The International Cessna^® 170 Association

Thanks for the quick response. I believe you are correct the damaged backplate would have caused flexing and possibly put more stress on the dome than it was designed for, causing the crack to start. Since you have had the prop dynamically balanced and the fit is normal, You should not have any problems with the new spinner. The one nagging question I have though, is why didn't the weld repair work? It had to be due to improper material being used or the spinner may be made of an unweldable alloy after all. You may want to consider that the repair you are having done now may give you the same results as before.

I was going to look up the part number for the spinner and ask Univair what ally they are made of but it appears their website is down. I guess it will have to wait until later.

Your links aren't working for me. I havedn't had any luck simply looking up their website online either. I do have their number, I will just call and ask about the part number you gave me.

I just spoke with Univair and they had to look up the engineering drawings (I'm sure that annoyed them since I wasn't buying anything) and they confirmed that their spinner domes and backplates for the C170. p/n U0550162 are made from 1100-o. This is pure aluminum with no alloys added and should be easily weldable. Not sure why it would re-crack unless they are using a rod that is not compatible with 1100. At least you can tell them what alloy it is so they can verify they are using the correct material.

Also, there is no need to anneal it as it does not work harden while forming and is non-heat-treatable. In other words it already is in it's softest form and will not change.

FredMa wrote:Also, there is no need to anneal it as it does not work harden while forming and is non-heat-treatable. In other words it already is in it's softest form and will not change.

Except that it will harden in the immediate vicinity of the weld.

Unless they are using incorrect materials for welding or an improper technique, there should not be hardening in the immediate area of the weld sufficient to cause a crack. 1100 is the easiest and most weldable alloy of aluminum there is. I should have stated that "It does not work harden easily". I was always taught to preheat the metal in a large area surrounding the area to be welded to minimize hardening and distribute any stresses created from the weld over a large surface area to reduce the likelyhood of cracks. I think a high school kid with about five minutes of training could weld 1100. If this shop can't, it may be time to look for another shop.

FredMa wrote:Also, there is no need to anneal it as it does not work harden while forming and is non-heat-treatable. In other words it already is in it's softest form and will not change.

I agree that 1100 it is not heat treatable.
But it is my understanding that it DOES work harden by cold working. (More slowly than the higher strength alloys.)

You are correct. I was correcting my statment apparently at the same time you were typing. Hate when that happens.

I don't know of any "FAA approved weld station" that will weld up a spinner. They have all steadfastly refused to do it, stating that welding a spinner is strictly taboo.

For that reason I had my Baron spinners welded by a non-FAA welder. That was 13 years and about 1500 hours ago and they're still on the airplane.

gahorn wrote:I don't know of any "FAA approved weld station" that will weld up a spinner. They have all steadfastly refused to do it, stating that welding a spinner is strictly taboo. ... .

I have not used K & K Precision Welding but it appears they have FAA approval for welding spinners. For $595 you can get an FAA-approved spinner from TCB Composite. They also list the mounting plate assy., p/n TCB0550162-3, but don't show a price or picture of it. They have a five year warranty. Anybody tried them?