Small end bush replacement

[Degsy]

Thanks for the advice guys

I'm taking the rods and pins over to a local motorbike tuning specialist tomorrow morning, the more local car engine people said the Small end bushes were a bit small for their machine to do a good job if the small ends are slightly out, he said some of the small Ecotech  Car engines can be a problem, anyway he recommended the motorbike guy as he does small stuff for them.

Swarfy I'll ask him to check the straightness and ovality.

One of the old big end shells did have tin foil behind it, that is a worry, I measured some kitchen foil and I recon it's about 0.0004"   I could do without the extra expense of new rods especially as I am thinking I will need to fit an SRM Oil pump.

Cheers

Degsy

[RDfella]

Degsy - if a big end is only slightly out, it can be re-machined.

[Swarfcut]

 Degsy  The foil may have been put there to compensate for wear in the shells, by supporting them closer to the crank. Another trick from the early days of motoring when spares were expensive, pockets empty, needs must.  Foil is sophisticated, more usual find is a slice of beer can.

 The big end eyes can be re-machined easily. This is established automotive practice, and will successfully recover all but the worst of abused rods.

 Don't condemn the oilpump just yet.    As long as the body is not cracked, bloated or scored, they can be coaxed back to life. There is a knack to assembling them, lots of hints already posted. They all bind to a degree, the skill comes in finding where the obstruction or tight spot is. If its seized, a good warm up with the heat gun can free off the parts enough to be dismantled and cleaned. Keep the gears in the same relative positions if you can.

 Swarfy. 

[Degsy]

Hi Guys

Good to hear the rods can be sorted if the big end journal is out.

Swarfy my need for a good oil pump is related to my concerns about the needle roller conversion and I'll post some more pictures later today for comment.  The old oil pump looks ok but I'm sure it is worn and is the SRM oil pump higher output? or just the same as the original when it left the factory, I need to check on this.

Degsy

[JulianS]

The SRM pump is quite different inside to the original item. The SRM gears are wider and have 11 teeth, the original 1956 on item has 14 teeth. The nose of the SRM is dowelled to the main body and there is an O ring seal between the body and the worm drive.

Fitted one 10 years ago, body does not distort, no roughness when turning with thumb and no wet sumping.

First 2 photos are SRM pump, the third a 1956 on original.

[Degsy]

Thanks, Certainly looks well made, I wonder what the flow rate is compared to the original.  If my original pump is working up to spec and the flow rate is the same then there is less incentive to change, I can live with wet sumping but I have concerns about sufficient oil flow with my old needle roller conversion.

Degsy
 

[bikerboy]

I have to say I have never had a problem reaming out small end bushes. An adjustable reamer, tiny cuts and its always been ok

[RDfella]

A hone is parallel and follows the hole it is working in, wheareas a reamer is tapered and thus virtually impossible to keep perpindicular to the job unless the latter is suitably clamped in a machine such as a mill or jig borer.

[Degsy]

Hi Bikerboy, RDfella

The bushes are with a local guy who is well recommended but does not answer his mobile and not always his email so If I can't get him by tomorrow I am just going round there and if the bushes are not done I'll bring them home and take my chances doing it myself.

Since moving back up to Scotland I am surprised how little choice there is up here for small engineering jobs.

Degsy

[UKlittleguns]

Hi RD,

Afraid you are a bit wrong about reamers.  Machine reamers and adjustable reamers are parallel.  Only hand reamers have a starting taper and that's just for the first inch.  Biker is right.  Never a problem with hand reamers.  Set in the bore and remove a tenth at a time.

[RDfella]

Having started life as a toolmaker I'm aware machine reamers are parallel. But that's not the sort degsy was referring to. Hand reamers (including all the adjustable ones I have, though I try to avoid using them for several reasons) are tapered. If they weren't you wouldn't be able to start them.

[UKlittleguns]

Hi RD,

Adjustable reamers are parallel, check your engineering books.  If they were not parallel you would never be able to set them with a micrometer or ring gauge.  That's why you don't "start them", you put them in the bore first and then start adjusting them.  I'm afraid that if your adjustable reamers are tapered then they are just knackered.
 

[jonny web]

[quote author=Degsy link=topic=16191.msg138341#msg138341 date=1619511370

I was tempted to just use the old bearings but when I put the new gudgeon pin in and hold it in a vice I get about 1mm play rocking back and forth at the big end (see photo)  that feels like too much?

[/quote]


Hi Degsy  Im in the same position really, as I have an A10 motor rebuild just started and am pondering small end bushes. As far as I can see, that 1mm rock of the rod equates as follows to play at the pin and bush. The pin to end of rod is about 6,5 in, and the bush about an inch wide. so if you centre the rod and rock it to one side, you get 1/2 mm of play or about 0.020 in. divide that by the ratio of length to width and you get about 0.003 in movement at the pin, this is about 0.0015 clearance all around between pin and bush. it should be about half that. But a 40 weight oil will take that up and there should be no problems. Bear in mind that this is a pretty low performance motor, and if you re anything like me and you have more than one old bike, it wont be doing that many miles either.. The question is whether you'll get the same degree of accuracy by trying to press in and ream the new bushes yourself. Mine look like the factory fittings and as there is no scoring on the pins or the bushes, I will put them back

[Minto]

Jonny web
That was a great explanation and bit of maths, thanks. Very reassuring.
Jase

[Degsy]

Thanks for the analysis Jonny Web, I agree, this will be a low mileage bike and I probably could have reused the small end bearings, it was the big ends that were a mess (see photo).  Anyway I did get them done professionally in the end and also had the conrods checked and they were fine. 

The conrods are on the crank and it is ready to go back in.

I reused the big end bolts and I know many will say do not do this but I could not find a source for new ones and there are some comments on the quality of the new bolts that are currently available.  I have not had any problems with my limited experience of reusing big end bolts.  I used Loctite 272 on the nuts which is high temperature and high strength so hopefully it can cope with the crankcase conditions.

Degsy