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Any way to expand an O-ring in a Bontrager pump?

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Any way to expand an O-ring in a Bontrager pump?

Old 05-07-22, 01:08 PM
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Any way to expand an O-ring in a Bontrager pump?

My Bontrager Turbocharger HP pump has issues, adding silicone grease only slightly improved things - pretty sure the plunger O ring is worn - getting a specific replacement looks like it's going to be problematic - are you aware of a way to swell that O- ring or do you think I'd be likely to find the right size in a typical O - ring collection at Harbor Freight, Home Depot etc.?
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Old 05-07-22, 01:15 PM
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I seem to remember using isopropyl alcohol to expand a rubber seal in a suspension fork once. Not sure if it's the same material, but might be worth a shot.
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Old 05-07-22, 01:22 PM
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Originally Posted by MyRedTrek View Post
My Bontrager Turbocharger HP pump has issues, adding silicone grease only slightly improved things - pretty sure the plunger O ring is worn - getting a specific replacement looks like it's going to be problematic - are you aware of a way to swell that O- ring or do you think I'd be likely to find the right size in a typical O - ring collection at Harbor Freight, Home Depot etc.?
Try a local Ace. Tru Value, or plumbing supply store. My ace has them individually at the service counter.
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Old 05-07-22, 05:03 PM
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Originally Posted by dedhed View Post
Try a local Ace. Tru Value, or plumbing supply store. My ace has them individually at the service counter.
+1. Also auto parts stores usually have a good supply.
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Old 05-08-22, 04:13 PM
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I would recommend first checking with your local older Trek Store (that has been around long enough to have carried that pump) and see if they have any leads and if nothing else do what dedhed recommended and check your local hardware or plumbing store.
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Old 05-08-22, 10:22 PM
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I'm pretty certain the o-ring is an standard industrial type oring. I can't see Trek/Bontrager making special o-rings but rather, incorporate an existing o-ring.

My local hardware store (Do It Best) has o-rings in those small parts pull out drawers. There are finite sizes, based on the cross section diameter of material and the ID of the ring. The various sizes are distinct enough that you can visually match your oring, just take it with you.

Often, shops have a measuring cone (a cone with the tip facing up) and you drop the o-ring onto the cone and where it lands, tells you the ID. Then there is a little notched, slip-in gauge at the base of the cone that indicates the diameter of the cross section.
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Old 05-09-22, 01:53 AM
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Originally Posted by MudPie View Post
I'm pretty certain the o-ring is an standard industrial type oring. I can't see Trek/Bontrager making special o-rings but rather, incorporate an existing o-ring.

My local hardware store (Do It Best) has o-rings in those small parts pull out drawers. There are finite sizes, based on the cross section diameter of material and the ID of the ring. The various sizes are distinct enough that you can visually match your oring, just take it with you.

Often, shops have a measuring cone (a cone with the tip facing up) and you drop the o-ring onto the cone and where it lands, tells you the ID. Then there is a little notched, slip-in gauge at the base of the cone that indicates the diameter of the cross section.
OP stated that the o-ring was worn, measuring it this way may cause one to get an improper size o-ring.
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Old 05-09-22, 06:21 AM
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Originally Posted by JoeTBM View Post
OP stated that the o-ring was worn, measuring it this way may cause one to get an improper size o-ring.
They'll be fine. The jumps are so incremental in O ring sizes and the cost minuscule you can buy a couple different ones and see which works best.
An "improper size" will often even work in many applications due to the nature of the material, especially in low tolerance applications.
No manufacturer uses a custom size O ring because they are made in so many sizes

O rings are sized by ID-OD-cross section. Measure your groove it sits in to get ID and width of groove will give cross section. OD is then standardized because of the cross section.

It's a bicycle hand pump, not a high speed, high pressure application.


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Old 05-09-22, 08:17 AM
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Originally Posted by JoeTBM View Post
OP stated that the o-ring was worn, measuring it this way may cause one to get an improper size o-ring.
Assuming the o-ring is worn to allow air to pass, you could not discern dimensional changes visually. The next size up or down will be obviously incorrect.

