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Going from 3" to 4" exhaust tips

SBMech

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Howdy,

Quick question, I am going to oversize the 3" exhaust ports in my transom to 4", going to lightning headers from stock Harman logs.

Should I glass in the interior of the through holes then install the tips or just install the tips with gaskets and silicone?

Obviously intending to use a gasket and silicone to seal the tips either way.

Thanks!

Josh
 

rivrrts429

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I would glass them and sand them. I have no idea as I've never done it but I'd think water would get to them easily and you'd have a rotten transom pretty quick.

Going to have to be extremely careful when drilling through the gel that you don't start chipping it. Since the 3" hole is already cut nothing you can do as a pilot.

Hopefully someone will chime in who's done it. I'm just thinking out loud and what makes the most sense.
 

Backlash

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You may want to put a layer of tape on the gelcoat around where the current exhaust openings are located. This should help reduce the chipping to your gelcoat (No guarantees!).

Then you could put a small piece of plywood on the inside of the current exhaust opening and another piece on the outside of the hull. Screw these two together using drywall or other screws so they clamp snug to one another. Don't drill into your transom!!! You are only connecting the two temporary pieces of plywood together. Essentially, clamping the transom and giving you a clean place to start drilling. Measure and mark everything carefully and make sure you use a new hole saw. Drill through from the outside of the hull too and do your best to keep the angle of the hole saw square to the transom.

Once both holes are drilled out, clean everything up. Depending on which exhaust inserts you are using, you may need to drill these holes slightly larger to give yourself room for the glass. Rough everything up with 36-80 grit wherever you expect to apply cloth and resin. Use vinylester resin and some 1708. DON'T use the crappy polyester resin from HD. You want this glass to seal the transom and you want it to have more of a tenacious bond to the core. Cut your cloth, mix up your resin and hardener, and go to town.

Work cleanly so your cleanup will be easier but take your time and do this right. It will only turn out as good as you'd like it to. :D.

You can essentially seal the holes with the cloth and resin to help reduce the chance of water getting into the transom core. Once you finish up, you can either roll or spray on gelcoat to match the current engine bay.

Don't use more resin than is necessary. That is a common mistake. It ads unnecessary weight and will only make that area more brittle. Extra resin doesn't provide any additional strength contrary to what most people think.

I think I covered it all but if not, ask away! :D
 

Jimmy

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Some arbour's have enough thread that u can get another holesaw to thread inside another
 

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74 spectra20 v-drive

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once you get the new hole in I would throw some resin on the bare wood and silicone the heck out of it. Growing up I saw a ton of rotted out transoms at my uncles shop. I would also use a plate on the back side of the transom as well to help seal everything and keep from digging into the glass. Usually the tips are just 1/4 inch bolts and they will dig in w/o a good size washer. I made a big ring for mine.
 

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138

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silicone..this is not the way to go...3m 5200 it is a pain in the butt but it works...clean up with wd-40.
 

Backlash

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^^ What he said.

The best way I know how to do this is to use bedding tape and then allow time between tightening everything down in stages. To do this, you need to bevel the surface where the fastener goes into the surface. Silicone doesn't work and 5200 is a PITA but can work. When I get to this part on the Schiaward project, I will go in to detail about the way I think this should be done.

Just brushing fiberglass resin on the core inside the freshly cut exhaust holes will not seal the wood core. You have to add glass in order for this opening to be sealed. Over time, as the wood expands and contracts, the resin will crack and flake away from the wood. Now you have a compromised core. And had someone used silicone to "Seal" the opening, you now will have the beginning of a rotted transom.
 

74 spectra20 v-drive

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Good call out on the resin only on the wood, I defiantly did not detail that well. we encapsulate every raw section of wood with glass and resin. My biggest call out was the backing plates as I see this a lot where folks tighten small diameter washers into glass and fracture the material. Even the stringer washers end up pulling into the glass.
 

Backlash

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Sorry didn't mean for it to sound like a "Call out." I hope it didn't come across that way! Just wanted to help a fellow boayer out. Nice engine compartment in the previous picture though! ;)
 

74 spectra20 v-drive

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No offense taken at all, I short cutted the response and if SBMech does as I described he ends up with an issue down the line... No worries at all here.
 

coolchange

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Some arbour's have enough thread that u can get another holesaw to thread inside another

I knoww this thread is oldb but.just for.reference.
Neat trick jimmy, I have to see if.my saw will do that.
Another that I use is cut a hole in a piece of 3/4 the finished size you want. Then use this as a template wherever the hole is, clamp it in place and use as a.pilot on the OUTSIDE of the hole saw.
This also.allows you to offset the hole if necessary.
 
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