Spark plug torque, Cold vs. Hot engine

[edmondthx1138]

I fly an RV-12iS, and Rotax has stated to only torque the spark plugs to 18 ft. lbs. on a COLD engine.

That's fine, but how would I replace the plugs after doing a differential pressure test? That requires the engine to be run up to operating temps for 5 minutes.

Either I've misread the documentation, or one must wait for the engine to cool down before replacing the plugs?

Any thoughts?

[gasman]

Originally Posted by edmondthx1138

I fly an RV-12iS, and Rotax has stated to only torque the spark plugs to 18 ft. lbs. on a COLD engine.

That's fine, but how would I replace the plugs after doing a differential pressure test? That requires the engine to be run up to operating temps for 5 minutes.

Either I've misread the documentation, or one must wait for the engine to cool down before replacing the plugs?

Any thoughts?

If you are doing a DPTest, then there must be something (maybe inspect the engine area) you can do as you wait for the engine to cool down...
It's worth the wait to do it right.

[bobg56]

I do my compression test cold and for the past 5 years and total of over 500 hours on engine I get on three cyl.'s 78/80 on the other one 77/80...doing cold is ok, getting motor warmed up will show slightly higher pressures if a cylinder is marginal, the 912 engines have really strong cylinders. I know mechanics who prefer to do cold tests on Lyc. and Cont. in order to see any weaknesses.

[edmondthx1138]

I guess the temperature range of cold vs hot engine needs to be understood.

[Bob Y]

The cylinder to piston clearance is so tight on the Rotax engines I was told by Lockwood that the engine would run even if the piston rings were removed. I’ve only performed one compression test on my so far, and I too get 78/80 & 79/80 for mine cold.

[azonslow]

I don't have any input on the hot vs cold torque settings but I've never found a Rotax document that says the torque is 18 ft lbs (216 in lb). The torque value in the Rotax MML iS (2023) is 142 in lb. That same 142 in lb value is shown for 912 ULS with what appears to be the new style spark plug that is used in the 912 iS and well as the 912 ULS. The old DCPR 8E spark plugs show 177 in lb in both the 2007 and 2012 MML for 912 ULS.

If my conversions are correct, 142 in lb equals 11.83 ft. lb, 177 in lb. equals 14.75 ft. lbs and 216 in lb. equals 18 ft. lbs.

[R100RS]

@azonslow - good catch, re. the numerous Rotax recommendations for spark plug torque. I have been installing DCPR 8E spark plugs to 142 in-lb in my cold 912 ULS. But, now I don't know it if is appropriate to use that value to the DCPR8E or whether it is only for the new style plug (with 177 in-lb for the DCPR8E).

Re. hot or cold, I wait until the engine is room temperature before installing plugs. I also do the differential pressure test at room temperature. I get 79/80 routinely on both of my 912 ULS engines (one with 300 hours, one with 800 hours).

[bobg56]

I use 15 ft.lbs I don't see where you could have a problem where there is none. They're not going to come loose. Make sure to use the heat paste on the threads instead of anti seize.