Within the community there is a wide variation of opinion about modifying bricks.  This topic takes up a thread that emerged in the MOCs discussion, which I felt it was better to continue separately.

I'd like to outline my personal reasons for I allowing myself to move from being dead against modding  to one who mods more than average.

1. In 1997 I was interviewed by TLG for a job as a Technic kit designer, having sent in my CV (resume) a while earlier.  One of the questions I was asked was whether I had ever modified any parts.  I took that question as an encouragement to do so.  I didn't get  job for 2 main reasons: firstly I needed an art and design degree and secondly I knew only a few words of Danish and my interview nerves made even those a struggle!  However, I took the encouragement home and that's when my modding career began.

2. TLG has a limited availability of moulds.  Moulds cost a lot of money to produce and they have to be used a certain number of times, which means there has to the be market for that many parts of each type.  A mould for some of the small, versatile parts I'd like to see (such as shorter Technic pins) would hold more parts than would a mould for 2x4 bricks, so the market for these parts would have to be great.  Technic is already not the best selling theme, so it is doubtful whether enough parts could be sold in sets to make their production viable.  This means that if I want a piece but TLG cannot make it for business reasons, I have to make it myself if possible.  This is OK for parts that can be cut down from others, but not for custom moulds, such as BBB train wheels.

3. Having modded quite a few parts, some of the ones I've made are so useful that I've made 50 and would never look back.  It's a choice between modding or being limited in what can be built.
a). The scenario of steam engine rods requires two liftarms to be attached, allowing them to pivot, with the whole thing being no wider than 8mm.  No current Technic pin is short enough but modded ones fit the bill.
b) 16x32 baseplates are currenly only available in sets.  I need 60 for my railway layout modules.  Cutting 32x32s in half not only makes the baseplates available, but it's cheaper than buying 16x32s at their former spares pack price.

4. My modding is not without rules.  I need rules to give modding a purpose:
a) The primary purpose of modding is to make useful parts that TLG should make but have not seen the light in order to make.
b) Since modded parts don't shine like normal parts, keep mods for aesthetics to a minimum.  I've modded a couple of tiles, but little else on the surface.
c) Never modify any piece that is essential for completing a set; if you ever have to sell the sets, you'll need the parts intact.  PaB and second-hand parts are fair game because second hand parts are sometimes damaged anyway, so you're getting a useful piece from a broken piece, and PaB and second hand LEGO has only the value you paid for the parts, no residual set value without instructions.  The latter goes for baseplates too.
d) If possible test a modding method on a broken piece that doesn't matter before you test it on a piece you want both halves of.
e) Only ever use glue when repairing a broken piece (it may be necessary to break a rail point in order to motorise it).
f) Never use paint.  Use the nearest LEGO colour of bricks if you are building a model of a real item.

Mark