kyamon · Apr 13, 2020

Hello everybody - this is a question regarding a 6-cyl engine in a 1974 109. I had it with a mechanic recently, and when he started the engine he immediately said "oh!" and told me there was a noise that was not supposed to be there, apparently a low rumble when idling - this is the only 6-cylinder of this type I have ever heard, so I don't actually know what noise he is talking about...
Anyway, he said that the crankshaft needed servicing, the main bearings for sure and possibly more. I decided to have a look myself, at first without removing the engine. With the oil sump gone I measured the end floats between the connecting rods and the journal shoulder (all 6 within specs), and then the end float of the crankshaft itself. I obviously could not do this accurately, but using feeler gauges I found that it may have about 0.002 in over tolerance (0.008 in instead of a max of 0.006). Is this alarming, should I take it out and add the proper thrust washers? Or is it actually tolerable despite the specified tolerances?
All advice appreciated!

Dippypud · Apr 13, 2020

kyamon said:
Hello everybody - this is a question regarding a 6-cyl engine in a 1974 109. I had it with a mechanic recently, and when he started the engine he immediately said "oh!" and told me there was a noise that was not supposed to be there, apparently a low rumble when idling - this is the only 6-cylinder of this type I have ever heard, so I don't actually know what noise he is talking about...
Anyway, he said that the crankshaft needed servicing, the main bearings for sure and possibly more. I decided to have a look myself, at first without removing the engine. With the oil sump gone I measured the end floats between the connecting rods and the journal shoulder (all 6 within specs), and then the end float of the crankshaft itself. I obviously could not do this accurately, but using feeler gauges I found that it may have about 0.002 in over tolerance (0.008 in instead of a max of 0.006). Is this alarming, should I take it out and add the proper thrust washers? Or is it actually tolerable despite the specified tolerances?
All advice appreciated!

The crankshaft endfloat can be measured with a feeler gauge, I've always done it that way, just make sure the crank is pushed as far as it will go one way and measure the clearance between thrust washer and crank on the other side with a feeler gauge ...

kyamon · Apr 13, 2020

Thanks - that is precisely what I did. I pushed it all the way back and measured the gap at the rear of the crankshaft, then pushed it forward and measured again. I get a value that way, but it is not very accurate - either way, it clearly seems more than 0.006 in and I wonder how much of a problem that is...

Dippypud · Apr 13, 2020

kyamon said:
Thanks - that is precisely what I did. I pushed it all the way back and measured the gap at the rear of the crankshaft, then pushed it forward and measured again. I get a value that way, but it is not very accurate - either way, it clearly seems more than 0.006 in and I wonder how much of a problem that is...

That I not sure a grown-up will wake up soon and come to help ...

@jamesmartin

Moron · Apr 13, 2020

kyamon said:
Hello everybody - this is a question regarding a 6-cyl engine in a 1974 109. I had it with a mechanic recently, and when he started the engine he immediately said "oh!" and told me there was a noise that was not supposed to be there, apparently a low rumble when idling - this is the only 6-cylinder of this type I have ever heard, so I don't actually know what noise he is talking about...
Anyway, he said that the crankshaft needed servicing, the main bearings for sure and possibly more. I decided to have a look myself, at first without removing the engine. With the oil sump gone I measured the end floats between the connecting rods and the journal shoulder (all 6 within specs), and then the end float of the crankshaft itself. I obviously could not do this accurately, but using feeler gauges I found that it may have about 0.002 in over tolerance (0.008 in instead of a max of 0.006). Is this alarming, should I take it out and add the proper thrust washers? Or is it actually tolerable despite the specified tolerances?
All advice appreciated!

The .002 you're quoting is nothing to worry about, But main and/or big end bearings could be the cause of your noise.......as could many other things

kyamon · Apr 13, 2020

Moron said:
The .002 you're quoting is nothing to worry about, But main and/or big end bearings could be the cause of your noise.......as could many other things

Thanks - that is a relief. I was planning to replace the big end bearings, and while it is open I might as well put in new ones for the big ends, I guess. I can't really further specify the noise since I don't know what it was... Hopefully I will hear an improvement after these replacements, but otherwise to me it just remains one of the many noises the car makes

jamesmartin · Apr 13, 2020

kyamon said:
Thanks - that is precisely what I did. I pushed it all the way back and measured the gap at the rear of the crankshaft, then pushed it forward and measured again. I get a value that way, but it is not very accurate - either way, it clearly seems more than 0.006 in and I wonder how much of a problem that is...

