced the torque setting to 81% - which was 16 N/M. When I tried to tighten the bolt the torque wrench clicked prior to the bolt turning. I then loosened the nut and reversed the trial - with the adapter I tightened the nut to 16 N/M. I then took the adapter off and set the torque wrench to 20 N/M - and when I tried to tighten the nut the torque wrench clicked prior to the bolt moving. This proves that the 81% setting is accurate - as the wrench clicks and says the bolt is tight enough with or without the adapter being used (and the correction applied).

So......that is the end of my guessing the torque!!!!! :o