Tight fitting motors, how do you solve the issue.

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Shade

Shade

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This is really a multi-level question...

First for existing built rockets I have several that have very tight MMTs. I have
difficulty getting Estes BP engines in them to the point where I feel I am going
to damage the rocket, any suggestions on how to deal with this? Some are
18mm some are 24mm, The rocket affected are both Semroc and Estes and or
components from either company.

Second, are there better/looser MMTs I can use for engines in general that will
fit all engines?

TIA.
 
Other than making sure you don't have glue or some other build up in the motor tube (which you would carefully & lightly sand out with sandpaper around a dowel if it is the issue), a a quick work-around is to remove some of the paper around the motor itself. Just not too much. Should only need a layer or two.
 
Shade said:
This is really a multi-level question...

First for existing built rockets I have several that have very tight MMTs. I have
difficulty getting Estes BP engines in them to the point where I feel I am going
to damage the rocket, any suggestions on how to deal with this? Some are
18mm some are 24mm, The rocket affected are both Semroc and Estes and or
components from either company.

Second, are there better/looser MMTs I can use for engines in general that will
fit all engines?
Click to expand...
Hi, Ron,

In my experience, the tight motor tubes are due to exuberant use of wood glue on the centering rings. It shrinks which creates a tight spot in the tube.

Many of the motors seem to be bulged in the lower half. I've always thought that was due to ramming into a worn tool which allowed the motor case to swell. Or, perhaps it could be due to moisture. Either way, no doubt, I've dealt with a few tight motors.

Besides using less glue on motor tubes, some 100 grit sandpaper works great for sanding a little OD off the motors.

HTH.

Doug

.
 
Peeling off some the engine casting is not my preferred method, for safety reasons, although
I will admit having done it. No glue on the inside of the MMTs, Liberal application of glue on
the MMT centering ring joint is likely, I have added several coats at times particularly with
24mm MMT's. will have to watch that in the future. Are there any glue know to shrink less?
I like my Ambroid, but it does have a tendency to shrink more than most, so I will evaluate
other glues.
 
Shade said:
Peeling off some the engine casting is not my preferred method, for safety reasons, although
I will admit having done it.
Click to expand...

Sounds dangerous when written out, but when done as noted (one or two layers...thin layers!!!), it should be fine. I've read this from other places as well and while I'm not crazy about doing it, it shouldn't cause any problems unless someone is going overboard with the paper removal (at which point the motor is loose in the tube and shouldn't be used anyway).

Also from what I recall, this issue may be caused by humid conditions swelling up the cardboard motor casing more than the motor tube. Can anyone verify?
 
The old Estes Mercury Redstone, with its molded plastic fin and motor mount unit, was notorious for way-too-tight fitting off the motor in the mount tube. I would usually run my Dremel around the inner lip of the motor mount tube to try and give it maybe a quarter-mm more clearance, but I still ended up having to use a dowel to push out the burned motor casing.
 
I keep a 1/2" dia. x 4" metal rod in my range box. If a motor is a little tight, I rub the metal rod up and down and around the motor and the edges. This tends to reduce the diameter of the engine by enough thousandths by compacting the paper case tighter to slip in easier. Also the metal rubbing puts a smooth shiny gloss on the paper as well, helping the motor slip in that much easier. I call it "metal waxing."
 
I assumed humidity swelling up the motors. Some of my older motors have this issue, but new ones are a perfect fit.
 
I sand the motor case or remove some paper wrap, never more than a layer or two. We have a saying in gunsmithing: modify the cheapest part that's easiest to replace if something goes wrong. The motor is expendable, the rocket is not.
 
Estes engines are often a bit tight in Estes tubes. Quest engines (both the old German and the new Chinese) are smaller and never stick. Rockets with Centuri-sized #7 tubes work better with Estes engines, because the ID is just a hair bigger; makes flying most of my Semroc fleet easier.

Getting the engine in doesn't usually trouble me, but getting the casing out is sometimes a bit of a bear. I have a pair of pliers with curved jaws that I got as part of a gift years ago, which is darn near perfect for removing 18mm engine casings, but even so it's still tough sometimes.

