1957 Colchester Spindle Adaptor to accept MT3 ?

The usual comment is that the spindle is "nearly but not quite" a #5 Morse .
Thank you for this detail. I will try to figure out how much the error is and if there is anything I can do about it.
A the difference in angle is about .92 arc minute. That predicts a difference of .0016 inch radius at 6 inches.
(I just drove to 3 auto parts stores - no Prussian Blue. )
I miss read the specs. This is wrong, see below.
 
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Thank you for this detail. I will try to figure out how much the error is and if there is anything I can do about it.
After some experimenting - This is confusing. The MT3 taper is 1:19.922 while the MT5 taper is 1:19.002. This says the MT5 is steeper, i.e. larger than the MT3. That is not what I am experiencing. I believe the large end is contacting first. Given the above the small end should contact first.
Thoughts on how to diagnose this?
(I just drove to 3 auto parts stores last evening - no Prussian Blue. I found some at a fourh store. )
 
If the taper was originally MT5 or supposed to be MT5, then either the maker got it wrong (which would agree with "the usual comment"), or someone tried to use a taper adapter and a boring bar to clean it up. You might look into trying to borrow or rent a taper reamer. Or the adapter is wrong.

And generally, MT sleeves for lathe spindles do not come with a tang. That will prevent you from feeding stock through the spindle. If that ever becomes an issue, I would just cut off the tang and make an aluminum remover bar.
 
I coated the inside MT5 of the spindle with Prussian Blue and inserted the MT5 to MT3 adaptor. Upon removal the adaptor was evenly coated with a uniform sheen of blue. There were no obvious high or low points. I repeated this 2 more times until there was insufficient Blue left on the spindle. I do not believe there is an issue with this spindle - adaprot pair.
As a point the spindle is only tapered for about 1.9 inches, that should be sufficient for small diameter work.
Follow on: I was watching a video on taper grinding. The technique is to make a very thin line of Prussian blue on the tapered part and then carefully rotate the part in the bore. The trick is to have a uniform friction fit. The good news is the new MT5-MT3 adaptor male to lathe spindle have a very good fit. The bad news is the male taper on the M3 collet adaptor that fits in the lathe adaptor has a terrible fit.
(I checked the MT3 female taper in the MT5-MT3 adaptor with another MT3 male collet and it had a great fit. )
I am now in search of a better MT3 - ER-32 collet holder.
Hopefully this Prussian Blue trick will help someone.
 
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You should be able to make a spindle adaptor from a MT 5 reducing sleeve ......the original spindle bush is 5 to 3mt,but some say its not an exact #5 .....anyway ,first step is to get a #5 sleeve and check it in your spindle ......the sleeve will be too long ,but thats not important,you can cut it to length ......you willl also need a bit of (preferably) bronze/brass bar to knock out the cutoff sleeve....I had heaps of old Master bits ,and was using the bushings as spacers for hold downs on the radial drill.....but where are they now? I may still have them ,but have recently scrapped 40 tons of old stuff as the land is sold.
Thank you. Yes, you are correct, Colchesters are typically not an MT5, they are MT 4 1/2. A 5 will fit and lock but it requires additional force to seat firmly. I am now on to the problem of the best way to adapt what is most likely a real MT5 to an MT 4 1/2. I do not have a grinder, but I do have a taper attachment. I was thinking of 600 grit emery cloth. The MT5 is larger at the small end, I was planning on 'counting' down, 10 seconds on the bottom fifth, 7 1/2 seconds on the next fifth, 5 seconds, and 2 1/2 seconds. I would not touch the large end, my calculations are I only need to take off between 1 and 2 thousands. I would then smooth with 1000 grit and test with Prussian Blue.
 
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