The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: Degsy on 25.04. 2021 15:40
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I was planning on replacing the small end bushes on my A7 engine since I am putting in new 060+ pistons, but my replacement bush is just a plain bush with no oil hole and no oil groove (see photo).
I guess drilling the hole is no big deal but I am not sure about making a groove, is the groove required? do I have the wrong bushes?
Also the new bush feels like it will need to be reamed out when I try the new gudgeon pin, it looks like it would be difficult even with the pin out the freezer and a warm rod and bush. Is it normal for the bush to need reaming out?
The new Gudgeon pin can be pushed into the old bush by hand.
Degsy
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Yes, new SE bushes always need honing to size. Drill the hole, don't worry about the scroll. I don't recomment reaming, as that's difficult to keep perfectly square and therefore risks having a tilted piston. Also, I find reamers are generally only accurate to around .001" whereas I like gudgeon pin to bush fit to be a moderate push fit.
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I've only ever seen plain bushes, and BSA state plain bearing sizes after reaming or grinding (as appropriate). For the A7 Gudgeon Pin Bush it's 0.6881"/0.6878." Ref BSA Service Sheet 702/June1959.
As you've already spent on the new bushes it would be a shame not to use them. Having said that the end result may not be that much better (if at all) from new pins in the old bushes. That small end eye on an A7 rod always looks a bit fragile to me, so take care if you do swap them out.
Swarfy.
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So the plain bearing is ok I just need to drill the oil hole, that's good.
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?
RDfella how do you do the honing if not with a reamer, I have an appropriate sized hand reamer on order.
Cheers
Degsy
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the rock is not so important unless of course it's ACDC!! it's the up and down slop that matters
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Hi,
Had to replace one of my small end bushes recently. Easy enough to get the block pistons etc off but taking the rod out for a 'proper job' was deemed a step too far.
Old bush pulled out with a simple nut bolt and spacer arrangement. Rod eye warmed with heat gun and new bush fitted.
I used an adjustable reamer. It was set to enter the new bush half way and then pinched up to just touch the surface. One turn, reamer out and pin fit checked. Reamer back in and process repeated. The nice thing about the adjustable reamer used in this way is that it self aligns with the bush and only takes out a tenth at a time. Took some 15 minutes but eventually gave an excellent pin fit.
Not for the purists amoungst you all but it worked a treat and saved an eyewatering amount of work.
Len.
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Hi again,
Just remembered how to post pictures.
(http://)
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Hi Guys
I already have the bushes and the adjustable reamer arrived in the post this morning so I think I will go for it and change the bushes. I read somewhere that you could use an old piston to help align the reamer, I tried it (see photo) and I am not sure it helps.
Len I think your careful incremental approach is the way to go. I hope the bushes come out and new in OK, should be easier as I have the conrods off the crankshaft.
Cheers
Degsy
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Hi Degsy,
I was advised not to try to ream the small ends myself without sufficient experience, as it is thought hard to achieve absolutely perpendicular alignment to the rods, or in other words parallel to the crankshaft. From the efforts of a previous "mechanic", I had one piston tilted to the right by 10 thou and a subsequent smoky exhaust - the small end was not parallel. Took me two attempts to remove the top end and replace the rings, re-hone etc without success before I discovered the piston tilt. Opinions vary but generally the tilt should be less than 2 thou over the piston width, which means about 0.5 thou tilt over the width of the small end bush. Now that is a tall order from hand reaming!
I asked the machinist if he could replace the bushes without removing the rods. He said "I want the rods". Although I do have the reamer, I decided to give him the rods, so had to dismantle a newly rebuilt motor to give him the rods.
He machined the small ends perfectly perpendicular, and I think it was well worth it.
No matter how you decide - as a final check, I was advised to get hold of a 0.750" Bright alignment rod and pass it through both small ends. If you are a good machinist, make your own alignment bar.
I purchased a 300mm length of 19mm bar ( 0.748" ) and a 19.05mm bar (0.750" ). That larger (oiled) bar squeezed through nicely with very little force. I took a photo for reference (Sorry about the background clutter).
Col
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Hi Col
Thanks for the good advice and you speak from experience and I don’t doubt that we should be working to these levels of precision and accuracy.
I joined the forum recently and I have read a lot about the need to work to tolerances of less than a thou, did these bikes really leave the factory with this level of precision, I can’t help feeling the quality control was not great back then and largely dependent on what kind of day the 1950s machinist was having (yes I have seen "Saturday Night and Sunday Morning" and wasn’t Albert Finney good in that!).
This is me just making excuses for planning on reaming the bushes myself and when my BSA goes bang one day half way up some Scottish mountain in the middle of nowhere I will regret not taking your good advice.
Cheers
Degsy
(Edinburgh, Scotland)
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Degsy - a reamer is a cutter, a hone uses stones.
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I recently tried using an adjustable hand reamer on new small end bushes for my A10. I had never done it before, but the concept seemed simple enough. The result was a complete disaster. I bought another set of bushes and took everything to an experienced specialist who will do it correctly. Looking forward to picking up the completed rods this weekend and resuming my restoration project. Good luck to you Degsy if you decide to go ahead! (Also, is anyone out there interested in buying a slightly used adjustable hand reamer? ;))
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*conf*Starting to have second thoughts about doing it myself, I'm looking for good machine shops locally
Cheers
Degsy
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Degsy. If you decide on the professionals, have the rods checked first for straightness and big end eye ovality. Make sure these aspects are in order before spending time and money on bush installation and finishing. Alloy rods are more "elastic" than conventional steel rods and are known to deform.
Standard size big end eye for small journal crank is 1.6055"/1.6060" Metric 40.78mm/ 40.792mm.
Swarfy
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yep, agree, my experience with an adjustable reamer was a disaster too on my gearbox, if you want to read about it!! but that was my own fault really for not understanding the correct way of using it ie VERY small cuts at a time!!
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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
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Degsy - if a big end is only slightly out, it can be re-machined.
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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.
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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
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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.
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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
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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
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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.
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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
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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.
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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.
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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.
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[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
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Jonny web
That was a great explanation and bit of maths, thanks. Very reassuring.
Jase
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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
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oh dear they do look a bit rough ! looks like maybe run on some v old oil, or maybe not enough, mine look pretty good, but i ll fit new ones anyway, same size- -20 save the journals a bit
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Hi
The bike has had a problem in the past with the lubrication system, see my thread "A7 Bobber Engine needle roller conversion" and this has starved the big end bearings.
I must close off that thread with a list of my final fixes for the problem.
Cheers
Degsy
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Degsy. That crank looks to be small journal (6 Bolt Flywheel) but the rods look more like big journal type, especially regarding the spanner clearance between the caps and nuts. This is always a dilemma for restoring an early crank when the small head castle nuts are rare and folks machine back the cap or add thick spacer washers under later nuts. What did you find?
Swarfy.
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Hi Swarfy
Yes it is a small journal crank (no tube in the sludge trap), the caps torqued down nicely and the rods feel good on the crank.
How could a large journal rod fit a small journal crank?
My 3/8 Drive socket fitted just fine on the nuts, are you saying the area around the nut has been machined wider, I'll have a look when I get a chance tomorrow.
cheers
Degsy
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Yes Degs, the cutouts for the nuts look bigger than the usual ones found on small journal rods. The cutouts are more in the style of a big journal cap. Rod forging is 67-391 for A7, which is what I would expect you to have.
A quick measure on a small journal cap has a 15.5mm diameter cutout, on a big journal cap its 19mm, making access that bit easier.
Swarfy.