The pliers look like this:

pliers.jpg
 
JERRYR708 said:
I call it "metal waxing."
Click to expand...
It's called burnishing. And a nice idea, burnishing a big motor. I use sand paper. Where I most often have trouble is right at the front edge of the motor, which can mushroom a bit. So I sand off the mushroom and go a little bit further to make a very small bevel. That's above the ejection charge, so if that's the only place sanded, there's no safety question at all.
 
Shade said:
... Are there any glue know to shrink less?
Click to expand...

Yes. I prefer to stay away from wood glues for a number of reasons and use epoxy instead. Epoxy usually has very little shrinkage, but it requires discipline to use it properly (weighing the resin/hardener and thoroughly mixing). For quick jobs I use Loctite 5 minute (actually it's about 3 so you have to work fast) or for things that I know will take time to position properly I will use Aeropoxy structural epoxy, which gives me a 30/45 minute working time.

As far as fixing the possibly pinched MMT, you might consider soaking the pinched area with thin CA then taking an appropriately sized wooden dowel with some 320 grit sandpaper taped around it and finish the process with 400/600 grit. That may be just enough to give some relief to getting the motors in and out.

Greg
 
Solomoriah said:
Estes engines are often a bit tight in Estes tubes. Quest engines (both the old German and the new Chinese) are smaller and never stick. Rockets with Centuri-sized #7 tubes work better with Estes engines, because the ID is just a hair bigger; makes flying most of my Semroc fleet easier.

Getting the engine in doesn't usually trouble me, but getting the casing out is sometimes a bit of a bear. I have a pair of pliers with curved jaws that I got as part of a gift years ago, which is darn near perfect for removing 18mm engine casings, but even so it's still tough sometimes.
Click to expand...

I agree with you on the ST-7 and Quest tubing, never have an issue with a tight engine in those.

Some engines seem to have the swelling right above the nozzle area where the propellant would be in the engine.
I don't peel the casing, that might get away from you and you could peel too much.
I keep a small sanding block with 220 grit on it. Light sanding and check the fit. More light sanding until it fits.
Take off as little as needed so the casing isn't weakened.

Try pulling the casings right after flight when the engine is still warm.
Fred Schector suggested that. It doesn't seem right, you'd think the casing would swell up with the heat.
The casing actually shrinks and is easily pulled right when you pick up the model.
This is great with BT-5 or BT-20 models with friction fit engines.

The top end of the casings can sometimes have a burr.
Estes used to recommend rolling it down on a hard surface.
I've used the 220 grit on a block to roll off the top before.
 
Last edited:
Solomoriah said:
Estes engines are often a bit tight in Estes tubes. Quest engines (both the old German and the new Chinese) are smaller and never stick. Rockets with Centuri-sized #7 tubes work better with Estes engines, because the ID is just a hair bigger; makes flying most of my Semroc fleet easier.

Getting the engine in doesn't usually trouble me, but getting the casing out is sometimes a bit of a bear. I have a pair of pliers with curved jaws that I got as part of a gift years ago, which is darn near perfect for removing 18mm engine casings, but even so it's still tough sometimes.

The pliers look like this:
Click to expand...

You don’t happen to have a brand or model number for those pliers, do you?

I’ve done some online searching and can’t seem to find it reasonably quickly.

Any info you can provide is apprecieated.

Thanks.

Cheers,
Jon
 
KenECoyote said:
Well if we're worried about peeling off too much paper, we should be equally worried about putting on too much pressure. :wink:
Click to expand...
You could even use the metal edge of a ruler with hardly any pressure. There would be enough cushion in the paper layers to absorb the very light ruler force. You would be doing more fast scraping up and down.
 
j.a.duke said:
You don’t happen to have a brand or model number for those pliers, do you?

I’ve done some online searching and can’t seem to find it reasonably quickly.

