Old Spark ignition engines.........What are the screw threads?

[Andrew Tinsley]

Hello Everyone,

                    Don't know if this is the right place to ask, but here goes. I am renovating and repairing my collection of US spark ignition motors, so they will be ready to use this season.
  A lot of the screws heads are damaged and I would like to replace them. I am assuming that the threads will be American National Fine or American National Coarse. Some of them don't seem to be either! So I have three questions.
  First, what other threads were commonly used? Do you have the equivalent of our Model Engineer or British Association threads (ME and BA) in the US?
  Secondly where can I get a selection of thread gauges suitable for use with the threads that I have? My current  thread pitch gauges don't include some of the pitches that I have come across.
  Lastly, can anyone recommend a source of screws that are suitable for replacing my damaged ones? Mail order is a preferrable method, as I live in the UK.
  As a separate question, some of the cylinder assemblies are held on by two screws, e.g Madewell, Vivell and OK Super 60. I have had problems with these screws coming loose during running. Even using threadlock on the Super 60 has not been a 100% solution. Has anyone got any suggestions for a permanent cure?

Many thanks,

Andrew.

[Jim Kraft]

Most of the screws are either 6-32, 4-40, 3-48, or 2-56. Some engines like the Anderson Spitfires use 5-40 for the head screws. 4-40's will screw in ok but are very loose. If you google McMaster Carr you can find just about anything that you might want in the way of screws. On most of my old sparkers I change to socket head bolts as they are much easier to use and keep from marring head fins and such. They will also let you tighten those two bolt cyl. jobs to keep them from coming loose. The OK Super 60 uses 6-32 for the cyl. hold down. Same with the Madewell. Maybe the socket head bolts, along with thread locker, would solve the problem with them coming loose.

[Robert Zambelli]

Andrew - I replaced the original slot-head screws on my OK Super 60 with socket heads. I had to bevel the heads just a bit to clear the cylinder fins.
Here's a trick I use in virtually all my threaded connection: Liberally coat the threads with RTV (silicone sealant) just before assembly. Then, insert and snug them down, wiping off the excess RTV with a Q-tip. Also, you can use a split lock washer for some added insurance.
In well over 300 flights with my OK 60, I have NEVER had to tighten the hold-down bolts.

Regarding thread sizes,  Jim is correct. Virtually everything is pretty much standard UNC or UNF. Most US, Japanese and Italian engines use a 1/4-28 UNF thread for the prop nut.

The only really different thread is the glow plug which is a 1/4-32 UNEF.

Bob Z.

[Andrew Tinsley]

Thanks guys,

                   Just the sort of information that I need! Perhaps I can get some suitable pitch gauges from McMaster Carr? I will do a quick search and find out.
  I am not too keen on socket heads, they don't look right on these old engines, but if they do the job, who cares! Thanks again for your help.

Andrew.

[Robert Zambelli]

Andrew - you should be able to pick up a thread gage at any hardware store (Iron monger???). Even Sears, if there are any in the UK.

There are usually three types: SAE, Metric and British Standard/Whitworth. Not too costly, buy one of each.

Bob Z.

[Steve Helmick]

Loctite makes a 'primer' for the thread lock stuff. It seems to work very well, and might be worth a try for your difficult cylinder bolts. Cleaning all the threads also gives much better results with Loctite.  Steve