Main and big-end bearings

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Main and big-end bearings

Postby landy » 14 Mar 2020 18:58

Hi all,

I currently have my Strada engine apart, mainly to sort out a gear selection problem. While inside I thought it as well to check the crankshaft etc. This engine was last apart about 20k miles ago and had the crankpin ground to suit 0.2mm undersize shells. The timing side main journal was good and a new standard bush fitted and was within limits.

Everything is pretty clean, very little muck in the sludge-trap and the crankpin and journal look fine. The white metal bearings all feel good, but as the white metal faces show some marking, I thought I'd fit new ones. Now the problems. The new standard main bearing bush is a 'rattling good fit' before even being pressed into its housing. The old one was made with a split which closes up when pressed in, the new one is a complete joined circle, so unlikely to close up and become a better fit. My only options seem to be to re-use the old one, or have the journal ground. I'm waiting to receive a new undersize bush and hope it is actually smaller than the journal.

The new big-end shells - 0.2mm undersize to suit the crankpin - are a slightly larger radius than the old ones and have to be sprung into the con rod and cap. One end of the shell then sits proud of the rod or cap and when assembled onto the crankpin, the rod binds up tight even with the nuts done up just finger tight. I suspect that because the shells don't fit the rod and cap properly, they are forced 'out of round' when the big-end bolts are tightened. I can't see an answer to this, apart from refitting the old shells as, if the new ones aren't truly circular then grinding the crankpin again won't help.

Lastly, the oil pump 'green' bushes now supplied are just a small neoprene tube and, unlike the originals, not a particularly tight fit in either the oil pump or the outer cover. Hopefully this won't cause oil pressure problems.

If anyone has any comments or suggestions about the above issues, I'd be grateful of your input. Many thanks, Ian
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Re: Main and big-end bearings

Postby George 350 » 14 Mar 2020 23:21

Hi,
Bearing shells: slight scoring marks are perfectly normal. Have you measured the ones you had fitted to see if worn? Might still be perfectly serviceable.
If you do a search on here (or it may have been google!) I'm sure someone posted the info as to the various bearing shells being Fiat parts. Might be a way forward for you should you need new shells.
As to the neoprene tube, I superglued mine to the oil pump after suffering the disaster of it not staying in place on a rebuild. Done lots of miles since so must be ok.
Hope your rebuild goes ok,
George.
George
350 sport 1978, 350 Strada 1978
650 Norton and 650 Enfield
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Re: Main and big-end bearings

Postby 3potjohn » 15 Mar 2020 09:04

I replaced my shells in 2016 and seem to think i used plasti-guage Should have photographed the process but there it is.
I think that at the time my neoprene seal was OK but if I did find a very loose fitting one I might consider a very thin wipe of three bond 1184 or similar around the periphery of a dry seal but that would have to avoid any excess at all.
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Re: Main and big-end bearings

Postby landy » 15 Mar 2020 11:08

Hi, thanks for your thoughts.

Unfortunately I haven't been able measure the original big end shells, a digital vernier really isn't precise enough for the small clearances involved. By feel and the lack of wear on the outer edges of them, I'm happy to reuse them. Looking more closely at the new ones, they are definitely out of round. The 350 con rod has a bore of 33mm when torqued up without the shells in place. The new shells vary from 34.5mm along the joint axis to 32.5mm at 90 degrees to it - the originals are 33.05mm all round.

The only reference I can find about Fiat shells is for the 500 which I think has a bigger crank pin and con rod eye.

Thanks also for the tips about the neoprene bush, I imagine it gets slightly compressed on length when the outer cover is on, which can only help.

