Prop, Metal to Wood

Your current bulkheads (front and rear) will work. The Univair kit includes bulkheads so if you want shiny new ones, go with the kit. If you are happy with the bulkheads you have, all you need is a spinner dome which will cost less than the full kit. Also, for the wood prop (which has a thicker hub) you do not use the spacer rings or the spacers that go on the bolts. That “spacer sandwich” is there on a metal prop installation to make the stack thickness of the metal prop the same as the wood prop is without spacers and rings.

You will need to trim the front bulkhead where it will conflict with the bulk of the wood prop – the thinner metal prop does not bump into the flange of the front bulkhead but the wood prop tries to occupy the same space, not good. It is a trim to fit type job.

Your current spinner dome is made from the same part as the wood prop spinner dome. They both start with the same uncut part. The cutouts for the prop blades are smaller in a metal prop spinner (due to the slimmer profile of the metal prop). The wood prop spinner is the same basic dome but the cutouts for the blades are larger. So, if you want to spend some time doing careful cutting work you can enlarge the blade cutouts in your metal spinner dome and make it into a wood prop spinner dome. Or, buy the part already cut from Univair.

Your current prop bolts will work with a wood prop — remember I mentioned that the “spacer sandwich” on the metal prop makes it the same stack height as the wood prop? Stinson did that so they could use the same bolts, bulkheads and spinner dome (except for cutout size) in both installations. As long as your bolts are in good useable condition they will work for either type prop.

The final thing you will need is a “moisture plate”. Stinson/Univair calls it a moisture plate but most wood prop installations call it a crush plate. It is a heavy metal plate, round, and drilled for the bolt pattern of the prop bolts. A wood prop is only clamped into position by the prop bolts. The transmission of power from the crankshaft to the prop is done by friction between the prop flange and the prop (and to a lesser degree friction on the front face of the prop where the moisture plate is gripping hard against it). Your prop bolts bear down on the plate and the plate firmly grips the prop hub in a clamping action between the prop hub on the back and the crush plate on the front. Bolt torque is therefore very important on a wood prop installation – if it is too loose the prop will work against the face of the flange and the resulting frictional heat can acutally burn your prop and can also ovalize the bolt holes in the wood as the prop works on the flange. If it is too tight the clamping forces will crush and break the wood grain and damage your prop. That is why wood props should be retorqued seasonally – wood expands and contracts with humidity so you should adjust the clamping force for conditions in order to avoid the prop being too loose or too tight in the sandwich.

You can buy a new moisture plate from Univair or maybe get a used one from someone on this list. DO NOT be tempted to use one of your “spacer sandwich” rings as a plate. I have seen such an installation. The ring does not have enough surface area or strength (it is thinner than a moisture plate as well as covering less area) to do the job well. Also, do not be worried that our plates are thinner than other plane’s plates (ours are 1/4 inch, most are 3/8 or even 7/16 inch thick aluminum plate) because our engine’s prop flange is much larger than other brands precisely so it will grip a wood prop with a large surface area. Also, we have eight bolts spreading the force around. Most wood prop installations have a smaller diameeter flange and only six bolts providing the force so they need a thicker plate to evenly spread the force.