8 thou isnt an issue and whats oil pressure like

kyamon · Apr 13, 2020

jamesmartin said:
8 thou isnt an issue and whats oil pressure like

Oil pressure looks good. I had a period with excessive oil consumption but that seems resolved now (nevertheless, replacing the valve guides is high-ish on the list).
The bottom of the oil sump was covered in goo, but I guess that is to be expected after (presumably) 45 years. I did not see anything that worried me, no chunks of metal or anything

kyamon · Apr 13, 2020

Are RTC2993 (with appropriate oversize) the correct con rod bearings for this engine (for some reason that page is missing from my parts catalogue)?
I have RTC1720 for the main bearings

jamesmartin · Apr 13, 2020

kyamon said:
Oil pressure looks good. I had a period with excessive oil consumption but that seems resolved now (nevertheless, replacing the valve guides is high-ish on the list).
The bottom of the oil sump was covered in goo, but I guess that is to be expected after (presumably) 45 years. I did not see anything that worried me, no chunks of metal or anything

you could allways remove a con rod cap and main cap to check bearing condition whilst sump is off

kyamon · Apr 13, 2020

jamesmartin said:
you could allways remove a con rod cap and main cap to check bearing condition whilst sump is off

Yes, that was my plan. That part seems rather easy, and if I remove them I can check if I need oversized bearings in case I order new ones.
My guess is that they are still the original ones (car has 36k on the odometer).

jamesmartin · Apr 13, 2020

kyamon said:
Are RTC2993 (with appropriate oversize) the correct con rod bearings for this engine (for some reason that page is missing from my parts catalogue)?
I have RTC1720 for the main bearings

rtc1720 is the main bearings and rtc1721 conrod bearings

jamesmartin · Apr 13, 2020

kyamon said:
Yes, that was my plan. That part seems rather easy, and if I remove them I can check if I need oversized bearings in case I order new ones.
My guess is that they are still the original ones (car has 36k on the odometer).

you cant fit a plus size unless crank is ground to suit ,bearings will be stamped on the back if under size

kyamon · Apr 13, 2020

jamesmartin said:
rtc1720 is the main bearings and rtc1721 conrod bearings

Thanks!

kyamon · Apr 13, 2020

jamesmartin said:
you cant fit a plus size unless crank is ground to suit ,bearings will be stamped on the back if under size

My assumption was that if I take them out I can tell (from a stamp) what size I need, is that also what you are saying?
Is Craddock a good place to order from?

jamesmartin · Apr 13, 2020

kyamon said:
My assumption was that if I take them out I can tell (from a stamp) what size I need, is that also what you are saying?
Is Craddock a good place to order from?

yes,if you do remove any put a pic up if you wouldnt mind

kyamon · Apr 13, 2020

jamesmartin said:
yes,if you do remove any put a pic up if you wouldnt mind

Will do - thanks for your help!

kyamon · Apr 13, 2020

jamesmartin said:
yes,if you do remove any put a pic up if you wouldnt mind

I don't know, but my feeling is that these guys deserve to be sent to retirement

The stamps say
G AX 530361 for the main and
G BX 272075 for the con rod
is there a site to look up these codes?

jamesmartin · Apr 13, 2020

original part numbers from the factory were often different to the part numbers used when buying the parts as service items later they will just be std shells,they are worn smooth so arent knocking id just fit a new set on both

kyamon · Apr 13, 2020

jamesmartin said:
original part numbers from the factory were often different to the part numbers used when buying the parts as service items later they will just be std shells,they are worn smooth so arent knocking id just fit a new set on both

OK, I will get standard sizes for both.
Another rookie question - the nuts on the bearing shells should also be replaced, I assume. Are they ERC1027?

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