Loose bushes in crankcase

[Jan]

Two of my SDs have loose fit bushes in the crankcase.

Do I need new bushes or can they replaced with Loctite?

Thanks,

Jan

[Taspiper]

Either would be an option BUT IMHO replacing the bushes would be better. We use Loctite products daily and although they have some uses such as this. I have found that 680 or high strength retaining compound, as would be used here, will let go after a short time when the bush is loose to start with and there is rotational and side loads coupled with heat and vibration. If the bush is still a firm fit you may get away with it.

[Charles uk]

To the best of my knowledge there are no new SD bushes available anywhere, if you don't have any.

It's possible to do it both of the ways you suggest but both require access to an engineering workshop & special tooling including a crankcase reamer.

[water bug]

Jan, I am having the same problem with my Seagull 170 crank case bushings. Art

[electrosys]

I've been a bit hesitant to suggest this, as I fully expect some scathing comments, but ...

If the bushes are so loose that they can be easily removed, then one rather crude way of enlarging their Outside Diameter would be to fit them (i.e. clamp them between nuts) onto a threaded rod to match their Internal Diameter, and run this between centres on a lathe. Then gently apply a knurling tool to the bush.

It sounds cavalier (and indeed probably is), but if all else fails it might be worth a shot.

Here's an example of this having been done on some 1" shaft which ran in a pillow block I rescued recently from the scrap yard - the cross-hatching is more precise that that from a knurling tool I agree - and in this example the shaft diameter has been enlarged by just 10 thou.

shaft.jpg (28.97 KiB) Viewed 809 times

Jeez - I'm getting this "Not downloaded yet" stuff again - what's up - is it something I said ?

[Charles uk]

I hope this doesn't come across as a scathing comment, first you have to answer the reason "why did they spin" because this didn't happen during the first 10 years of use.

They spun because the main bearing clearance was too much due to wear, allowing the lubricating oil film to be pushed out of the way, giving a load bearing surface with no or insufficient lube, generating heat through friction which is conducted into the cast aluminium crankcase which expands at a greater rate than the bronze bearing, reducing the interference fit, when the lubricating oil gets hot enough to be pushed out of the way enough, the bronze bearing will grab onto the crank & the bearing will spin, wearing the crankcase, & further reducing the interference fit until you can remove the bearings with a push from your finger even when cold.

Repairing this problem is often outside the owners engineering ability as sometimes the crankcases are so badly damaged that the require boring & sleeving, the sleeve can then be bored to the correct fit for the old bearings providing they are not too badly worn on the ID, for bearings that are no longer available new & where new crankcases with bearings already fitted are not available it's possible to reduce the bearing internal diameter with a silver steel polished mandril of the correct internal diameter by using a cross hatched knurling tool & a heavy duty lathe.

In the case of the majority of Seagulls where new crankcases are still available for a few pounds all this effort is not cost effective

But for Seagulls of the types being discussed here SD's, 170s/125s & all pre war Seagulls where bearings & new crankcases have not been available for 12 & 40 years or more, all this work might well have to be done, as well as any post insertion reaming that might be required.

Perhaps Waterbug, Art, might give us a photo illustrated diary of his bearing replacement, that would explain the task much better than I can.

[Taspiper]

Electrosys, yes you could do this and it will probably work. We have done the same many times over and sometimes even using just a centre punch to run over the outside surface. These methods work but should only be considered a temporary measure until a proper repair can be affected.
Charles mentions the right method of doing the job properly, boring, sleeveing, making new bushes etc.
I guess your looking for the most cost effective short time repair from the sounds of it so only you can make the right decision on which way to go. Knurling and Loctite may last a very long time if the motor is used little.

[Charles uk]

Rolf

I did some on an SD that was so slack on the top, the flywheel magnets with a push could touch the laminations, I knurled them on a length of 5/8 stainless that was 1 thou undersize, after I had to push the 5/8 through the bush with the tail stock & had to flypress the bearings into the crankcase with bearing retainer as lube, the crank was tight in the bottom bearing but wouldn't go into the top.

So I had to bolt it upside down to an angle plate then tighten up the crankcase bolts then emery the stainless until it would fit into both bearings so I could clock it for center/vertical then ream it back to 5/8, the crank ran real good with no play, but all this took a saturday morning in a full workshop not a problem when it's a homer but if someone was paying for this, in your money that's around $150 & an unhappy customer, though the next one should be quicker.

So I vowed if there was a next time & it was possible to buy a new crankcase that was the way to go, failing that buy or make new bearings.

