are the internal and external rams interchangable

Don't realy understand this question ?!

i need a set af trim rams for a trs my old ones are internal lines and most of the ones im finding are external lines

Yes, they still fit !!!!!!!

No, it is not recommended by Mercury.The internal line style uses the high pressure low volume trim pump.The external line type uses the low pressure high volume trim pump. Mercury states in the manual that using the incorrect trim cylinders will affect trim operation and may cause damage to the trim system. The high volume external type also uses larger diameter lines from the pump to the manifold on the bottom of the gimble assembly.

Here's a pair.
http://cgi.ebay.com/ebaymotors/TRIM-...item5ad9b2f525

thanks