Exhaust trailing arm hole question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Cristiann8 · 07-31-2020, 03:20 AM

Is a 3 inch hole to drill into the trailing arm crossmember for 2 1/2 inch exhaust enough? I bought a 3 inch hole saw but it looks quite small but I want to make sure before I start drilling my crossmember.
Posted via Mobile Device

**SIXTY9** · 07-31-2020, 09:07 AM

I used a 4" hole saw on mine just to make sure it had enough clearance. I did add a 1/4 plate to make it stiff.

**Grumpy old man** · 07-31-2020, 09:17 AM

That's tight , I used 4" for 2 1/2" pipes and it was a lot easier to mark the hole location and use a cut off wheel to cut thru the cross member and get a tight fit on a short section of pipe to weld in to place , The 4" pipe came from Metal supermarkets and they cut and shipped it to me for less $ than I could have driven there to get it across town .

SCOTI · 07-31-2020, 12:29 PM

They also have bolt-in kits out there for simplicity if that helps....

https://azproperformance.com/collect...outs-63-72-c10

LS short box · 07-31-2020, 04:57 PM

I cut mine with a 3.5" hole saw and reinforced with a flat plate of steel 3/16" with a 3.5" hole cut in it.

**70STOVEBOLT** · 07-31-2020, 05:23 PM

I used a 3 1/2 hole saw on mine. Unless you are going to have 500+ HP you don’t need to reinforce it.

gmc684x4 · 08-01-2020, 12:47 AM

I would say re enforce the hole for a professional fit and finish

LS short box · 08-01-2020, 04:51 PM

My frame was bare when I cut the holes for the exhaust so it was easy to do. I didn't know what kind of strength I removed when I made the hole so the reinforcement plate seemed like a good idea and I kind of like to do fab stuff. My LS1 did 380 rwhp on the chassis dyno so about 450 at the crank?

07-31-2020, 03:20 AM	#1
Cristiann8 Registered User Join Date: May 2018 Location: Porterville Posts: 53	Exhaust trailing arm hole question Is a 3 inch hole to drill into the trailing arm crossmember for 2 1/2 inch exhaust enough? I bought a 3 inch hole saw but it looks quite small but I want to make sure before I start drilling my crossmember. Posted via Mobile Device

07-31-2020, 09:07 AM	#2
SIXTY9 Senior Member Join Date: Jan 2013 Location: North Charleston SC Posts: 504	Re: Exhaust trailing arm hole question I used a 4" hole saw on mine just to make sure it had enough clearance. I did add a 1/4 plate to make it stiff. Attached Images Last edited by SIXTY9; 08-01-2020 at 07:11 AM.

07-31-2020, 09:17 AM	#3
Grumpy old man Senior Member Join Date: Dec 2013 Location: Gods country East,Tn Posts: 8,545	Re: Exhaust trailing arm hole question That's tight , I used 4" for 2 1/2" pipes and it was a lot easier to mark the hole location and use a cut off wheel to cut thru the cross member and get a tight fit on a short section of pipe to weld in to place , The 4" pipe came from Metal supermarkets and they cut and shipped it to me for less $ than I could have driven there to get it across town . Attached Images __________________ 1967 Factory short bed - Old school '71 - 350 / 4bolt / 487 heads / Edelbrock C3BX Muncie M-22 4 speed / Hurst Comp plus Factory 12 bolt posi 3.73 / 255-70-15 Smoothed firewall / Factory cowl induction Power disc brakes / power steering / 3.5-5" drop

07-31-2020, 12:29 PM	#4
SCOTI Registered User Join Date: Apr 2001 Location: DALLAS,TX Posts: 22,064	Re: Exhaust trailing arm hole question They also have bolt-in kits out there for simplicity if that helps.... https://azproperformance.com/collect...outs-63-72-c10 __________________ 67SWB-B.B.RetroRod 64SWB-Recycle 89CCDually-Driver/Tow Truck 99CCSWB Driver All Fleetsides @rattlecankustoms in IG Building a small, high rpm engine with the perfect bore, stroke and rod ratio is very impressive. It's like a highly skilled Morrocan sword fighter with a Damascus Steel Scimitar..... *Cubic inches is like Indiana Jones with a cheap pistol.*

07-31-2020, 04:57 PM	#5
LS short box Registered User Join Date: Jun 2015 Location: Carlos MN Posts: 2,132	Re: Exhaust trailing arm hole question I cut mine with a 3.5" hole saw and reinforced with a flat plate of steel 3/16" with a 3.5" hole cut in it.

07-31-2020, 05:23 PM	#6
70STOVEBOLT Senior Member Join Date: Jun 2005 Location: Winona Lake, IN Posts: 6,414	Re: Exhaust trailing arm hole question I used a 3 1/2 hole saw on mine. Unless you are going to have 500+ HP you don’t need to reinforce it. __________________ 70 C/10 SWB 402/TH400/3.73 "The Needy Beast" 200,000 Mile Club Disc Brake Club Owner installed options: Front Sway Bar Power Steering Power Brakes Cigar Lighter Courtesy Lights Deluxe Side Markers Wiper Delay Sliding Rear Window Power Windows Power Locks Sniper EFI 2015 Silverado 1500 LS 4.3/6L80/3.23 lowered 2" front & rear

08-01-2020, 12:47 AM	#7
gmc684x4 Registered User Join Date: Jul 2012 Location: Whitehorse yukon Posts: 1,218	Re: Exhaust trailing arm hole question I would say re enforce the hole for a professional fit and finish

08-01-2020, 04:51 PM	#8
LS short box Registered User Join Date: Jun 2015 Location: Carlos MN Posts: 2,132	Re: Exhaust trailing arm hole question My frame was bare when I cut the holes for the exhaust so it was easy to do. I didn't know what kind of strength I removed when I made the hole so the reinforcement plate seemed like a good idea and I kind of like to do fab stuff. My LS1 did 380 rwhp on the chassis dyno so about 450 at the crank?



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.