15 replies to this topic

[dsgoody]

I’m after a bit of advice; I’ve recently lifted out my engine (1275 A+) due to a gearbox fault. While the engine is apart I’m going to check the condition of the main and big end bearings. I’ve got a Plastigauge kit and I’m showing a clearance of 2.5thou on the main centre bearing. Could somebody advise if this is within tolerance?

 

If not; I'm looking at re-shelling it. How could I tell if the crank needs a regrind?

[MRA]

0.001" is acceptable 0.0025" is for a tractor...

 

0.0005" to 0.002" is factory

 

part 2....

 

measure at various points around the journal to see if it is round or tapered, then with a surface gauge test the surface finish to see if it falls within requirements.

 

I use a micrometer to 5 decimal places and a talysurf  for the surface finish.

Edited by MRA, 29 February 2016 - 05:35 PM.

[ACDodd]

The crank needs to be measured using a micrometer. Factory main sizes are 2.0011 to 2.0017", if its less than that it needs a regrind. In practice the big ends are usually the first to go out of tolerance. In performance engine building it is also acceptable to increase the running clearance a little. At 2.0005" the engine will still run ok but the life expectancy of the bearing will be less.
The inside diameter of bearings varies usually, 2.003 to 2.0035".
Your clearance would indicate the bearing and journal are likely worn.

Ac

Edited by ACDodd, 29 February 2016 - 05:38 PM.

[MRA]

As AC says above race bearings require more clearance, do not be tempted to think you have a race crank just because the clearances are larger, in a race engine with wide duration cam timing the engine has little power at lower revs so the load on the bearings is less and the extra clearance poses no issues, however on a road going engine you will want to be a little tighter as the engine will work from fairly low revs and the extra clearance will kill the crank and or bearings very quickly, this is because the oil pressure will not be high enough to support the difference in radii between the bearing / smaller crank journal combination.

 

The closer you get to the same diameter the better the crank is supported, but conversely you need the extra clearance for a faster spinning crank, but by then the oil has had time to build up a higher pressure and is more able to support said crankshaft.

[dsgoody]

Many thanks for the detailed response as always, much appreciated. Long story short the crank has seen better days.

It will be whipped out tomorrow and off to a local Mini expert who has some machining contacts. What's the going rate of a regrind these days?

[MRA]

the going rate can vary depending on where you take it, from as little as £7 per journal up to £20 per journal

[MontpellierVanMan]

Could the Usual Suspects help me with this one ?

I have an 1275 A+ 12H motor, ostensibly out of a 1989 Metro, and was checking the Mains and Big-End clearances with Plastiguage whilst fitting new standard shells.

The Big-Ends are visually good, and come in at 0.0020" / 0.0020" / 0.0020" / 0.0015" - which I understand is spot-on ? The Haynes gives 0.0015" to 0.0030", and our own ACD quotes .0025" as best.
The journal diameters measure up at 44.44mm to 44.45mm, albeit just with a reasonable-quality Vernier, which compares OK with the 44.44 - 44.46 given in the Haynes.

So far so good.

However the Mains - which look good too - throw up a width on the Plastiguage of 0.0020" / 0.0015" / 0.0020" which I understand is way over the 0.0005" to .0015" desired range and would suggest it needs re-grinding immediately - but ONLY on the Mains ?

They measure up at 50.83 to 50.84mm, again with the Vernier, when Haynes quotes 50.81 to 50.84mm, so that's OK, and in inches that equates to 2.001" to 2.002", when ACD quotes 2.0011" to 2.0017" as factory sizes, which makes sense and would suggest the crank was OK.

I'll blag a pair of better mikes this weekend, and look at the bores on the Mains, but to begin with -

1. Could someone confirm that there was no change to late A+ blocks/cranks and running clearances that would explain why I've got so much clearance on the Mains,

and 2. Is it a common occurrence to find a crank that has good Big-Ends but requires a grind on the Mains only ?

Edited by MontpellierVanMan, 11 August 2016 - 08:03 PM.

[ACDodd]

Use a mic. It is usual that big ends need grinding not mains.

Ac

[Spider]

Before we go too far, can I suggest you close the Haynes manual on this. It's clear to me that it's very incomplete in regards to this information.

 

From the factory and from around 1986 on, there was small variances in the Crank Mains and Big Ends..

 

Have a look on the crank webs for a dab of paint, there maybe none, or could be Red, Yellow, Green or Blue.

 

Have a look for that and we'll go from there.

 

While AC may have noted that the Big Ends wear, I've actually notice the Mains wear more than the Big Ends, due in part to oil being centrifuged away from the Mains and in to the Big Ends.

Edited by Moke Spider, 12 August 2016 - 07:49 AM.

[ACDodd]

The reason big ends need grinding more often is not due to wear is to do with the aftermarket bearings being slightly larger than the original factory shells. The aftermarket big ends suit a journal of 1.7504 to 1.7509" rather than 1.7497" to 1.7504" as quoted by the factory. Ergo if your journal measures 1.750" its too small and needs grinding to suit the new undersized shells. My prefered big ens size for a re ground road crank is 1.741". This is bigger than factory specs, but gives a good long lasting journal and bearing shell combination. I even re grind brand new cranks to suit the shells as the factory journals are too small. Clearances have to be rich for success.

