Why can't I Dry-fit PVC fittings?

sjb8112 · Jun 14, 2005

What is the trick to being able to dry-fit PVC fittings before final assembly? I am working with 3" PVC fittings from Lowes, plumbing up the waste line to the new toilet location in our bathroom remodel. I'd like to put the three fittings together and mark them before I get out the cement so that I can be sure the slope of the line is correct and that the closet bend is going to be in the exact right position, height, and level once the cement sets. But, the fittings just don't slide together -- If I really work at it, I can get them seated about halfway, but that's not good enough.

Once I put primer and cement on them, they slide right together and seat all the way, so I don't think there's anything wrong with the fittings.

I only get one shot at this, and if I mess it up I'm going to be cutting into the ceiling and wall in the room below the bath to get at the tee that I ruin on the stack.

hj · Jun 14, 2005

pvc

That is the way it is supposed to work. PVC joints are called "interferrence fit" which means they are tight when dry so there is no excess space that has to be filled with cement when they are assembled. If you remove any roughness on the end of the pipe, you may get them together a little further, but if they fit all the way, when dry, the joints would not be good enough once it is assembled.

Sponsor · 2022-01-29T12:20:58-0800

Jimbo · Jun 14, 2005

Well, this is why Do-it-Yourself sometimes becomes Do-it-Over. Some things are learned only be experience. Your only cost will be to replace some fittings if you don't get it just right the first time. If time is valuable to you or at a premium, then this is where you hire someone who will get it right the first time.

This is not meant to be a criticism of DIY, because at home I tackle any job in any trade. Some jobs go better than others, and I just accept that fact.

CaptWally · Jun 21, 2005

The reason PVC cement is called "solvent weld" is exactly the reason the facts have been stated by knowledgeable people before I got to the thread. PVC fittings are not 'glued' together, they are welded with the cement. The glue is not actually a separate entity connecting two parts, rather it is a solvent that chemically liquifies the PVC itself creating a bond that in effect is melting the parts together.

The parts do not fit together dry because the tolerances are so thin to make an effective joint. When the 'glue' is applied, the parts slide together all the way because the plastic has actually been melted at the point of connection. Once that connection cures, it is essentially one solid piece of PVC, not two pieces glued together.

If you want to ensure a proper fit you must measure and cut where the fittings will be in the final assembly, not in a dry fit situation.

SteV8e · Nov 16, 2006

I ran into that dry-fit problem too. My application is different because I am using the pvc to build frames. My son and I are building a trebuchet and the frames will be 2" pvc. The treb will be made of two complex 3D frames that are a mirror image of one another. Each half includes a double wye, 4 90elbows, 4 45elbows, 2 Tees, and 2 adapters with square-head plugs (the frame will be filled with water for lauch stablility).

Because of the complexity and not dry fitting, it is hard to build. If I had 3D CAD models of the fittings, I would know the exact lengths of the pvc pipe needed. Does anyone know if there are CAD models available?

I am doing this project with my son to teach him some engineering and applied math. He is interested in hurling objects long distances.

Thanks

Prashster · Nov 16, 2006

PVC pipe is also slightly flexible so usually putting the pipe next to the fitting and marking will give a good enough measurement.

Jadnashua · Nov 16, 2006

Many of the fittings have a definite lip inside them. You should be able to insert the pipe in the fitting until it bottoms against that ridge or lip once it is primed and slobbered with the solvent.

Don't overload the pvc, it can shatter into some nasty shards if you exceed its strength. It will bend a bunch first, though ,and is sensitive to temperature. Also note, it changes length a fair amount with temperature.

hj · Nov 16, 2006

measure

That is why they make rulers so you can calculate the fitting's dimensions as far as insertion depth.

Phil H2 · Nov 16, 2006

I don't know if PVC fittings have standard dimensions (galv/blk iron have standard running length dimensions, copper fittings do not and they vary from one manufacture to the next). If you know the fitting manufacture, you could check their website to see if they have CAD models. Spears Manufacturing has dimensions for their PVC fittings on the web (http://www.spearsmfg.com/).

SteV8e · Nov 20, 2006

Phil:

Thanks for the link to Spears; it has detailed dimensions for the fittings. I had received some specs from McMaster-Carr for the manufacturere Nibco, but not as detailed as the Spears drawings.

I had to delete the ) from the end of the Spears link to get it work.

I think that I will need to make the ProE CAD files myself. Let me know if you would like me to send them to you.

ToolsRMe · Nov 20, 2006

Can you use ABS instead of PVC?

Gary Swart · Nov 20, 2006

You can't dry fit ABS either.

ToolsRMe · Nov 21, 2006

Gary Swart said:
You can't dry fit ABS either.

I've installed a central vacuum system in my house and routinely dry fit the ABS stuff.

Now that may be different than the ABS you use for water plumbing since I needed to get that stuff at a vacuum cleaner store.

Live and learn.

Mikey · Nov 21, 2006

My central vac used PVC, not ABS, but it was dry-fittable as well. In fact, I installed a large part of the system by dry-fitting and taping the outside of the joints with electrical tape. (It was temporary, waiting for some other construction to be completed.) It's a special thin-wall pipe designed to maximize the inside area for the outside diameter. Fits are sloppy, but once welded they're pretty strong.

Gary Swart · Nov 21, 2006

The PVC used for vacuum systems is not the same as that used for drains. Go back up this thread and read HJ's reply regarding how PVC pipe/fittings are made and fit. Fitting PVC is really no different than fitting copper. I use a marking pen when working with PVC and copper to mark where I will cut the pipe. It's really not rocket science.

SteV8e · Nov 28, 2006

I was not able to find any 3D CAD models for PVC/ABS fittings, so I made the ones I needed. Let me know if you would like me to share them with you.

One might not use them when laying out a drain, but with something more sophisticated (central vac) or a frame like I am building, they will be very handy. I will know the lengths of all my cuts and do them all up-front; no dry fitting needed.

I've tried to attach .pdf files to show a fitting and the frame, but the file is too large and the distiller I am using doesn't allow me to change the file size.

student1 · Dec 31, 2006

krifos said:
If it is really that critical, try cutting off of the 75% of each hub on elbows tee,s etc. With most of the hub removed you should be able to make a dry fit model of what you need . After this you can replace the "test" fittings with new one's and be confident that you will have good results. Pvc is cheap enough to allow you to do this.

That ain't gonna do you any good. The reason you can't dry-fit is because the socket is tapered down to an interference fit. My 2c.

Randyj · Dec 31, 2006

like hj said... measure the depth of the socket and use this to calculate the length needed. If you slop the glue on really good it will slip all the way to the stop... not rocket science.

hj · Dec 31, 2006

frame

One thing, if you cut two of everything the two sides will be equal, and it is likely that some pipes, such as the legs, will be the same on a single side. I cannot see how a 1/4" even would make a difference, especially since I do not think you will be launching dishwashers or washing machines.

Bob NH · Dec 31, 2006

The most important aspect of getting consistency is cutting the pipe sections very square and to accurate length, and chamfering the ends slightly so they will bottom out in the fitting.

That kind of accuracy requires a miter saw setup.

If you check the fitting dimensions, most of them have a socket that has a consistent center-to-center add to the straight pipe length. That add depends on the size of the fitting.

The biggest error in trying to make a precision structure is that the angles can't be held accurately without tooling. You might want to set up a jig on a sheet of plywood to ensure that you get accurate angles.

Why can't I Dry-fit PVC fittings?

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