I think 18T has defeated me so want to test a easier 19T. Using a upwards tensioner. Would I definitely need to lengthen the chain? half or full link?

i can switch from a 17t to a 20t on my inbred without lengthening the chain, so would imagine you are ok with just a 1 tooth difference.

I have a black used once OnOne 19t cog and an unused Silver Surly 19t cog at home if you are looking for one and have not bought one yet?

You may well get away without having to lengthen the chain if you are only moving from a 18t to 19t. Upwards tensioner (Singleator) has always worked best for me.

I can switch from 16 to 18 without changing chain also. In a slot drop out I reckon the wheel moves forward by less than 10mm by going bigger if that's any help.

Thanks

MrTall – ygm