Ian
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Re: Main and big-end bearings

Postby brianh0_4 » 16 Mar 2020 10:48

Ive just fitted big end shells, and wondered too what was wrong. Really jammed on the big end journal, so went up to full torque, and they seated properly in the con rod, and all was perfect.....
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Re: Main and big-end bearings

Postby EVguru » 16 Mar 2020 11:03

Paul Compton
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Re: Main and big-end bearings

Postby mad muller » 16 Mar 2020 19:52

had similar issues when doing engine rebuild , when end caps fully tightend, the con rods were grabby even after a reground crank had been done to spec , used very fine emery paper and oil; to take a bit off the crank pin, you take your time on this bit and remeasure by bolting up the rods with the shells fitted, keep the crankpin perfectly round by counting your passes with the emery paper and turn the ceank as you go , use plastigauge to measure clearance fully torqued up , i went with two thou imperial for the clearance being old school , all this came soon after i bought the bike and is still on the road in use ten years later i may add with great help from club members , hope this is some help.
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Re: Main and big-end bearings

Postby landy » 17 Mar 2020 12:34

Thanks for the replies, interesting and encouraging.

However, assembling the big-end with the new shells and fully torqueing the nuts, the rod is absolutely solid, can't budge it at all. After some careful measurement, the crank pin measures at 29.85mm, the new shells torqued up in the bare rod are between 29.65 and 29.75. Probably explains why it's tight.

So it looks like if I use the new shells, the crankpin will need a very slight grind to suit. Maybe the previous shells aren't quite 0.2mm undersize and the crankpin was obviously ground to give the correct clearance with them.

Thanks again for your assistance, Ian
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Re: Main and big-end bearings

Postby EVguru » 17 Mar 2020 12:42

I think at one time, NLM had shells remanfactured and they were done in imperial sizes with an unofficial 3rd undersize.

I sold Stewart a set of original factory shells (still in their blister packs) so he had a set as samples to get another batch of shells made.

It's best practice that Journals are ground to suit supplied bearings and cylinders are bored to supplied pistons.

We no longer have the luxury of OEM parts, so the numbers in the book cannot be slavishly relied upon any more.
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Re: Main and big-end bearings

Postby landy » 22 Mar 2020 14:36

That sounds quite possible. I think the last shells I bought from NLM in 1999 had been manufactured for them. The first set sent to me had been made oversize in error, rather than undersize and had to be replaced.

Can anyone provide the nominal new diameter of the timing side plain bearing journal please. Mine is 46.35mm, which I believe has never been ground.

Many thanks, Ian
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Re: Main and big-end bearings

Postby EVguru » 22 Mar 2020 14:56

landy wrote:Can anyone provide the nominal new diameter of the timing side plain bearing journal please. Mine is 46.35mm, which I believe has never been ground.


That equates to 1.825" which sounds right. Yes, just found a handwritten note in an old Bluebook copy with that dimension.
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Re: Main and big-end bearings

Postby mad muller » 25 Mar 2020 15:31

Found box that replacement big end shells came in, from nlm they were made by king bearings 0-20 metric size and had the crank ground to suit but still had issues as already mentioned. Hope this is of some help.
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Re: Main and big-end bearings

Postby EVguru » 25 Mar 2020 15:34

mad muller wrote:Found box that replacement big end shells came in, from nlm they were made by king bearings 0-20 metric size and had the crank ground to suit but still had issues as already mentioned. Hope this is of some help.


To suit the stated undersize, or to suit the actual bearings?
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Re: Main and big-end bearings

Postby mad muller » 25 Mar 2020 15:46

Yea prob they were ground to bearing size stated on box and they took that amount off the crank and never tried fitting them you are right in what you say I then had the fine tune myself as I didn't think much of the place I took the crank to they were car not bike engineers you live and learn.
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Re: Main and big-end bearings

Postby landy » 09 Apr 2020 09:11

Hi all, thought I'd post an update on this as I have the engine rebuilt and back in the frame. Paul's youtube videos were invaluable and enjoyable viewing.

The crankpin required only a very light grind - approx. 1.5 thou - to give the correct clearance to the big end. The new main bearing bush measured at 0.5mm smaller than the nominal standard for the journal, so the journal was ground to suit, i.e. approx. 20 thou.

Having spoken to Andrew Jones at Mdina, he only stocks two sizes of main bearing bush, which are listed as standard and undersize. From my experience, the standard bush is bigger than a standard unground journal and therefore unusable. The undersize one is -0.5mm, so any cranks needing to be ground would have to be to that size and effectively means that one life of the journal is lost as the -0.2mm bush is no longer available. After that, I guess the only option would be a roller bearing conversion.

Thanks for your thoughts and guidance along the way. Ian
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