Any info you can provide is apprecieated.
Click to expand...
Everywhere I've seen them online, they've been part of a set of large needle-nosed pliers of various shapes; when I received them as a gift, that's how they were given to me.

... everywhere, except here:

https://www.amazon.com/dp/B000VJPQ68/?tag=skimlinks_replacement-20

Under 5 minutes with Google, and there it was. I even know what they are for now.
 
Last edited by a moderator:
Those pliers are often referred to as hose pliers, cable pliers, or spark plug wire pliers.

As for motor fit, a 3/4" flap wheel in the end of a drill or dremel at low speed in the motor tube will remove little enough material at a time to give a good fit. Here's an example: https://www.amazon.com/dp/B000NV4B1K/?tag=skimlinks_replacement-20

You can find those at pretty much any hardware store. 1/2" for 13mm, 3/4" for 18mm, 1 inch for 24mm and 29mm.
 
Last edited by a moderator:
Peeling, sanding, burnishing and rolling? That seems like a lot of extra work for each flight.

All Estes motors are slightly larger at the nozzle end. You can easily insert the front end of a motor into BT-20 or ST-7 but you cannot insert the nozzle end.

Go ahead, try it. I'll wait.

The solution is very simple: Sand the first 1/4 to 3/8 inch of the inside of the motor mount tube with a tiny piece of 220 grit sandpaper over your fingertip. This gives the clearance needed for the fat nozzle end. The rest of the motor mount tube will hold the motor perfectly aligned. I don't have a problem with glue girdling because I don't put a centering ring in the center of the motor mount.

For built rockets with tight motor mounts, I removed the nozzle from a spent casing, glued it to the end of a wooden dowel and put it on a drill. I put talcum powder on the casing and in the motor mount and then spin the casing in the MM with the drill. The talc acts as a fine grinding compound without taking off too much paper.
 
Joe_Shockcord said:
Peeling, sanding, burnishing and rolling? That seems like a lot of extra work for each flight.

All Estes motors are slightly larger at the nozzle end. You can easily insert the front end of a motor into BT-20 or ST-7 but you cannot insert the nozzle end.

Go ahead, try it. I'll wait.

The solution is very simple: Sand the first 1/4 to 3/8 inch of the inside of the motor mount tube with a tiny piece of 220 grit sandpaper over your fingertip. This gives the clearance needed for the fat nozzle end. The rest of the motor mount tube will hold the motor perfectly aligned. I don't have a problem with glue girdling because I don't put a centering ring in the center of the motor mount.

For built rockets with tight motor mounts, I removed the nozzle from a spent casing, glued it to the end of a wooden dowel and put it on a drill. I put talcum powder on the casing and in the motor mount and then spin the casing in the MM with the drill. The talc acts as a fine grinding compound without taking off too much paper.
Click to expand...

I'll have to say that suggestion sounds like a very good solution. :clap:
 
There's a reason I don't peel paper off motors. When I was in junior high school (some 38 years ago) I either had a Red Max with a tight motor mount or the teacher in charge of the rocket club had a batch of swollen motors. Either way, one day after school we had a launch and I couldn't get the motors in the Red Max without peeling off a couple of layers of paper. The first flight worked fine but I peeled too much paper off the second motor and it burned through the side of the motor mount and body tube. I won't let that happen again.
 
Joe_Shockcord said:
...I peeled too much paper off the second motor...
Click to expand...

To be honest, I often take early learning errors with a grain of salt. I take a look back at the early stuff I had built/done (even as a teen) and I'm amazed they worked at all. :wink:

I've read from multiple places about peeling the paper off the motor and I could almost swear I've read it in one of my rocket instructions, but can't recall for sure or which one.

Here's a site which mentions it off a quick 'net search:
https://www.rockets.co.nz/tips.htm
"Never force the motor into the mount. Instead carefully remove the labelling sticker from around the motor, or no more than two wrappings of paper from around the paper casing. Lightly dust a 'peeled' motor with talcum powder before inserting into the mount, and get into the habit of relabelling the motor with a pen so you can still identify it, if, for some reason you decide not to launch. "
 

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