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CAUTION with sizing your shell bearings on Crank - don't go too tight.

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400 Cafe Racer:
Hi Guys,
I mentioned when congratulating Nurse Julie on her Engine Rebuild thread, and on my "Splitting the crankcase problem" thread, that I was was going to investigate a mechanical "Screech" sound that lasted for several seconds after resarting the bike while it was warmed up after my second ride.

My hunch from my experience, was that it sounded like the main bearings were tight and could seize up if not rectified.

Recent History
The PO had carried out a full engine rebuild with the majority of work done by an engine specialist (THIS WAS NOT TRIG AND JULIE I must emphasise), just leaving the PO to do final assembly. This work was done in 2015 and the engine has run 3000miles since. Advice on which shell sizes to use on which journals was made by the specialist.

Having experienced the noise I consulted the PO who thought it might be the timing chain adjustment, but this "Screech" sound was far too solid and friction sounding to ignore. I decided NOT to run the engine again and do a full strip and inspection.

Having just split the casings and inspected the journals and shells it looks as though the centre bearing has been the main offender with journals 2 and 4 not far behind. See photos. In the order of 1 to 5.

It was noticeable that 2,3 and 4 had little oil film on the crank or the shell (oil squeezed out due to lack of clearance)and you will see the highly polished face on these journals and shells. Compare these with 1 and 5 which are relatively unpolished and wet with oil, showing  that a good hydrodynamic film of oil was  present to "float the bearings" 1 and 5 rather than grip them.

I think this has only shown up because of the heat sinking and oil draining from the heads and barrels after a ride causing the extra heat transfer and differential expansion on these components at the time, and the fact I started the engine again shortly after.

Journals 2,3 and 4 will now come under closer scrutiny with Plastigauge measurement to see the actual clearance measurement.

To be continued as time allows.

Dennis

PS my next post will show the Shells 1 to 5
 

400 Cafe Racer:
Shells 1 to 5

K2-K6:
Have you inspected the camshaft bearings yet?

400 Cafe Racer:
Hi K2-K6,

Yes thanks,
The camshaft bearings, Camshaft, Cam followers/rockers and all valve gear all appear to be near pristine condition. I think the lubricity of the oil along with regular changes have resulted in no replacements needed, as was the case when the engine was rebuilt by PO. The only minor flaw I can see fnom initial inspection is that when the cylider head cover was refitted at some previous point, the operative did not engage the teeth of the rev counter drive gear by "feel" while progresively tigtening down the cover. This has resulted in some minute chipping of the camshaft teeth edges of the worm drive. This looks as though it was dressed off and has been used without further problem. There are no marks on the cable drive gear.

I believe one of the main reasons for rebuild at that time was someone before had insalled an oil scaper ring badly, this caused a wear groove in the bore, which resulted in a new liner being fitted on the rebuild by PO.

All pistons are marked HONDA.

Thanks

Dennis

400 Cafe Racer:
Hi Oddjob,

Thanks for your input and the important points you make, which I will check out. I will possibly get time tomorrow to lift out the crank and do an inspection behind the shells to examine further.

In general everything seems very clean internally with no particulate matter visible. No bits in the oil filter or pump pick up screen. Pistons seem rather sooty, with just a hint of slight leanness (white carbon fringe) on the edge of No1 Exh valve edge and centre electrode. There is probably some fine tuning I need to do later. In general it seems to be set up slightly rich and was running between 12.4 and 12.7 A/F ratio on the Inovate A/F gauge.

During the "Screech" time you could feel the vibration "grab" of the crank on the shells. Luckily it has not resulted in  spinning a shell. I shall measure up the crank and examine closer when I get more time to fully assess if there is any refacing needed. Everything will get a detailed assessment in due course as I want make sure all is order when I rebuild.

Due to family circumstances I am coming to a busy period and it may be several weeks before I can get back to it, so apologies in advance for delay on the findings or outcome.

PS
I have decided not to lift the crank out at present and disturb any "Evidence" associated with the cause, but rather do the Plastigauge measurement with everything "as is" to capture true measurement of the clearance before anything is moved. This should help identify what clearance currently exists on a cold engine and how much thermal expansion has caused the crank to tighten. I believe that this could be quite valuable for future reference. Given the configuration of the engine design, the 2 inner cylinders on the air cooled block are always going to run hotter.

Thanks

Dennis

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