4" Prometheus - MMT Issue

[DAllen]

Got my MMT all glued together and it's time to insert into the aft end of the BT. Does not fit very well. Insert MMT from the forward end and it fits great and slides in nicely until the centering rings get to the fin slots then it gets really tight. So it seems like the ID of the BT becomes smaller where the fins slots are. Anyone else encounter this and what did you do to deal with it? I've been hand sanding...A LOT and I am looking for ideas of how to deal with this - other than hand sanding.

 

[jFlds]

I just run a palm sander around the CR until it slides in better. Had the same issues with multiple glass kits. A little 5 inch orbital is worth it's weight in gold. I keep one in the trailer for field issues.

 

[crossfire]

This happens a lot with fg kits. Just keep sanding and test fit often.

 

[David Schwantz]

I also have found the same. I like to test fit everything before gluing. I will mark which CR goes where on the MMT and inside the tube. This way if hand fitting is needed it is much easier

 

[rfjustin]

Is the inside of the tube heavily scuffed up, especially right on and beside the fin slots? I'd be more inclined to sand more inside the tube than on the rings at this point.

 

[jFlds]

I just run a palm sander around the CR until it slides in better. Had the same issues with multiple glass kits. A little 5 inch orbital is worth it's weight in gold. I keep one in the trailer for field issues.

Make sure you've got gloves and long sleeves on if you go into that tube to sand.... Unless you enjoy getting sharded by a filament or 5!

 

[mtnmanak]

Cutting the slots in an FG tube causes the tube to flatten out slightly right around the slot. This get far more pronounced in larger tubes. I have gotten some 6" and 8" tubes that have flattened out at the fin slots as much as 1/8" or more. I usually CNC my centering rings to get a perfect fit, but if you use the CRs that come from the kit maker, you can do a couple things to get them to fit better.

First, check the diameter of the CR compared to a spot between the fin slots. If the CR is too big, you need to sand down the overall diameter of the whole CR until it fits that diameter.

Then, I mark out the fin positions on the CR and lightly flatten out the CR at those points until you get a decent fit. If you do this, I like to number the fin slots and their corresponding positions on the CRs to ensure I remember how they fit back into the tube.

If you reduce the entire diameter of the CR to fit the flattened out areas at the fin slots, you will almost definitely end up with decent sized gaps between the CR and body tube in the areas between the fin slots.

I have tried to use all manner of clamps to reshape the body tube, but I was never successful. It was always easier to shape my CRs to match the tube.

 

[Sooner Boomer]

What might help is to index the CRs and the body tube. This way the rings go into the tube in the same orientation each time. Only sand where you need clearance, and you shouldn't end up with a ring that's too small all the way around. I also index the fins. Fin marked #1 always goes into slot marked #1. The fit may be different with each component.

 

[DAllen]

Helpful info right here. Yeah being out of round at the fin slots seems to be exactly what I am encountering and that makes sense. So I will index the slots and the MMT. I already have lines on the MMT for where I want the fins to attach to so indexing will be easier.

 

[crossfire]

Helpful info right here. Yeah being out of round at the fin slots seems to be exactly what I am encountering and that makes sense. So I will index the slots and the MMT. I already have lines on the MMT for where I want the fins to attach to so indexing will be easier.

I have often wondered if the tube kind of collapses a bit due to the fin slots. Cutting slots as to weaken that area of the tube.

 

[DAllen]

Ok so I did a combination of sand off the inside with a 4" wire brush on a 12" bit extender on the drill plus a very slight flattening of the CR's at the fin slots and daggumit it now fits very nicely. Snug but it does slide with encouragement which is exactly what I think it needs.