Hi Dave.
I am hoping you can help me out. I picked up a used 91AC and have a couple of questions.
First of all are the screws on the valve cover. Do you know what size they are? The one on the front is only long enough to screw in about three threads. ANd these threads are striped. Under them there is plenty more thread. I just need a longer screw. Our LHS in 20 miles away so l want to see if he has the screw in stock before l go out there. In the manual it only lists the part # and not the actual size.
Next is prop size. I am putting this on a world model chipmunk. What would you recommend for a prop size and pitch. Between the manual, what other people have told me and what l have read in the forums l am getting confused. Plus l don't want to buy 10 props just to find the one l want and have a bunch that l may never use again. We are at about 2400' ASL as well.
Next is compression. I have a OS 91 surpass and it seems to have a fair amount more compression. Am l right comparing the two? The YS is suppose to have very little run time and it looks quite clean inside and out.

So when you get a minute let me know what you have to say.

Thanks

Hi Tim

Not sure about the valve cover screws.

Most guys turn a 14x10 or a 15x8 on these motors. In any case prop it to run about 9000 and it will be happy. You should still be able to turn those props at that rpm at your altitude.

Try 2.5 mm on the valve cover screws.

Thanks to Chad for the info, it is correct.

Good Flying,
Dave