Seacock Replacement Part 3 

Groco now offers backing plates - maybe a time saver for some. Thanks for this series Wayne!

I just replaced my thruhulls and backing plates. This is a good video on the backing plates, however there was some issues. Compass marine is a good resource for backing plates and thruhulls. G-10 fiberglass plate is an abrasive. It really messed up blades. Cut it more like steel and aluminium, slow. On my jigsaw I put it on the slowest speed and hold the trigger only halfway. Your looking for 100-150 rpm or less. Regular metal cutting blade. IF you go fast the blade overheats and loses the teeth.it can get red hot that way, which loses the temper and it stops cutting. For backing plates you want 5/8" thick. Most of mine are 3/4" and I have some 1" plates Cut the holes with a drill press. Do everything you can to make these perfectly straight. Slight angles will hurt you later. Use the correct size holesaw for the specific thruhull. There are a couple charts online. Don't go with the closest size. Remember that the teeth of the hole saw are wider than the the plug. So if you find one that fits the thruhull it's too big. There are two ways to install the backing plate recommended by compass marine. 1st epoxy the plate in place and drill the holes through the hull. I aligned the plate with a 1/4"bolt through the center hole. If I was replacing and had a bigger hole I used the plug on the bolt also to fit the hole. After the epoxy is hardened use the holesaw from the outside. Put the new seacock on and drill the flange bolts through. Epoxy those bolts into place and fair. As an option with the thicker G10 you can drill into the backing plate only and tap for 5/16" flange bolts. Bolt the seacock in place. I found slight angles of the bolts or thruhull made life miserable. Version 2 is my recommendation having done both. Make the backing plate circle with jigsaw. Using a drill press with clamps cut out the center hole with the correct size thruhull. Put the plate, thruhull and seacock together, you may have to use a 2x4 piece with the same holesaw hole as a spacer. Use the correct size transfer punch to mark the holes through the seacock. Make sure this is accurate. Use the drill press to drill out the flange holes. Use the drill bit for 5/16" threaf tap, Size F I believe (I got the tap and bit as a set). It's important that you take the time to ensure this is straight and 90 degrees from the plate, slight angles will make it hard to get the seacock on. Take a 3/4" or 1" forsner bit and center it in each hole, make sure the plate is clamped well. Cut out a pocket in each with the forsner bit until it's deep enough to hide the head of the bolt. Take the tap and put it into a cordless drill thread each of those holes. keep it straight, I can't stress this enough. Screw in your bronze bolts, and tighten them in with a socket and ratchet. G-10 is really strong, unless you used thin material it's not going to strip the threads. If preparing in advance than fill the pockets with thickened epoxy and sand smooth when cured. I gave the bolt head a quick sanding with a disk sander to make sure the epoxy could grab hold better. Spray the thruhull with mold release or hairspray and let dry. Cover the thruhull with teflon or packing tape, needs to be tight, test fit in hole on the boat. Don't use grease! At the boat make a batch of thick epoxy with microfibers and cabosil. Butter it on the back of the plate really thick. With the thruhull in place, put the buttered backing plate in place over it. Pressing down to squeeze out excess epoxy. Make a fillet in the squeeze out along the edge of the backing plate, and scrape off any excess. Next day More than likely you will have to unscrew the thruhull as the epoxy likes to squeeze into the threads and make tight threads. I found heating the inside of thruhull from the outside of the boat helps a lot. I used a groco thruhull wrench. Run the hole saw back thru the hole to remove the epoxy threads or excess. I found this 2nd method was more work and took time, but considerably better in regards to alignment. The alignment is key as i can't state how miserable and frustrating it was to install everything on the last step when the hole or bolts are misaligned. I can't send a picture with this unfortunately

Great videos Wayne! Thanks for the great info... on another note, for your families sake, where a Respirator when cutting and sanding fiberglass!!!!!! ;)

Thank you for this series on seacocks! As I am thinking about undertaking this task for the first time, your videos are perfect for understanding what to do and how to do it.. But I still have a few questions.. First: I do understand how a backing plate will strengthen the whole thing, but when I look at the big and thick base of your seacock, I wonder why you would need it.. It kinda looks like the seacock has its own integrated backing plate no? Wouldn’t that be strong enough? And second: As I assume you that you will have a perfectly reasonable and valide answer to the previous question, and that, consequently, I will probably be putting backing plate when changing mines, I would like to know the preferred way to fit the backing plate to the hull. Do you sand down the backing plate so that it matches any curves in the hull or do you grind the inner hull to make it flat? Thanks again!

Is the adhesive enough to keep the bolts from turning in the backing plate when tightening the nuts that hold the seacock flange down?

Wayne, what are your thoughts on using Starboard as a backing plate for new seacocks?

Hey great videos.. What material are the backing plates? is that a pre-made fiberglass board

Wayne, that looks like super glue! What is the name and brand of the product? Cheers

What is wrong with the audio on your videos can see you talking but only get your backing track