Diff. identification

[Einar]

I've just taken out the diff. of my Vitesse due to awful noises - probably need a new one. I positively know that the diff is not an original Vitesse diff. I recall that the previous owner told me it was a Spitfire 1500 diff. The number underneath is FR 3978. Can anyone verify that this is the original diff. to Spitfire 1500, ratio 3.63:1. The diff has only 4 studs to fit trough the swing spring (not 6 studs)

[heraldcoupe]

The FR prefix means it's a Factory Reconditioned unit, the number won't mean anything in terms of the vehicle it's intended for. Back to first principles I'm afraid, count the number of turns in and out,
Cheers,
Bill.

[Einar]

OK. A newbie question  :-/ How do I count turns? Can you explain?

[GT6Steve]

Basically it means to turn the input shaft one full turn and count the number of rotations of the output flange.  Clamp off one output flange so it doesn't rotate to confuse the count.  If you don't feel comfortable interpolating the output from a single rotation you can turn the input five or ten full turns and then do the math.

For example, If I turn the input one full turn and the output turns a bit over 3 times I can see this will be a 3.27 diff.  A bit over 3 and a half turns out will suggest a 3.63,  Not quite four full turns out will be a 3.89 while a bit over four turns suggests a 4.11.

If you turn the input ten turns and count a bit over 36 turns on the output a division by ten will give you a 3.63.

Make sense?

[CharlieB]

Steve I think you have that backwards?

For the output shaft to turn one full turn you will need to turn the input shaft 3.27 times for a 3.27 diff, 3.63 times for a 3.63 diff etc

Also if you clamp one output shaft the other will turn twice as much.

[Einar]

Thanks - just what I observed: It almost make sense, however I observe the opposite - I rotate the drive flange (input?) a bit more than 3.5 times then the output shaft & flange turns 1 time??? This is if I do not fix one of the output flanges! If I fix one of them, the output turns almost 2 times, i.e. the half of 3.63.....!?? Anyway I think I've identifed a 3.63 diff

[CharlieB]

So to clarify, turn both output shafts together one full turn & count the number of turns of the input shaft. 3 1/3 turns = 3.27 diff, 4 & a bit = 4.11 etc.

[Einar]

Before I may change the entire diff I would like to inspect it a bit. I’ve taken the diff out of the car and believed to identify the noise in the region of the drive flange, i.e. the front of the diff. Is it easy (and possible) to dismantle the diff in the front to check for any wear in i.e. bearing at the drive flange end?

[CharlieB]

I don't know much about diffs but I do know the 1500 diff has a collapsible spacer & if you dismantle the front flange you have to put the nut back on exactly the same number of turns as it came off. So mark the nut & flange & count the turns to remove it.

You might as well replace the oil seals before you fit the diff.
Edit: Just re-read your post, you're taking it out not putting it in! (shake it all about!)

[GT6Steve]

Thanx Charlie, you're right.  I knew something seemed wrong when I wrote it.  I just do it and never think it through what I'm doing at the time ::)

At least it got sorted in the end and Einar has his answer

[Pete Lewis]

if the input is noisy then it could be  the main thrust bearing on the pinion has failed you will need to reset the pinion mounting distance to suit the new bearing thickness, if youre unsure about ratio checking then dont attempt to re build your diff   it is specialised ....  the outer bearing for overun thrust is easier as it does not affect the dimension settings of gear mesh,,    but the problem of pre loading and spacers will be there,   two choices really   either lucky or a  nightmare  
have fun  Peter  

[Einar]

Thanks! No, I will not rebuild it myself - the only thing I will check is just the input bearing. According to drawings this seems accessible without dismantling the diff.

[herald948]

heraldcoupe wrote:

The FR prefix means it's a Factory Reconditioned unit,...

It's exceedingly "rare" that I would question Bill, but I find on the Web that both Club Triumph and Amicale Spitfire note the use of FR as the prefix for the Spitfire 3.63:1 diff.?

[Deleted User]

FR is indeed the prefix for the Spitfire 1500 diff.

Not all 3.63 CW&P's use collapsible spacers, beware!

[heraldcoupe]

My mistake, too much time spent around FR engines!
Soprry for misleading anyone,
Cheers,
Bill.

[Einar]

No problems - I've identified my diff, and learned how to detect the ratio and got some advices of what I should look for regarding the failure ;)