I got an ancient Merco .49 Multispeed in a bulk buy.   The 5-digit telephone number on the tag indicates pre-1965. 

I'm trying the decypher the bolt size.  It's something in the area of 4/40, 5/40, or 3 mm, but not exactly any of these.  Was another thread standard about in Merry Olde back then?  I Merco still around?  How about a head gasket for a bore of .875?

I'm a bit perplexed.  The bolts are a bit buggered, maybe from the previous owner looking inside, bit the piston top looks un-run.   I'm itching to bark this beauty out on the stand right quick.

Hi, I've got a few of the merco 49 engines and love them. They have a real nice 4-2-4 run, good power and a great stunt engine. I'm going to use one in my stiletto, I use one in a score and pulls the plane with out a problem. I believe you should use twenty-five per cent castor oil. Have a great day, Gary (Try search on this forum, type in merco and I believe you will get some info on this baby)

The engine runs like a champ, 10,200 RPM down to 3,800 and back.  The inside looks brand-new.

The tag from Joe's Hobby Center carries the phone number WEbster 3-6567, a number some of the older flyers around here can still recall.

Per Rustler's site, these units were produced 1961-1963.  The phillips heads are an odd-ball size too, explaining why they're buggered-up, but not why the previous owner took 'em loose.

That sounds good.

I've heard of the Whitworth Thread for years, but this is the first time I've owned one.  And they do mike out to .120".

I used my digital camera and Photoshop as a high-price magnifying glass and confirmed the 40-pitch.  I'll canvas the industrial suppliers in towne to see if any still supports Whitworth.

I've alread confirmed that 4/40, 5/40 and 3MM are not close enough.