The other variable about o-rings is the material. Some materials are better suited for different chemicals or environmental conditions, but for this applictin, the cheapest and most available rubber will be more than sufficient,
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Old 05-09-22, 08:28 AM
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Originally Posted by MudPie View Post
The other variable about o-rings is the material. Some materials are better suited for different chemicals or environmental conditions, but for this applictin, the cheapest and most available rubber will be more than sufficient,

Most hardware store/plumbing are going to be SBR or Buna N (Nitrile) and more than adequate. Auto parts may have a Viton (FKM) for fuel applications, but would be overkill for a pump.
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Old 05-09-22, 08:49 AM
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Originally Posted by MyRedTrek View Post
My Bontrager Turbocharger HP pump has issues, adding silicone grease only slightly improved things - pretty sure the plunger O ring is worn - getting a specific replacement looks like it's going to be problematic - are you aware of a way to swell that O- ring or do you think I'd be likely to find the right size in a typical O - ring collection at Harbor Freight, Home Depot etc.?
Trying to swell an o-ring does not end well. The whole o-ring swells which means that it doesn’t fit anymore. As others have said, go to a good hardware store…probably not a Big Box. Ace has a better small bolts/small hardware selection and you don’t have to buy 13 of them.
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Old 05-09-22, 09:00 AM
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Just get a new air chuck. They aren't very expensive at all. The last one I purchased was only about $4.50. And it included a hose that happened to be longer than the one on the pump.

You might have to search a little. There are $50 and higher air chucks out there for those that need the prestige and self-adoration.
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Old 05-09-22, 05:15 PM
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Originally Posted by Iride01 View Post
Just get a new air chuck. They aren't very expensive at all. The last one I purchased was only about $4.50. And it included a hose that happened to be longer than the one on the pump.

You might have to search a little. There are $50 and higher air chucks out there for those that need the prestige and self-adoration.
I'm referring to the O-ring that's on the plunger that rides inside the pump body. The chuck is good afaik.
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Old 05-10-22, 07:39 AM
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Originally Posted by MyRedTrek View Post
I'm referring to the O-ring that's on the plunger that rides inside the pump body. The chuck is good afaik.
Yeah, I really need to slow down and read. But heck, that wouldn't be me if I did! <grin>

In that case, pretty much similar advice. Get a new floor pump.

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Old 05-10-22, 08:12 AM
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Parker Hannifin, industrial air tool supply, Grainger, if you can’t find it at the hardware store.

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Old 05-11-22, 03:09 AM
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Found two "Trek" pages on this but nothing gives dimensions

https://www.trekbikes.com/us/en_US/e...-ring/p/26018/

https://www.trekbikes.com/us/en_US/e...ommet/p/13891/
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Old 05-11-22, 06:41 AM
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Greases are made to very slightly expand most all o-rings materials for improved fit and service life but not repair a worn o-ring, if you use them periodically your o-rings will last much longer.

Dow Corning Molykote 55 Oring Grease 5.3oz - DC 55

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Old 05-11-22, 07:33 AM
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Originally Posted by easyupbug View Post
Dow Corning Molykote 55 Oring Grease 5.3oz - DC 55
Will the Molykote also work on thicker rubber bushings as well, like in a Presta pump head/chuck?
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Old 05-11-22, 08:04 AM
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Speaking as an engineer who deals with o-rings pretty often... There's a lot to know about o-rings but you don't need to learn it for this.

