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MG Midget and Sprite Technical - Reaming small end bushes
Has anyone ever managed, or have a way of accurately reaming new con rod little end bushes? I had a crack myself but the resultant hole is nowhere near square on to the rod. The gudgeon is a good fit, but visibly on the wonk! Doh! (Paying someone to do do it is an option, but I always like to have a go myself first!). Cheers, Malc. |
Malcolm Le Chevalier |
When I've had it done by an engine builder he said he farmed the work out to another local shop that had the specialised honing equipment to do it square and accurately to size, it important to get it right. |
David Billington |
Yes I did mine recently. Very hard work but good result. I hope you didn't use an adjustable reamer! I bought a used fixed one on eBay for £12. It's a spiral flute one - straight flutes won't work (I tried!) because the cutters catch in the join where the bush is split. The taper on the flutes should ensure a pretty accurate hole, square to the rod. At least it did for me.
Nearly forgot, ensure you have a hand reamer and not a machine one. The former is designed to keep square as you work through the bush. It is extremely hard work. It took me 40 minutes to do each bush, but the exercise is good. I have just done the valve rockers as well, and these didn't seem to need as much taken out. Les |
L B Rose |
Interesting. Thanks Les. I used an adjustable hand reamer as I wasn't able to obtain a fixed reamer of the correct size for the 1500 bushes (they are 13/16"). The pic shows how far out I am. Maybe my reaming is just a bit sloppy! The bushes seem to be quite a way undersize. The ID needs reaming out 20 thou. Malc. ![]() |
Malcolm Le Chevalier |
Reamers don't cut well until they have been sharpened - no idea why that should be, but new ones are useless! |
Chris at Octarine Services |
My Haynes manual gives a size for the small end bush of 0.8126" to 0.8129" so a 0.0003" window. A common fixed reamer tolerance is H7 and that has a range of 0.8125" to 0.8133" so could ream just under of somewhat over given the original spec. You would need an H6 tolerance reamer and that could produce a hole of 0.8125" to 0.8130". H7 in that size is -0 micron to +21 micron, H6 is -0 to +13 micron.
Personally if I were to do it I would do in my mill and set-up to bore the bush slightly below size then ream out the last few thous, that would ensure that things are square and the rod centres are correct but then I have that option. I wouldn't do it though as I don't have any bore measuring equipment that can accurately and consistently measure a bore to ten thousandth of an inch accuracy. I can see why the engine builder farmed the job out. |
David Billington |
Chris, Maybe you're buying cheap reamers, I don't have any issue with new quality reamers straight out of the box. |
David Billington |
I can't measure the the bore and was just going on feel of the pin. The old bushes defiantly had some play between the bush and pin. Yet the manual says the pin is "thumb pressure push fit" or something like that. Which I translated to "can be assembled without a press and with no discernible play between bush and pin". Malc. |
Malcolm Le Chevalier |
Malc Why not get pre finished bushes |
William Revit |
Malcolm If it was adjustable was it straight or spiral flute? Most cheap ones are straight. Problem is that 13/16 spiral flute reamers are more expensive. I don't know what a machine shop would charge, but if you have to spend say £50 on the tool it might be marginal whether you save anything by going DIY. |
L B Rose |
Willy: The bushes are an odd size and I can't find anything that might be a better pre-sized fit. I am exploring getting some machined from aluminium bronze. If I can get them machined to within, say, 4 thou ID, they could be honed the rest of the way.
Les: It was a straight flute adjustable reamer. It did seem to stick a lot to start with - even with plenty of lube. It got better the further I went, but I suppose the damage was already done! It is about time I took out a bank loan and set up a proper machine shop in my garage! :-D Cheers, Malc. |
Malcolm Le Chevalier |
Malcolm,
Consider lapping them with an expanding lap such as mentioned here http://www.gadgetbuilder.com/Lapping.html and getting something to make sure what you're using stays square to the small end bore. The usual sprung brake cylinder hones can be problematic as they can result in the bushes being bellmouthed so best avoided. Regarding William mentioning pre-finished bushes I have never seen that type of bush available pre-finished, plenty of types are but I have never seen the type used in small end bushes supplied and certainly not in the tolerance specified. |
David Billington |
ACL do them here Full circle steel backing but only in some sizes I'll check What are the id/od(or tunnel) measurements and length & do they have oil holes, although they could be drilled if req. I take it that it's a .8125"(13/16") pin by the earlier sizes quoted so od and length Is this for the 1500 Triumph engine |
William Revit |
15/16 OD, 7/8 length |
Malcolm Le Chevalier |
An update on this... I have decided that it is going to be easier to balance the spare set of rods I have (with OK little ends) than sort the little end bushes in this set of rods (which have been previously balanced). In short, I give up for now :-D Malc. |
Malcolm Le Chevalier |
Malc
Sorry mate, I forgot to get back to you Went and had a chat with my old machine shop mate on Monday and ACL bearings here has shut down- Well sort of, they were bought out by ACL Distributors USA and are now only doing the ACL Raceseries bearings which are limited production for basically the US Nascar supply being mostly GM-Ford V8 bearings and some high performance Japanese performance car bearings as well The supply of normal everyday engine bearings are outsourced from ,he thinks Taiho bearings and trade as ACL/Taiho bearings With these bearings ,they come in normal fit which I Imagine is what you had and are usually around .020" undersize and have to be machined after fitting But some sizes come in semi-finished and are bored to size but typically end up around .001"-.002" too tight when fitted and need a light hone to correct size These semi finished bushes have SF on the end of the part no The old ACL part no for your bush, which is no longer available from ACL was ---RB5297 and was machined to size Other manufacturers use this same number but there bearings are typically the .020" undersize and would need machining I've had a look on the ACL/Taiho website and can't find 1500 midgets and also the part numbers are different to ACL's old Nos. but they do appear to have semi finished bushes in some sizes So -All up I'm afraid I can give you not much help---I can chse it up further through ACL if you want but I don't think there is much hope there willy |
William Revit |
This thread was discussed between 14/02/2018 and 21/02/2018
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