JonathanE Posted September 7, 2017 Report Share Posted September 7, 2017 Has anyone tried to strengthen the gearbox to engine flange on a 3 synchro "narrow" flange box with a circular strengthening plate so as to sandwich the cast gearbox flange between the strengthening plate and the engine? I was thinking of doing a CAD drawing of the gearbox side of the flange - probably broken into two or three segments - and getting it/them laser cut form 8mm aluminium (on the basis that the thick flange box is 6mm thicker than the narrow flange). Interested to konw if anyone esle has tried this or any variation and if so, with what success. If anyone has a .dwg or .dxf of the flange and holes they would like to share, I would be grateful; otherwise, if I do draw it, I can share it with anyone who wants. Thanks Jonathan Quote Link to post Share on other sites
Charlie D Posted September 8, 2017 Report Share Posted September 8, 2017 Jonathan, Back in the 1970’s when I lived in Greece I was planning a trip back to the UK. Not long before I’d stripped the thread in the clutch slaver cylinder where the flexible hose attached and as a new cylinder was impossible to find in Athens I’d Araldited the hose into the cylinder. In order to test the effectiveness if the joint I pumped up the pressure in the cylinder and pressed with all my force on the pedal. There was a bang!!! Not the hose coming out of the cylinder, but the gearbox flange breaking off….. I made a cardboard template of the bottom half of the flange, extending about 3" each side of the break. I took the template to a local blacksmith and within an hour he’d made a perfect replica from a piece of ¼” x 1” mild steel bar. (Yes, he’d bent it into a curve along the 1” part, with perfect smoothness, just using his forge and a hammer.) He’s even drilled out the holes where I’d marked them. I think it cost about 1$US The car was my regular day-to-day drive for 5 more years and the bracket (and Araldite) worked perfectly. Having said all that I had no choice. In your case I wonder why you want to strengthen the flange. It’s worked OK for tens of thousands of cars without problem in the past. (Unless you want to test the strength of your clutch slave cylinder…) Charlie D. Quote Link to post Share on other sites
JonathanE Posted September 8, 2017 Author Report Share Posted September 8, 2017 (edited) Thanks Charlie. I have no particular problem with my flange but I note that Triumph strengthened the flange in later cars and I have read in this forum, in the Restoring TRs book and elsewhere that the later wider flange gearbox casing is more desirable - the implication being that the narrow flange has a weakness (although the reason for prefering a later box may have something to do with synchromesh on first gear) which Triumph addressed withe a new casting. I was just thinking of addressing it a different way. It is encouraging to hear that your mild steel solution worked. Thanks again. Jonathan Edited September 8, 2017 by JonathanE Quote Link to post Share on other sites
Ian Vincent Posted September 9, 2017 Report Share Posted September 9, 2017 (edited) The original gearbox that was in my 3a was a thin flange one and it had been broken and welded back together around its junction with the bell housing. I replaced it with a saloon box with O/D. Rgds Ian Edited September 9, 2017 by Ian Vincent Quote Link to post Share on other sites
JonathanE Posted September 12, 2017 Author Report Share Posted September 12, 2017 That is interesting Ian. Do you think that a strengthening plate would have preventing the thin flange case from breaking on yours and was your motiviation for changing to a saloon gearbox the stronger flange, better internals or the overdrive (I surmise from your post that the cracked gearbox did not have OD)? Quote Link to post Share on other sites
RogerH Posted September 12, 2017 Report Share Posted September 12, 2017 Hi Jonathan, simply increasing the thickness of the flange will not work. It would stop the flange cracking from bolt hole to bolt hole but have no effect on keeping the flange attached to the bell housing (perhaps it would be a small help). You would need to make the backing plate out of a decent Ali alloy and then weld all along its base to attach it to the bell housing.. Or you could simply weld little triangular fillets either side of the attaching bolt holes. This would then connect the load on the flange into the bell housing. Do you know the failure point of failed bell housings (3, 9 or 12-o-clock.) Roger Quote Link to post Share on other sites
stuart Posted September 12, 2017 Report Share Posted September 12, 2017 Hi Jonathan, simply increasing the thickness of the flange will not work. It would stop the flange cracking from bolt hole to bolt hole but have no effect on keeping the flange attached to the bell housing (perhaps it would be a small help). You would need to make the backing plate out of a decent Ali alloy and then weld all along its base to attach it to the bell housing.. Or you could simply weld little triangular fillets either side of the attaching bolt holes. This would then connect the load on the flange into the bell housing. Do you know the failure point of failed bell housings (3, 9 or 12-o-clock.) Roger The fillet idea is what the factory did when they up graded the bell housings on the later all synchro boxes, I too have seen plates made up to go round the gearbox after a flange break, Rhino Macs one had just that sort of plate as there was at large piece of the flange broken off. Stuart. Quote Link to post Share on other sites
RogerH Posted September 12, 2017 Report Share Posted September 12, 2017 Hi Stuart, that is quite impressive. Probably caused by the clutch slave operation with a loose or absent bracing arm from the sump. I hope you refitted the tapper bolt in the fork before refitting !! Roger Quote Link to post Share on other sites
