Crank shaft end float question

[Dug]

In another post, someone was asking about clutch judder. They ran thru a few things but what caught my eye was the statement
"The crankshaft end float is 8 thou (i.e. within spec)"
Not wanting to change the path of the original question, this is being asked here.
Ok, here is one I can't resist asking a group type of question. "If" the acceptable end float of a 948cc thru 1275cc engine is 0.002-0.003 with full 360 degrees thrust washer contact at the main cap. How is 0.006-0.014-end float on a 1500 engine with only 180 degrees found to be ok?

Is this one of the main reasons the 1500 was prone to connecting rod failure? Is it also why the 1500's have such a history of spinning the thrust washers?
My feeling is yes. I’d be freaked with 0.008-end float.
Anyone of 1500cc knowledge know a reason why the factory manuals list so much clearance and range? And why when you look in a Spitfire manual the end float is listed as less than the end float given in most MG manuals for the same engine?
"dug"

 

[aeronca65t]

With the half-circle thrust washers, it's probably impossible to maintain the tighter clearances of the A-series engine unless you changed them often.
And on a street car, that's not practical, so they probably just spec'd bigger tolereances. That's probably the bottom line.

Even when new, the 1500 would "run" it's rod bearings if kept at top speed for over 45 minutes. It's a crappy, long stock engine intended to have torque and decent emissions (unlike the much more durable, older 1297 Spitfire engine)

In my 1500 racer, I swap rod bearing and thrust washers about every 25 hours and change the oil and filter after every race.

[I'm building a 1275 A-series engine to replace the 1500 this year]

 

[dklawson]

For the record, those I know who race 1275 A-series engines do not retain the standard 0.003" endfloat. They run closer to 0.006" clearance and their argument is that there is too much whip of the crankshaft and that motion is bad for the thrust bearings if the standard endfloat is used.

 

[Hap Waldrop]

The best way to keep the thrust washer in a Spit/Midget engine is to keep your foot off the clutch while sitting at redlights. On the Triumph race engine no one ever does anything special with the thrust washer in the Triumph engines becuase there no real need for the racer to use the clutch other than changing gears or starting off form the grid, and the engine are freshen up enough to just replace them. On the street, people have came up with some very clever idea, like pinning the thrust washer, or even milling the rear main cap to accept tow more thrust washer, but if you just not ride the clutch you'll triple their life.

On A-series and B series engines, race and street, the thrust washer don't get much of a workout, I hardly ever replace them in a rebuild, I always check the endfloat upon disassembly to see where they are at, and 99.9% of time they are right on spec, and get reused. Alot of the time one of the most silly things you can do on A-series engine is repalce the thrust washer, you end up creating a problem where none existed before.