As far as I know, the only way (assuming no mouse lines installed) is to remove one end or other from the boom.. Drill out the rivets, pull the casting clear and away you go. Just rivet the end back on when you're done.
I tapped the holes and used s/s/ bolts when I did mine, so that I could always remove the casting easily in the future.
I don't know whether you have a pulley system inside the boom, but if you have, the above method is the only way. If it's simply from one end to the other, you might get away with removing the boom, canting it at around 45 degs, and sliding a light, weighted line down. Use this to pull the proper line through afterwards.
It's actually a lot easier to do than it sounds, though getting the rivets done if you don't have access to a 6mm puller, is a pain.
Tape two mice to the existing reef line at clew end, pull it through almost to tack end, fish second mouse out of second reef exit using a specially contructed tool (raw ingredients, 1 wire coathanger). Release second mouse and wiggle it free, add more mouse line to it, pull whole lot back to clew end, Hook out (coathanger) second mouse through second reef line slot at tack end, detach bits where necessary. Hey presto, original reef line back in position, and a mouse for a second line ready and willing to heave through new pennant. Easier to do than to describe.
Hello
Just a shot in the dark, can you not use your exsisting pennant to pull through 2 small guide lines. use 1 for the original pennant and draw the next one through with the spare.
Keith
I reckon you should use the existing line to thread through two mouse lines and draw them out using a special tool to move the second one to the empty roller.
Yes, I agree with you and other posters who've suggested similar methods, but these will only work if there is no multi-part tackle for the outhaul. This exists on my boom (and no doubt many others) so pulling a mouse line through with the existing reef line won't work. The mouse line will be on the wrong side and, from experience with my boom, virtually impossible to get to the correct side without end access.
My earlier post assumed the out-haul tackle was there, but if Fergus has just straight through lines, the simple way's the best!
A previous owner had left a part coil of fencing wire on board which did this job perfectly. It fed past the outhaul block nae bother and then wheeched the pennant through. Might even work with a halyard.
Just last week, I rethreaded the (moused) reefing lines after taking them off for washing. To make rethreading easier, I removed and subsequently refitted the two friction levers (which I never use) at the gooseneck end. Imagine my surprise when I realised the flattener multi-part tackle was now adrift inside the boom!. Have a feeling it's a case of removing the casting, assuming I really want to use the outhaul.
Can't it be fished out?
With our boom removed and in the garage (very sensible place to do this!) I managed to put too mousing lines down, and through the correct exit with screwdriver and hooked bit of metal. I wouldn't want to do the job on the boat though ...