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Rotor Bumpers

Designer: Siegfried's Call


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Many types of materials are used as personal preferences by our clients. Each material has positive and negative qualities and we found it time to share our experiences with them as well as offer them as potential solutions here. If the rotor bumper material is too hard, the result may be a louder valve action in both directions and can also potentially lead to the development of side play. Side play is when the rotor bearings start to wear in an elliptical pattern causing it to lose centricity in the housing or casing. When rotor bumpers are too soft the valves are extremely quiet, but the result is a very bouncy action. The strength or stiffness of the springs also play a significant role in how the material feels. For example, a relatively soft bumper material with heavier springs may feel great, but if the horn has a lighter spring action they may bounce or shutter too much. For accuracy of play and a positive overall feel, experimenting with different bumper materials may provide for the discovery of a much greater noise reduction and positive action for your horn valves. It is important to note that not all bumper materials fit the same stopping arm plates as like sizes may not stay in place with some plates. Also note that after installation, a careful adjustment of the bumper needs to be made by trimming the exposed radius to match the hash marks located on the end of the short spindle bearing under the valve caps. Although these hash marks are only a guide, a pro shop would take a quick peek inside the bore using a bore scope to qualify the result. If you have never trimmed a bumper before, we suggest visiting our shop to witness a demonstration. It is easy to scratch the bottom plate with a knife if in a hurry or caution is not heeded.

Buna N - tech stuff: Use in oil applications. Temperature range is -20° to +212° F. Durometer hardness is A70. Buna-N is black.

Sound Advice: Pretty hard by feel with very little bounce, if any. Noisy. Not tested long term for durability and rotor oil resistance.

FDA-Compliant Buna-N - tech stuff: same as Buna N but is white. We use this product as a standard in the shop for all of our horns, and also with cleanings, set up changes, etc. Hard, no bounce, durable, and alignment is the same a year after it is installed. Has a touch of noise, but gets better within a couple of weeks. Fantastic product for horns.


Soft Buna-NTemperature not rated. Durometer hardness is A50. Color is black.

Sound Advice:  This is a good middle of the road bumper, not a lot of bounce and very little noise on most horns. However, it needs to be replaced every 6-12 months.

Neoprene—tech stuff: Resists Freon, oils, sunlight, and weather. Temperature range is -40° to +212° F. Durometer hardness A70. Color is black.

Sound Advice: Very durable over time, great for student instruments with lower maintenance expectations. Very little bounce and slightly noisy.

Silicone—tech stuff: DISCONTINUED Highly resistant to ozone and weather. Temperature range is -67° to +400° F. Durometer hardness A70. Color is red-orange.

Sound Advice: We have discontinued this product. After exclusive testing at our bench, we have found that this material nearly doubles in size after being exposed to oils. It also breaks down significantly. This means that alignment issues will result, and we cannot recommend this product anymore for music instruments.

Soft SiliconeMax temperature is 392° F; min. is not rated. Durometer hardness is A50. Color is black.

Sound Advice:  Softest feel with a little bounce particularly with lighter springs. Very little noise, if any. Fairly durable and resistant to oils. Our shop favorite feel.

Soft Viton® FluoroelastomerTemperature range is to 392° F. Durometer hardness is A55. Color is black.

Sound Advice: After testing by far the best product we have found for horns. A bit tough to cut but well worth the trouble.

EPDMHas excellent resistance to water and steam. Temperature range is -40° to +212° F. Durometer hardness is A75. Color is black.

Sound Advice:  A bit on the harder side, but fairly durable. Cuts well.

Our current inventory of the above materials is available in a 3.25" cord strip and is .210" in diameter. (we have found this diameter works for most applications)

For horns with more than 4 valves we recommend buying 2 packets. We have also prepared them pre cut as a set. A set is to be used for up to 4 valves.

In our shop, we primarily use Soft Viton. Again, we change this based on our feel at the bench and customer feedback.



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