Hitchcocks Forum

Hi everyone. I am replacing the big end in my 1990 350 Bullet. I live in Southern Ireland, the importance of this to this thread is, that I cannot find anyone to hone out and polish the fixed bush in the con rod to take the floating bush. Where in UK can that be done? There needs to be zero tolerance in the fit. Who has done this job and can offer direction and advice please. Thanks.

Al, I did this job as part of the rebuild of a 1955 350. Well my pal Chris did! He is not a trained engineer but learned how to do it by experience. Our rod was tight when the outer bush was pressed in and he used a mandrel (made of wood) and wrapped with very fine wet and dry (used dry) to hone the inner track. This may not sound very scientific but it worked a treat and he has used the same technique on many of his own bikes, pre War Model Js, and they get used hard. I asked about possible inaccuracy and he said there is that risk but care and consistency will pay off. The alternative is to ask T & L Engineering of Elstow near Bedford, UK to do it for you, I'm sure they can and I recommend their work. You can post it to them, of course. Contact, [email protected] or check their website.

REgards, Mark

Mark, many thanks, that is exactly the kind of thing I was looking for. Job done, as you say! We are very handicapped here in Ireland for all the historical engineering background inherent in UK bike history. Cheers. Al

Hi Al,

Why do you need to "hone" the bush exactly?

If you pressed the bush into the conrod by yourself,just make sure that you haven't cocked it over and put it out of round. It's much easier to get another bush, and try again!

If you know it's in round and you really want to deburr/polish it for what ever reason, I've used a soft scotch-brite wheel that's roughly the same diameter as the bush in my drill press. it may not be the most scientific way to do it, but the bullet 350 isn't a scientific engine!

Regards,
Nick

The reason you might need to hone the outer track (for those that are unfamiliar with this process,) is that if it is a tight fit in the rod, and they often are, the inner diameter will shrink slightly causing the floating bush to be tight or even not fit at all. If the outer track has gone all the way through the rod it's unlikely to be "cocked" but worth checking. The nylon brush treatment sounds like a good idea, Chris used the paper and mandrel because that's what he was taught by an old-timer and it does work.

REgards, Mark

Taking a few tenths (of a thou) off the inside of a bush is fairly easy. Take a wooden dowel, cut a slot down from one end and slip in some Emery cloth. Fit dowel into a drill (air drills are best) and spin the Emery inside the bush. Fine Emery well remove metal pretty slowly which is what you want. Just keep checking the fit and, if it gets hot, let it cool down before checking the fit again.

Oh, just realised someone already described this technique😀

Crankpin clearance - 20 ~ 37 microns = 0.00078" ~ 0.0014" (planely .78 ~ 1.4 thou)


Bush to conrod clearance - 57 ~ 82 microns = 0.0022" ~ 0.0032" (planely 2.2 ~ 3.2 thou)




Make sure that the 'eyes' in the flywheel are not tapered... ?


Many are!!! In which event an oversize crank-pin will be needed: 0.002" interference fit.

Nick: LOTS of the big-end eyes are Oval!!!! Replacement rod liners need MAJOR 'lapping' to bring into truth!




Al: Oversize crank-pins are not so easy to come by, (H do not stock them??) If you do end up requiring one I have them available from 0.002" - 0.012"


Std crank pin dia is 1.002 parallel to fit a 1.000" 'eye'. IF the eyes have fretted tapered, (Did you 'run' the main bearings too, if so the eyes WILL be tapered) then you will need to have the 'eyes' in the flywheels reamed true again. O/S pin size required being that finished diameter plus .002"

Wow that's crazy Tim!! why is this? something I'm definitely not used to building Lycoming aircraft engines for a living!!!