Craig, if you have any os engines you might try one of the bolts in your magnum, then you can take that bolt with you when you go shopping..

Good Luck

Take the engine to the Ace hardware store and fit the bolts to it.

Sorry Steve, lock washers.  The engine comes with M3x40mm machine screws, split lock washers and a fairly thick gasket.  If you omit the lock washers or gasket, 40mm screws bottom out on the muffler before it gets tight against the exhaust port.  One option is run a 3mm tap through the muffler so the screws can pass all the way through(none of my mufflers were tapped completely through but this may vary).  I don't trust the split type lock washers and for the past 20 years I have used outboard motor nitrile gasket sealant on exhaust parts.  That has solved muffler screws coming loose but on the .52 a 35mm screw works better than the 40.  If you use the lock washer and supplied gasket, 40 is fine.  If that doesn't make sense I can post pictures.  As I recall they do include everything to use a rear needle or carb mounted needle for those of us who refuse to change.  George

Don't much feel like starting a new thread...BUT this has got to be the MOST aggravating situation in my recent C/L retread experience

I have accumulated a bunch (75) engines since my return in 2012
some notable many will never see a muffler (26 Cox ) ( 12 odd Torpedo, Johnson, Nelson, Fox combat) engines

BUT I also have many OS FX/FP/LA, Magnum, and Fox 35/40 with mufflers and in some cases original questionable fasteners for the back plates and heads

2/56, 3/42, 3/48, 3/56, 4/40, 4/48, 5/40, 5/44 are all hard enough to Gage and differentiate with BUT
then to add in the Metric sizes and all of a sudden it becomes exponentially difficult to manage

That said, I treat some screws with a higher order of interest and concern than any of my 33 year long wife's .35 to 3+ carrot diamonds!

FV4, I am pretty sure I've never seen a 3/42, 3/56, 4/48, or 5/44 in my life as a machinist. They are an option, but nobody uses them. One night at Boeing, I had to single point an internal 1/4-60, to produce a small batch of microphone adapters. No tap existed in all of Boeing. The lucky thing was that there was also no gauge. I had to hand grind my own cutter, to make things more fun. 

One of the weirdest things I've ever run into was my first (early) Double Star .60bb. Lovely engine, but the screws were all wobbly in the threads. Luckily, the crankcase threads were fine, but the screws were obviously single-pointed on a lathe, instead of rolled like we are used to. I conferred with Tom Knoppi, my F2C flying friend. He said that is how they train apprentices...all their screws were terrible. When their teams came to the West, they returned with boxes and boxes of machine screws. They still have the occasional snafu, like the B.40's that had oversize prop shaft threads, just last year.

George is absolutely right about the split type lockwashers...avoid them. They will break and fall out from under the head of the screw. Always at a bad time, too. I avoid muffler gaskets after seeing what happened when a club member had his G.51 (on muffler pressure) blow the gasket. A constant leak is not usually a problem...the outlet is just a bigger leak, after all. However...there's always a however...on a "tuned" pipe or tuned muffler (MACS, and modern OS stock mufflers), I suspect you want to have them sealed, but I also haven't tried to find out. I use a wee bit of the red silicon gasket stuff that Brett recommended. Permatex, I think. Note that if you use muffler pressure (MP), a change in the amount of leakage makes the engine go quite lean. Might be a good idea to make a few flights without MP to make sure there's not going to be a problem.  Steve

Hmmmm. I just go down to Hi-Strength Bolt or Tacoma Screw and buy a box of what I need. A hundred might be $6 or so.  Steve