O320, 160 hp engine. Rv7a, the bulk head has a crack which is the same diameter as the prop hub. A friend who is an a&p said it was due to harmonic vibration and has seen it before. I have a Catto 2 blade fixd pitch but the bulk head is from Vans. Plan on ordering a new Bulk head from Vans. 2 questions: is the bulkhead a standard 13 inch(iI guess I should measure it) and has anyone seen this happen
Ohooooo yea.... Very common. Spinner bulkhead cracks are common. I can't picture your exact crack description.
This bulkhead is just sheet aluminum that was formed soft and heat treated (I am guessing that is Van's process). I don't remember if it's 6061 or 2024 but forming causes residual stress unless heat treated will crack. It is possible Van did not HT properly or at all? Without sending the metal to the lab you don't know.
Sheet metal flex will obviously fatigue and cracks given enough cycles. You did not say how many hours.
BURR - Sharp or rough edges is a place for a crack to start. Deburr and chamfer and polish the edges and make sure there are NO scratches or gouges. Think about the washer and bolt that holds it down. Did they leave marks on the old bulkhead?
The spinner it self may not be balanced. It's just fiberglass with wet layup. If some part of spinner is resin rich or has extra ply overlap on one side, it is not balanced. How to statically balance a spinner? Don't know but it may be worth some effort to make sure it is statically balanced. At least make sure the thickness is even and selectively sand removing some material or add material (primer). The spinner thickness should be even.
I never installed a fixed prop spinner but you have two bulkheads? One front and one back?
The spinner may have been mounted slightly cockeyed. Not enough to notice but enough to cause rotational forces or vibrations.
Clearly prop balance is important.
Engine mounts get tired or were never good to start with. I am a big fanboy of LORD engine mounts with the inner deflection damper (silicon filled spacer). The price on these has gone stupid high... but they are still worth it.
Preload - The spinner should slide on and screw holes line up with out pushing on bulkhead. Pre-load is a no no.
If you get it perfect you should not crack. However perfect is not possible. Keep an eye on it. You can't see bulkheads with cowl on easily at least. However every oil changes a good inspection is important. OIL CHANGE is not oil and filter. When ever anything is opened up visually inspect everything with strong light and even magnification in some cases and look for cracks, not just spinner the whole firewall forward area. Cracks start small and grow. It does not happen in one flight.