First you should try hard to find some ready sized tube, it may be available if your lucks in. The size of drill you need would be likely to be available in around 80mm long series which would need drilling from both ends and if you really wanted one to reach 150mm it would be an expensive special. I could have a look to see if we have something at the museum which would do the trick tomorrow, we do have a huge number of extra long drills which have been gifted to us.

A reamer that size might work, but increasing the bore 0.5mm, 0.0195″ would be hard on a reamer three times the diameter.

Drill Service have a 4.5mm extra long series drill with a flute length of 150mm which is not overly expensive:

https://www.drill-service.co.uk/products/drills/extra-length-drill/del-hss-drill-extra-length/

But discussion of the various methods is a bit pointless (pun intended) until the OP can describe what he means by “quite accurate”. What needs to be accurate; the finished diameter, concentricity with the OD, surface finish or staightness?

Julie

Addendum: Peter types faster than me!

I think a 3mm dia by 150mm long boring bar might be a tad too flexible to mount between centres…

Can you modify the design so the assembly does not rely on the truth of the full length of the tube?

There are drills designed for making gun barrels – try one of those. In fact 4.5mm = 0.177 inches…

On 29 October 2025 at 22:20 duncan webster 1 Said:

 Might be worth trying a D bit made from 4.5mm silver steel, but there is a fair chance of it grabbing.

This could be worth a try, when you harden silver steel it “grows” very slightly if you polish the un-hardened part to reduce the size slightly and run slow with plenty of oil it may work.

Andy

OK…. for a hudraulic cylinder try to source thick wall steel tube ( Metal supermarket – usual diaclaimer do this material – I recently purchased 1/2″ bore 1″ OD thick wall from them ). and if you need to use a slightly different diameter and re make the piston to suit, much easier.

If you must drill… Try this.

Turn down the end of the drill and thread it and make an extension shaft. Instant long drill. Drill in stages clearing the flutes every couple of mm of depth once the flutes are hidden in the bore. I have drilled out a 3 foot long tube that way many years ago.

Drill a fraction under size then make a ‘long reamer’ as per above. Then ream the bore the same way.

THEN –  whether you drill it or get ready sized drawn tube you must to this…..

Hone it / polish it.

Make up a simple hone using smooth wet and dry abrasive wrapped round a long bar, tack the end with tape and wrap it around a few times making it a snug fit in the bore. lube with light oil and polish the machining marks, a brisk-ish in and out motion during honing helps to average out the asperities and produce a nice smooth bore.

Have done it many times to make longer cylinders and it works a treat.

It might be a lot easier to find some tube of about the right size and make a new piston. Macc sell 6mm od 5.1mm id brass.

Lots of the above suggestions will work on bigger tube, but this is very small

An alternative might be two concentric tubes.
Here’s a couple of possibilities.

Bill

Assuming you are using a lathe, I think you are liable to find the bore becomes slightly oversize, especially near the ends, due to slightly imperfect alignment of headstock, workpiece bore and drill.