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.
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**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: 2,131	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: 2,131	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?



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