Saw this one in another post and seemed a bit off topic from the original post if I repied there so thought it was worth a new post. I found some info from an old post on Stuka SW.
Maybe it could have been seen as a smart ass reply then but when info is posted it should be helpful to others and correct. It related to the #5-40 screws in many of the ST engines which no doubt the American market influenced the choice of screw thread standard used during the 70s as crank threads were also standardised at 1/4" UNF.
Many of the older Japanese engines (Enya at least) in the up to late 60s at least used the old SI or similar thread system where a couple of the threads had a pitch of 0.1 more than the ISO Metric system, eg M5 x 0.9 and M3 x 0.6 rather than the now standard M5 x 0.8 and M3 x 0.5.
Back to ST, the early head screws were as Steve H states, M3 x ?? at least in the 40 size of around the mid 60s but from 1972 (about) they seemed to change to #5-40. The only ST I had at that time was a Blue Head 60 which had what looked like #5-40 or 1/8" Whit! They both screwed in but I didn't bother measuring the originals accurately even though all the instruments were on hand to do so. Also the ST 46 engines I had from 1975 on were also the same thread. As #5-40 soc heads were hard to get here, we used 1/8" Whitworth which of course has the 55 deg angle against the 60 deg. Not correct but never had a failure with them.
Then in about 2004, the subject came up on the other forum re what size are the ST head screws hence my reply then after suggesting the size but was deemed wrong at the time!
These were my findings then to one of the replies to the post:
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>>With all due respect to everyone involved, the pitch on a ST46 head screw is not 40 TPI. Take a 4-40 or a 5-40 and see if the threads line up with an ST head screw. They don't. In about a 1/2", it's off by about 40-45% of a thread. Not quite peak to peak, but definitely not 40 TPI. I can't imagine that it was intended to be 40 but just off-tolerance - it's too far off.
I think we all agree that a 5-40 can't be put in place of a 3x.5, not without doing something silly.
That's close enough to use as a replacement with no issues (since they only engage for a little over 1/8" on the ST) but they aren't the same.
####<<You all had me concerned that I had given some wrong info on the screws used on the heads of the Tigre 46. I know that it wasn't the original intent of the subject but it did come up and I feel that we must still clean it up for those who have responded.
You see, I did feel rather down over the subject and after getting a nice cuppa for Mama and getting her to put her feet up after a long day, I headed for the shed!
I have a number of all sizes and vintage ST engines including the G 60 Bluehead R/C engine. I pulled the head screws from this as well as the ST 60 and a 46, all of pre 80 vintage. All head screws "appeared" to be the same pitch and all O.Ds were around 0.119" dia. As I never really trust the O.D of a thread as definite identification I take other measures.
I know that most of you have basic measuring tools but how many of you who have answered this post have thread micrometers and optical comparators in your workshop. I thought so! Possibly none. As I do happen to have these, I decided to go all the way with this one and check the screws in hand for full identification. The optical comparator has 50:1 magnification and the micrometer scale on it is 0.01 mm. The findings were:
1. The O.D was around 0.119" on all three screws.
2. The pitch diameter was 0.1047" (the basic pitch diameter from the book for a 5-40 UNC is 0.1088")so you have 0.004" clearance on the basic size of the nut.
3. The thread angle is 60 degrees.
4. The measured distance over 10 threads is 6.34 mm which when converted to imperial will show that there is only an error of 0.0004" over these 10 pitches. If you care to work that out, 6.34 mm is 0.250" which when divided by 10 is 0.025" which incidently is 40 t.p.i. !!
From this info, you can only come to one conclusion and that is that the Italians used 5-40 screws in these engines.
HH
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Hope this info helps those who are still using ST engines from this era. Good luck finding the M3 x 0.6 screws for the old Enyas!! .................. getting as rare as hens teeth over here!
HH
Topic: Screw Threads in Many Super Tigre Engines (Read 1036 times)
