12 replies to this topic

[the boy howard]

Im currently building a 1380 engine and ive fitted the crank and thrust washers but i dont seem to have any movement in the crank, im told that i should have upto 6thou?
The crank spins freely without any binding to speak of, can i just let this go do you think? Im just going by feel rather than a dial guage.

cheers

Lee

[mini93]

what are you using to push the crank back and forth? it will still require a bit of a push really, screw driver between crank and a bearing carrier is how mine was done

[adcyork]

6 thou is a very small amount of movement to judge by feel. It is also a very inaccurate way of checking the end float. You do need about 6 thou to allow for the crank flexing as it turns.

Invest in a dial guage, it'll come in handy when you need to do the cam timing later on.

[Turbo Phil]

You want 3-4 thou end float ideally, some people use a bit more if it's a real screamer. Measure with a feeler blade between the web of the crank & the thrust washer, there needs to be some clearance. If there's non at all you need to make some by thinning the thrusts down slightly by rubbing the back of it on some fine wet/dry on a flat surface.

[Cooperman]

With a new engine aim for about 0.002" up to 0.005". Over that use the slightly thicker thrusts to bring it down again. Those thrust bearings are a weak point on a Mini with a heavy-duty clutch which is why you should never start any Mini with the clutch pushed down or sit with the clutch down and rev the engine.

[the boy howard]

ok, thanks for your replys everyone, very helpfull. Ive measured it with feeler guages inbetween the centre bearing and thrust washers using a screwdriver to lever the crank side to side. I can get 2 thou one side but nothing on the other so am going to wet and dry the back of the thrust washers to sort this.

cheers again!
Lee

[Cooperman]

If, with the crankshaft prised to one side you get 0.002" one side and nothing the other, then you have a total of 0.002" end float, which is exactly what you want to start with. Don't linish anything off the thrust bearings, they are fine as they are.

[the boy howard]

Aaah right, i havn't rubbed them down yet. I can only get the reading of 0.002" on one side even if i use a screwdriver to lever the crank the other way and try the other side i cant get the feeler guage in. The crank seems to turn ok, as you would expect really, no real tightness. I presume that after a bit of use the thrust bearings "bed in" and it will be ok?

Lee

[bmcecosse]

I say 2 thou is NOT enough - for a 1380 engine which is presumably going to make a bit of power. Get it up to 5 thou!

[Cooperman]

Sorry to disagree.
The power has nothing to do with the effect of the end float. A 1380 won't rev any higher than a 998, and that could be the only issue. Heat/expansion is not an issue as the engine is 'all-steel/cast iron' so the expansion rates are effectively the same for crank and block. Now, if you had an all alloy block, that might be different as differential expansion between the steel crank and the alloy block could be an issue due to the aluminium across the main bearing support expanding more than the width across the crank webs.
I always set 0.002" float on my rally 1310 cc 'S' which revs to 7000 (in extremis, I hasten to add!). It's been done like this since 1995 and my earlier cars, going back to 1964 have always had a nominal 0.002" as the ideal, but minimum float. The engine is mstripped and checked regularly and after about 6 rallies i find the float has increased to around .004" to .005". New thrusts bring it back to 0.002" every time and with a nitrided crank crank wear is not an issue.
The old BMC Workshop Manual gives the end float as 0.001" to 0.005", so 0.002" is twice as much as specified by BMC for all Minis including the 1275 'S'.

[bmcecosse]

I stick with my view! It needs to be able to get some oil in there. Maybe the fact your engine wears out to 4 or 5 thou - it's trying to tell you something!

Edited by bmcecosse, 04 August 2009 - 09:04 PM.

[Cooperman]

It's telling me that with a full, heavy competition diaphragm and with lots of gear changing and 'spooled-up' starts on tests, the thrust take one hell of a bashing. Thrust wear has always been an issue on rally Minis with comp clutches.
So long as you don't start a Mini engine from cold with the clutch down, plenty of oil gets in there with 0.002" float. It's fed from centre main which is awash with oil, as well as the oil spray from the crank rotation as well.
The problem is not the clearance as a geometric figure, it's the end load which squeezes the oil out from the thrust loaded side. It doesn't matter whether you have 0.001" or 0.005", when you push that heavy clutch down, you'll squeeze a lot of oil out from the thrust face side and that's how the wear occurs.

[mini7boy]

It's telling me that with a full, heavy competition diaphragm and with lots of gear changing and 'spooled-up' starts on tests, the thrust take one hell of a bashing. Thrust wear has always been an issue on rally Minis with comp clutches.
So long as you don't start a Mini engine from cold with the clutch down, plenty of oil gets in there with 0.002" float. It's fed from centre main which is awash with oil, as well as the oil spray from the crank rotation as well.
The problem is not the clearance as a geometric figure, it's the end load which squeezes the oil out from the thrust loaded side. It doesn't matter whether you have 0.001" or 0.005", when you push that heavy clutch down, you'll squeeze a lot of oil out from the thrust face side and that's how the wear occurs.

this is yet another case where just because someone got away with something unsound, it doesn't mean others should try it.

Believe it or not, A-series cranks flex at higher RPMs such that the side and thrust clearances are "used up" when the crank flexes. Too allow for this crank flex, much more than .002 crank end float is required.
I would never use less than .005 on a fast road car and was told by an ace Mini race engine machinist and engine builder to use closer to .009 in my race engine. This is how I set up my engine.
Rod side clearances should also be set up generously, again to allow for crank flex at high RPMs.

A three main bearing crank in a long stroke engine causes a lot of crank flex. It requires special attention because of this.

Knowledgeable Mini drivers learn to minimize the time spent with their foot on the clutch pedal so as to preserve the thrust bearings and the crank thrust surfaces. ESPECIALLY with stiffer clutch diaphragms.