Just take the disassembled pump to the hardware store and figure it out. You need the right thickness and it should be stretched just a little when installed on the piston so it's not loose, and then when you put it into the pump cylinder you should have a little but not too much trouble. If you have a choice on hardness just try to get one that feels about the same. Buy a couple of them in case you tear it when you install it. Clean everything really well and grease it before you put it back together for good. It's possible the cylinder has gotten scratched or rusted as the thing was deteriorating so don't get your hopes up it'll be good as new for just as long.
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Old 05-11-22, 08:04 AM
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Yes, i get very good life, does not take much. There are a lot possible failure modes for o-rings and bushings in industry but in a pump plunger or a chuck the likely failure mechanism I would say is abrasion with the plunger wall and the presta and/or schrader, this friction can be helped with a lubricant. In the plunger I would also expect over time another failure mode would be compression set as the walls flatten the circular o-ring and it takes a permanent flattened set, this is where a very slight expansion of the o-ring material could help.
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Old 05-11-22, 08:06 AM
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Originally Posted by easyupbug View Post
Greases are made to very slightly expand most all o-rings materials for improved fit and service life but not repair a worn o-ring, if you use them periodically your o-rings will last much longer.
Lubricant will reduce friction and, thus, wear on the o-ring but no lubricant should change the characteristics of an o-ring. Having swollen many o-rings because of chemical incompatibility, I can tell you that a swollen o-ring is not something you want. Any change to the physical characteristics of the o-ring from swelling to softening to hardening will result in improper performance of the o-ring. The result may be only a slight decrease in performance or it could be disastrous.
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Old 05-11-22, 08:45 AM
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As a follow-up, I got an O-ring at an Ace, applied silicone lube and it works great now.

Actually I got two, have one as a spare.
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Old 05-11-22, 09:00 AM
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I have been clear in the OP's case he needs a new seal, a lubricant will not help him. That said in my discussion with smd4 on preventative maintenance, not repair, or chemical incompatibility, you can read from the Dupont Resource Center "MOLYKOTE® 55 O-Ring Grease is a silicone-based material that helps ensure positive lubrication and sealing by slightly swelling rubber O-rings and seals" which is standard industrial preventative maintenance practice that enhances the sealing and life.
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Old 05-11-22, 03:32 PM
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Originally Posted by easyupbug View Post
I have been clear in the OP's case he needs a new seal, a lubricant will not help him. That said in my discussion with smd4 on preventative maintenance, not repair, or chemical incompatibility, you can read from the Dupont Resource Center "MOLYKOTE® 55 O-Ring Grease is a silicone-based material that helps ensure positive lubrication and sealing by slightly swelling rubber O-rings and seals" which is standard industrial preventative maintenance practice that enhances the sealing and life.
I think you are misunderstanding what the Molykote is doing. From the Dupont technical sheet:

DESCRIPTION
MOLYKOTE® 55 O-Ring Grease is a silicone-based material that helps ensure positive lubrication and sealing by slightly swelling rubber O-rings and seals. This product is heat-stable and oxidation-resistant and is serviceable from approximately
-65 to 175°C (-85 to 347°F).
How to use
MOLYKOTE® 55 O-Ring Grease should be applied using brush, grease gun, or automatic lubrication system. MOLYKOTE® 55 O- Ring Grease can be used in centralized lubrication systems. Do not mix with other greases.
MOLYKOTE® 55 O-Ring Grease has been shown to swell natural rubber. However, compatibility of the lubricant may vary with the plasticizer content of specific materials (especially elastomers). Small-scale compatibility testing should be conducted prior to the use of this product in any application.
MOLYKOTE® 55 O-Ring Grease should not be used with the following materials unless thoroughly tested for your specific application:
• Silicone rubber (SR)
• Polycarbonate (PC)
• Acrylonitrile-butadiene- styrene (ABS)
I read this as it is meant to swell natural rubber o-rings. In my experience, that is a vanishingly small proportion of o-rings available and in common use. Buna, neoprene, and Viton are far more common. I’ve had a lot of o-rings swell and it’s never good for the sealing job they are meant to do.
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Old 05-11-22, 11:02 PM
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Btw, the reason I think the original O-ring got worn down in the first place is some corrosion developed at the bottom of the tube. Not sure how that happened but I began to feel this rough spot at the bottom of the plunger travel. Before putting the new O-ring in I attached varying grades of wet-dry sandpaper to the end of a dowel with doubled over duct tape and worked it against the rough area until it disappeared. 320, 800, 2000, 3000 grit. Wiped it out with paper towels also attached to the end of the dowel and shot aerosol parts cleaner down there until the towels came out with no more residue, then re-lubed with Sil-Glyde.

The whole travel feels buttery smooth and the pump really moves some air now all the way from the top of its travel.
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