Trimming Bump-Stops

[icechains55]

Hey guys, so I've heard that you're supposed trim the bump-stops (rubber thing in bellow that slips over strut) on lowered cars, but I'm not sure how much to trim off. I'd say that for a 1.5 inch drop, they should be trimmed 1.5 inches, but I thought I'd check here first. Also, I'm using kyb bump-stops on Bilstein struts and the actual strut on the bilsteins are way thicker than the openings in the bump stops. so I've decided to bore out the hole so it'll fit. Anyone ever done that before? How did you do it? TIA

 

[industrialfish]

Don't trim them and don't bore them. You should only need to trim them if you are going REAL low. I'm dropped 1.75ish and I've never hit the stops with Vogtland springs. Just grease them up, or not, and twist them on there. You don't want them loose or you can get crap on the piston under the bellow (attached to bump stop).

 

[wstefan20]

Your best option is to purchase upgraded bump stocks from fat cat motorsports. From experience, he knows his ****, and they work so much better than anything else. But no, don't cut them down

 

[spazzz]

Hey guys, so I've heard that you're supposed trim the bump-stops (rubber thing in bellow that slips over strut) on lowered cars, but I'm not sure how much to trim off. I'd say that for a 1.5 inch drop, they should be trimmed 1.5 inches, but I thought I'd check here first. Also, I'm using kyb bump-stops on Bilstein struts and the actual strut on the bilsteins are way thicker than the openings in the bump stops. so I've decided to bore out the hole so it'll fit. Anyone ever done that before? How did you do it? TIA

you Have to trim them, finding the oems are futile. I honestly can't remember how i did it. I think I cut the first section off (small cone side), hollowed it out with a sharp steak knife (sawing).
I had a solid reason at the time for cutting it down and common sense says you HAVE to bore it out.

I have Bilsteins on the rear with SSCs.

They need this done to go around the shaft and to fit in the boot holder

 

[spazzz]

Brainfart

Bilsteins have internal bumpstops so the old one is trimmed to hold the boot into the shoe

 

[industrialfish]

you Have to trim them, finding the oems are futile. I honestly can't remember how i did it. I think I cut the first section off (small cone side), hollowed it out with a sharp steak knife (sawing).
I had a solid reason at the time for cutting it down and common sense says you HAVE to bore it out.

I have Bilsteins on the rear with SSCs.

They need this done to go around the shaft and to fit in the boot holder

Stop right there. You may have had a reason at the time but it was still wrong.

If you are slamming your car you probably will need to trim them to get any travel, but otherwise do not trim them.

If you are worried about bottoming out your strut, your problem isn't your bump stops, it's too light of spring or strut. Trimming the stop will increase how much your strut can compress, right up until you blow a piston seal from over compression.

Trimming your bump stop means that when you DO bottom out, it's more violent because now there is less material to absorb it.

Don't bore the bump stop either, it needs to be tight on the piston because the top part of the bellow connects to it, preventing debris from collecting on the piston.

 

[industrialfish]

Brainfart

Bilsteins have internal bumpstops so the old one is trimmed to hold the boot into the shoe

Ok, this is important lol.

My other comments still stand, in general applications.

 

[icechains55]

you Have to trim them, finding the oems are futile. I honestly can't remember how i did it. I think I cut the first section off (small cone side), hollowed it out with a sharp steak knife (sawing).
I had a solid reason at the time for cutting it down and common sense says you HAVE to bore it out.

I have Bilsteins on the rear with SSCs.

They need this done to go around the shaft and to fit in the boot holder

That's what I'm talking about. The piston on Bilsteins is about 1 and 1/2 inches in diameter, while the opening in the KYB bump-stop is maybe 3/4 inch at best. There's no possible way to get the bump-stop over the pistons without boring it out to fit. And even if I bore it out its still gonna fit tight as hell. No amount of lube will get 3/4 inch around that 1.5 inch shaft.
So I trim the small cone side? about how much? I have an 05 Grand Prix on 18in rims and the bilsteins will be on all 4 corners lowered on Canuck springs which are basically the same as SSC if I recall.

 

[spazzz]

You only need the big end to sit inside the boot.
You take the top metal piece, insert boot and then jam the trimmed bumpstop in.

So you need the big flat ring side that fits in the top metal cover of the boot. Not much else is left acter trimming that will fit over the shaft once the plate bottoms out.

Thanks, Industrial for hollerin at me. We're buds and you know it.

 

[icechains55]

Soooooo how much of it do I trim? in inches lol

 

[spazzz]

Soooooo how much of it do I trim? in inches lol

I can honestly say um?? a lot.

I only have the rears but I imagine the fronts will be the same. Let me put it this way, the plate, washer, strut boot keeper or whatever its called bottoms out on the piston which is rounded. I kept the ring part for inside the metal cap and beveled as much as needed to encompass the top of the piston, somewhat like a human body appendage socket.

If you put the bump stop in the boot cap and trial and error with the strut you will know just by seeing it and trimming.

It's all coming back...I remember when I was done just saying to myself, why did I just use the boot keeper part.

The way I did it had a little more meat.

Do you have the struts bump stops and boots in front of you or are you using the old stops and boots?

 

[spazzz]

cut them like this then hollow out enough to sit on the strut.

 

[wstefan20]

That's what I'm talking about. The piston on Bilsteins is about 1 and 1/2 inches in diameter, while the opening in the KYB bump-stop is maybe 3/4 inch at best. There's no possible way to get the bump-stop over the pistons without boring it out to fit. And even if I bore it out its still gonna fit tight as hell. No amount of lube will get 3/4 inch around that 1.5 inch shaft.
So I trim the small cone side? about how much? I have an 05 Grand Prix on 18in rims and the bilsteins will be on all 4 corners lowered on Canuck springs which are basically the same as SSC if I recall.

You do realize that the bilsteins have an internal bump stop right? This is the bump stop you really need to be changing to get the correct ride quality. In a perfect world, you would get the shock re- valved to compensate for the drop and dyno tested to ensure the correct compression rate and rebound rate so the bump stop was correctly sized.

Basically, without re- valving, you'd want to go for shorter and harder internal bump stocks not add external if you want your shocks to last.

Here's what the internal bump stocks look like stock:

 

[icechains55]

Didn't know they were on the inside. I guess i'll cut and bore the ones I have mostly just to hold the boots on. They're brand new KYB bump stops and boots. the last time I did this was 10 years ago, and that was on KYB GR2 struts with eibach springs and I just reused the stock bump stops. Bilstein is a completely different breed of strut. Thanks for all the help though guys.