[electrosys]

Hi Rolf - well, I haven't got this problem with bearings myself, it was simply a case of having seen a shaft with cross-hatching to provide an interference fit for a bearing - a technique I've never seen before - and then hearing about bushes spinning in a crankcase and making a connection between them based on wishful thinking ... !

Charles gives a good description of the events thought to precede and cause bush rotation - which I must confess I found at first to be somewhat counter-intuitive: that a bush which is badly worn is then grabbed by the shaft following friction heating - I'd have thought that the wear would have prevented that (?), but obviously not.

Indeed, talk of loss of an oil film made me realise that I don't actually know what these bushes are made from. Are they solid bronze then ? I had assumed that they were sintered, but haven't looked at any that carefully. Presumably sintered bushes with their oil impregnation would be much less likely to heat-up when worn than solid bronze ?

'best
Colin

[Taspiper]

Rolf

I did some on an SD that was so slack on the top, the flywheel magnets with a push could touch the laminations, I knurled them on a length of 5/8 stainless that was 1 thou undersize, after I had to push the 5/8 through the bush with the tail stock & had to flypress the bearings into the crankcase with bearing retainer as lube, the crank was tight in the bottom bearing but wouldn't go into the top.

So I had to bolt it upside down to an angle plate then tighten up the crankcase bolts then emery the stainless until it would fit into both bearings so I could clock it for center/vertical then ream it back to 5/8, the crank ran real good with no play, but all this took a saturday morning in a full workshop not a problem when it's a homer but if someone was paying for this, in your money that's around $150 & an unhappy customer, though the next one should be quicker.

So I vowed if there was a next time & it was possible to buy a new crankcase that was the way to go, failing that buy or make new bearings.

Fully agree, sometimes it's just not worth the pissing around unless you really want too. A morning of my work is $280.00 these days and that still doesn't cover the overheads

[Taspiper]

Electrosys, I do not have enough seagull experience to tell you what the bushes are made from. I can tell you though that I favour sintered bronze in applications that do not get adequate lubrication.
I had an industrial dryer fan where we replaced the bush with a standard bearing type bronze and it lasted a couple of days, when we replaced it with a sintered version it's now been years and still going.
I would assume standard bronze would be ok in crankcases where they get constant lubrication from the fuel. Charles may know more.

[electrosys]

Hi Rolf

I hate 'not knowing' stuff, so have been Googling around this morning: FWIW, it appears that sintered bronze, although still a bearing material of choice for electrical motors etc (which we know about), is rarely seen these days inside internal combustion engines - I found one reference to sintered bronze in the book 'Fundamentals of Motor Vehicle Technology', and another in a website devoted to the Ford Flathead V8, where Ford had experimented with s/bronze con-rod bearings, along with Babbit metal, lead silver, and cadmium silver - this was early 1930's to the mid-40's - which pretty-much coincides with the invention of s/bronze, so I guess Ford were simply trying out a new material.

Found a lot of info about polymer bearings too - great claims are being made for this stuff - but that's a whole new ball-game.

Cheers,
Colin

[Taspiper]

About ten years ago we were making plastic bearings for everything but it seems to of gone out of fashion a bit. Temperature and moisture seemed to be the biggest problem.
I made a hydraulic cylinder bush that was a nice sliding fit on the rod when made, next day(winter) I had to machine it off the shaft it had shrunk on so tight. By the time you take into account all the tollerances the fits were too sloppy for any use.
Sometimes old technology is still the way to go.......Seagull....

[Jan]

Thanks for posting the replies and the 'Why did they spin' explanation by Charles L.

So it wasn't a trick to provided 'Ze Germans' with captured unreliable outboards for operation 'Seelowe'.

I will go for the Loctite option but will keep the knurling tool idea in mind.

Thanks,

Jan

[water bug]

Great discussion, and information. Charles asks that I do a Photo diary on repair of this very problem ( spun bearings on my 170 crank case). I have bought an Excellent crank case including bearings form John (SOS)> BUT I may just try and repair the original C'case to see what can be done. Top bearing is a finger push fit in the crankcase, which seems could be saved using high strength Locktite. That bearing ID is .002" clearance with the crank OD. A little much, but possibly useable. ( I DO not race My Seagulls). The bottom bearing just falls out, being .006" worn on the OD. ID is still .002" to the crank. SO this one will need a new made bushing , Knurling, OR Possibly bushing the OD of the bearing to a .001-.002" interference press into the crank case. Then as Charles says , it will need ID reaming to the desired .001" - .0015" crank clearance desired on the ID. That reaming to be effective , needs to be both crank case halves bolted together, and "line" reamed as an assembly. WHEW.... It may take more cogitation, (thinking) before attempting. Will report if/when I do something. Art