rhino_mac Posted September 12, 2017 Report Share Posted September 12, 2017 Roger, I fitted a new gearbox and overdrive to JSK. Or should I say, Stuart fitted the gearbox I supplied him - the idea of me ever fitting a gearbox is laughable. The photo is of the old gearbox once removed that I offered to you for parts before some scrote nicked it off the drive! Quote Link to post Share on other sites
Ian Vincent Posted September 12, 2017 Report Share Posted September 12, 2017 The welding on my broken box was a pretty good piece of work. I doubt that it would have fractured again. I changed the box because I wanted an O/D and I'm not too bothered about originality as long as it looks right from the outside. Rgds Ian Quote Link to post Share on other sites
JonathanE Posted September 13, 2017 Author Report Share Posted September 13, 2017 I have been wondering what (apart form over zealous use of the clutch pedal per Charlie D) causes the flange to crack and break. Is it something that happens in use (do the two front engine mounts permit more movement than the single gearbox mounting) or is it a slight misalignment on assembly so too much pressure is put on the flange as the bolts/studs are tightened. The How to restore TRs book shows a piece missing at the top, Stuart/Rhino's picture looks like that break may have been clutch pedal related. I think that a bolted on plate to sandwich the thin flange between the flange and the block could possibly help prevent breaks like that shown in the book, but I can see that it would not be much help at all against breaks in the lower half where the flange is more or less self supporting and just a very thin pressed cover. I had only seen the TR book picture of a broken flange when I started thinking about this and until I read CharlieD's post, I had not really thought about the flange breaking there. Quote Link to post Share on other sites
RogerH Posted September 13, 2017 Report Share Posted September 13, 2017 Hi Jonathan, the crcking at the side where the Clutch servo fits is almost certainly caused by the operation of the clutch. The GB housing is not bolted to anything other than the stay from the sump flange area. If this is loose or missing the flange takes the load. It is amplified but the radial length of the slave support bracket. The top failure looks more interesting. The bell housing is only supported at its top half of the circumference. If the side bolts and starter motor bolts were loose then it would crack very quickly but it is unlikely they are all loose. However as the engine is support well up front and the GB as far back as it can go there would be a big bending moment in the middle - just where the attachment is. With all this bending force it isn't beyond reason that the bell housing is deforming (imagine squeezing a toilet roll centre tube) the sides midway down the bell housing pushing out and the bottom coming up. This would put a decent bending moment on the top flange.. The fillets, ST introduced, on the sides go some way back and the flange increased in thickness would suggest they had the same idea. I wonder if they increased the wall thickness of the bell housing. Roger Quote Link to post Share on other sites
stuart Posted September 13, 2017 Report Share Posted September 13, 2017 The breaking of the flange back in the day was due to enthusiastic driving, as ST cars were used a lot for rallying etc hence why when the opportunity came for the new box to be introduced with all synchro then they beefed it up to cope. There is I believe a slight change anyway in the bellhousing from very early TR2`s to later 3/3a ones anyway. Stuart. Quote Link to post Share on other sites
RogerH Posted September 13, 2017 Report Share Posted September 13, 2017 The breaking of the flange back in the day was due to enthusiastic driving, as ST cars were used a lot for rallying etc hence why when the opportunity came for the new box to be introduced with all synchro then they beefed it up to cope. There is I believe a slight change anyway in the bellhousing from very early TR2`s to later 3/3a ones anyway. Stuart. Hi Stuart, the high speeds and bumpy roads would certainly put a strain mid-way between the mounts and cause the bell housing to deform. Roger Quote Link to post Share on other sites
dingle Posted September 13, 2017 Report Share Posted September 13, 2017 I think a lot the flanges that are broken at the top are caused by trying to use the bell housing bolts to pull the gearbox in place after a clutch replacement. Berry Quote Link to post Share on other sites
RogerH Posted September 14, 2017 Report Share Posted September 14, 2017 Hi Berry, that could certainly happen if one was too heavy handed. It would be nice to see the fracture face of one of these failures. That would show where the crack started and how the forces were applied. Roger Quote Link to post Share on other sites
Lebro Posted September 14, 2017 Report Share Posted September 14, 2017 I think a lot the flanges that are broken at the top are caused by trying to use the bell housing bolts to pull the gearbox in place after a clutch replacement. Berry Yep, it could easily happen if you do not get the splines lined up. Bob. Quote Link to post Share on other sites
dingle Posted September 14, 2017 Report Share Posted September 14, 2017 Yep, it could easily happen if you do not get the splines lined up. Bob. That's what I was thinking. Especially, if it is the person's first experience with replacing a clutch and gets frustrated with not being able to get the trans. to mate with engine. A few years ago, I bought an extra OD trans. that had the top flange repaired. It was even of the thick flange variety. Berry Quote Link to post Share on other sites
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