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Con Rods ST#1, #5A

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John Purdy06/11/2018 19:34:08
178 forum posts
59 photos

I have a question for anyone who has built either the ST #1 or the #5A engines. A good many years ago I acquired a number of ST castings including a #1 and a #5A. They were out of their original packing, in separate boxes but were essentially complete. The 5A had some machining done ( not particularly well done) but the #1 was untouched. I have started machining the #1 and having finished the big end bearings yesterday I started looking at the connecting rod casting and it seemed to have a lot of excess material to machine away, so I dug out the one for the 5A and comparing them both with their drawings it appears to me that they might have got switched. In the picture the rusty one on the left was with the #1 castings, the other with the #5A.( all the #1 castings had a coating of light surface rust but the #5A were clean) Can anyone confirm which one is for which engine? Thanks.


JasonB06/11/2018 19:52:44
16256 forum posts
1717 photos
1 articles

I think it is more to do with when they were produced than what engine they are for, currently both conrods share the same part number.

John Purdy08/11/2018 20:12:43
178 forum posts
59 photos

I had checked Stuart Models web site, spare parts section, and had noted that, as you say, the con rods for both now have the same part number so are probably the same casting ( a cost saving move when Stuart Models took over perhaps ?). I don't know when my castings were bought but I think probably late '60s early '70s making them Stuart Turner castings. I have the parts list for the #5 (but none for the #1) and it lists no part number for the con rod and for all the castings that do have part numbers they are totally different from the current Stuart ones.

If no one can identify them positively I think I will use the non rusted one for the #1 as it is the smaller and the finished dimensions of the #1 are smaller all round except for the distance from the pin to the bottom of the foot which is the same (3 3/8" ) for both.

Edited By John Purdy on 08/11/2018 20:15:18

Edited By John Purdy on 08/11/2018 20:15:41

John Purdy27/12/2018 21:41:05
178 forum posts
59 photos

I have been machining the cylinder for the #1 an before I finish the bore I decided to check the rings I have. When I got the castings the piston rings were in multiple pieces so I bought a pair of 2" dia rings from one of the usual suppliers. On checking them I have found that when closed up to 2"dia the ring gap is about 35 thou which I feel is excessive. When closed up to zero gap the dia is 1.975". I have roughed out the bore and it is currently 1.800" so I have lots of leeway to make it whatever. My question is what is the normal gap used for an engine like this? In all probability I'll only be running it on air and not expecting it to do any real work.

Edited By John Purdy on 27/12/2018 21:41:44

JasonB28/12/2018 07:00:05
16256 forum posts
1717 photos
1 articles

Are you sure you have not got 50mm rings = 1.968" ?

I'd go with about a 0.005-7" gap

not done it yet28/12/2018 13:25:37
3349 forum posts
11 photos

I think JB is right. Your 2” rings are, more than likely, smaller than you expect.

How are you measuring the diameter? If not by fitting it to a ring of the proper diameter, you will get misleading results. Think here, if you are ‘squashing the ring to diameter, in one direction, it will elongate in the perpendicular axis.

One cannot (should not) fit oversized rings in a smaller hole by filing the ring ends to give the correct gap, so the converse is true - a smaller diameter ring will have far too much gap but will also not seat properly against the cylinder wall.

I only did, or do, ICE rebuilds where the minimum ring clearance is 3.5-4 thous. per inch bore diameter for engines that are not ’hotted up’.

Look on the Hastings website (or any other ring manufacturer or engine builder) for more detailed info,

John Purdy28/12/2018 19:34:10
178 forum posts
59 photos

Thanks for the replies.
Jason the rings were bought from Reeves in Mar '93 and were listed as 2" dia. Your 5-7 thou gap is about what I thought would be about right. I'll go for that and we'll see what bore I end up with.
My measuring is quite possibly suspect so I will just slowly open out the bore till the ring just goes in then a little further to get the 5-7 thou gap.

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