Ac

Edited by ACDodd, 12 August 2016 - 09:15 AM.

[MontpellierVanMan]

Thanks for the quick input lads.

 

Measured by the local grinder (Mitutoyo) the Mains come up at 50.83 / 50.82 / 50.83 mm - bang in the middle of the original range.

 

Glyco-Mahle quote (their data-base, this morning) 0.02 to 0.63 for the Big-Ends, and 0.024 to 0.073 for the Mains. (that's 0.8 thou to 2.5 thou for the Big-Ends, 1.0 thou to 2.9 thou for the Mains.

 

Smaller on the Big-Ends than on the Mains, then.

 

Haynes say the opposite ............. 0.038 to 0.081 for the Big-Ends, and 0.017 to 0.058 for the Mains.

 

Smaller on the Mains than on the Big-Ends for Haynes.

 

Haynes, in whom I have zero faith, their proof-reading being sh-t and their lazy manuals worse than useless (plenty of MoT advice, but - gearbox ? "a difficult and invloved job for the DIY home mechanic").

 

If Glyco-Mahle are right, then, my 50.82-3 mm crank with new shells and 2 thou is OK.

 

However, as AC says, you get the impression the shells are "big" - you would expect new shells on a 50.83mm journal to be showing 1 thou not 2.

 

What does Our Man Down Under reckon ?

[Spider]

Working off Factory Info for the later (post 86) A+ Engines, Mains can be anywhere from 50.81 to 50.84 mm or 2.0005 to 2.0017", so a 0.0012" variation however these are 'classed' and colour coded in selective ranges, hence why I was asking if there's any coloured paint on the web(s) of the crank.

 

Given running clearance is from 0.0007" to 0.0023", a 0.0012" variance is significant and hence why it's so important to check these properly and with great care, not just for size, but also ovality and barrelling / hour-glassing.

 

Again, later A+ Big Ends are from 44.44 to 44.46 mm / 1.7497 to 1.7504", a 0.0007" variance and running clearance is 0.0015 to 0.0032", so less critical here, but all the same, needs to be checked carefully.

 

If the Crank is or is to be reground, then this is academic.

 

Now this might give some insight as to what AC has found in regards to the Replacement Bearings being 'tight'. A and A+ have different size cranks and not directly interchangeable, though sometimes can be done and gotten away with.

 

A Mains are 2.0005 to 2.001", running clearance is 0.0005 to 0.002"

A Big Ends are 1.7504 to 1.709", running clearance is 0.001 to 0.0025".

 

From what I've noticed, some of the after-market Bearing Manufacturers did not update their sizes from A to A+.

 

Like AC, I don't like to run 'tight' clearances. The factory machining of the blocks just isn't that good to start with and all it does is cook the oil and rob power.

 

For Mains I run from 0.0018 to 0.0021" Running Clearance and Big Ends from 0.0017 to 0.002" depending on what the Engine is intended to be used for.

 

While I think of it, IMO, the 0.002 to 0.003" End Float on the Crank is WAY too tight, I prefer to run 0.004 to 0.005". Again, the factory machining isn't good enough to run any tighter and is not easily corrected.

[Spider]

From the factory and from around 1986 on, there was small variances in the Crank Mains and Big Ends..

 

Have a look on the crank webs for a dab of paint, there maybe none, or could be Red, Yellow, Green or Blue.

 

 

Working off Factory Info for the later (post 86) A+ Engines, Mains can be anywhere from 50.81 to 50.84 mm or 2.0005 to 2.0017", so a 0.0012" variation however these are 'classed' and colour coded in selective ranges, hence why I was asking if there's any coloured paint on the web(s) of the crank.

 

 

Here's a new Crank I dug out. You can see it has Yellow dabs of paint on the Webs next to the Mains

 

 

So, the Yellow Paint tells me that i) All Mains on this Cranks are sized the same (Bonus!) and ii) that they are between 2.0013 to 2.0017". The Thickness of the respective Bearing Shells to go with this Crank would be 0.0713 to 0.0717".

 

Has your Crank got any similar paint dabs and what colour is it?

[MontpellierVanMan]

Being a f-wit, Moke, I think there may have been some vaguely olive-green paint that irritated me and got cleaned off.

Well-meaning, cack-handed ?

In reality, these would have been used to signal to assembly how to match cranks to shells - no ? Or to rods ? Otherwise no point in painting them ? That was how Ducati were doing it at about that time, either blue shells or red shells, chosen after the grind.

I didn't manage to erase the YGG stamps off the crank-casing though - yellow, green, green ?

I'm going to press on with assembly, by most authorities' reckoning the tolerances I've got are fine, if at the wider end, and it's not destined to ever do again the 200K km the previous incarnation did in my hands.

[Spider]

The stampings are indeed Crank Main Sizes. Y = Yellow (as you've worked out), sizes as above. G = Green, these were 2.0009 to 2.0013", with shells 0.0717 to 0.0721" thick.

 

Oh and apologise for the Imperial measurements, but I'm sure you can translate.

 

The Tunnels in the Blocks were (supposedly) the same size and by this time, they were hone finished, so there's a good possibility that they are pretty good, the sizing variance was only related to the cranks, which by this time, were outsourced to Mitchell Shakletons.

Edited by Moke Spider, 15 August 2016 - 08:09 AM.