How to cut MGB inner fender for RV8 headers

skootch13 · June 09, 2026, 02:37:51 PM

Hi guys. I will be cutting openings into my inner fenders for RV8 style headers. I have two questions. One, Is there a pattern somewhere I can use?

And two. What tool do I use to cut the fenders? Air powered jig saw? What blade? One of these? https://www.harborfreight.com/5-Amp-14-Gauge-Metal-Shears-64609.html?event_id=184606&hftm_sc=&hftm_source=google&utm_source=google&utm_medium=cpc&utm_campaign=21901270715&campaignid=21901270715&utm_content=169550067559&adsetid=169550067559&product=64609&gad_source=1&gad_campaignid=21901270715&gclid=CjwKCAjw857RBhAgEiwAI-1yKE29hA3AWLFjhpS31ZCkRu8JMae6tp1xVEGCsp4rMufHV9ZqYqyxLRoCSFAQAvD_BwE

BlownMGB-V8 · June 13, 2026, 04:21:49 PM

I would lean towards using a holesaw. You could cut out some paper circles and stick them where you think the opening needs to go, overlapping them where needed. Each one only needs support for the pilot hole. Then other methods can be used to smooth and enlarge the opening.

Jim

skootch13 · June 17, 2026, 01:31:27 PM

I like that Idea, Jim. Slow. Methodical. Can be controlled. Then grind away rough edges.

MGBV8 · June 17, 2026, 01:54:19 PM

Back in the day, there was a pattern to get one started. The exact hole location will depend on engine placement & header design, of course.

BlownMGB-V8 · June 18, 2026, 09:40:29 AM

A die grinder and large carbide burr speeds things along nicely.

Jim

jackrobbert · June 19, 2026, 07:41:42 AM

I'd avoid cutting until you've test-fitted the headers if possible. Even with RV8-style manifolds, there can be slight differences between manufacturers, so a generic template isn't always accurate.

For the actual cutting, an air or electric jigsaw with a fine-tooth metal blade works well. Many people also drill a pilot hole first and use a nibbler or a small angle grinder with a thin cutting disc for the tighter sections. Just take your time and cut slightly undersize—you can always trim a little more to get a neat fit.

Once you're happy with the opening, don't forget to smooth the edges and add some edge trim or paint the bare metal to prevent rust.

BlownMGB-V8 · June 19, 2026, 07:55:51 AM

If you are a dab hand with a zip blade you can slit a length of steel tubing and fit it over the edges of the cutout and then braze or weld it into place, giving a professional rounded edge to the opening and making it stronger. Just make sure it doesn't jump out of the vise.

Jim

skootch13 · Reply #7 - Re: How to cut MGB inner fender for RV8 headers

Quote from: BlownMGB-V8 on June 19, 2026, 07:55:51 AMIf you are a dab hand with a zip blade you can slit a length of steel tubing and fit it over the edges of the cutout and then braze or weld it into place, giving a professional rounded edge to the opening and making it stronger. Just make sure it doesn't jump out of the vise.

Jim

I will bring the car to you.