Lager1

This 91 new to me runs like butt on cold start and all the way thru warm up, then runs OK when hot. Wanted to take a look at the innerds of the distributor cap and rotor and I cant get the metal cover off? The hex head screws are right up against the recessed holes in the aluminum cover. Looks like they have never been removed and its possible since the car only has 80K on the clock. Looks like Im going to have to go " cave man" on the cover to get to the screws. The alum cover might not survive my next step.

FostersPerformance

Wernt they torx screws?.

Either way, use the correct bit and impact driver (that you use a hammer with). Maybe even fill that cavity with some PB blaster and let it soak.

NavAir

This is the way to do it, although, if those screws haven't been out since 1991 expect some dissimilar metal corrosion (the cover is aluminum, the screws are steel). Be patient, if you break them off, it will take some effort to get the remainder out...

Lager1

Man,, I wish they were torx screws,, Nope just plain hex head screws, maybe 7-8 MM or 5/16? I cant get a socket on these screws no matter if I use metric or SAE due to the interference of the alum cover meaning too close. If got all sorts of sockets cause I was a tech for over 45 years. Have no idea how anyone would have such a thin wall socket to to fit in there.. Cave man time I guess ?

Lager1

Yep,, if they were Torks,, Looks like Im going to have to drill the head of the screw off, then either reinstall a Torx or a Phillps.. Thanks for the reply.. really appriciate it.. I have NO idea what was OEM or some one replaced these screws. with hex heads.. Gotta love an old car..

s carter

Have ground more than a few socket walls thin in the past years. But if you're patient could try to cut a groove in for a screwdriver. Or use a reverse drill bit, more than a few times I have gotten screws out with them.

Incorvettei

Yep, cut the heads off, tap and remove

Lager1

Now , there is an idea,,,, Thanks for your imput.

Lager1

LOL,, last resort,, might have to. I gotta get this cap off.. Thanks for the suggestion, see your a cave man too ?

slammin

I believe the factory screws are M4 x 0.7 button head torx using a T20 wrench. My 91 was missing the cover. I checked the tapped hole size and I am using stainless steel screws. It sounds like yours has had the holes re-tapped. An M4 screw is .001 inch larger than a 5/32 diameter. If you can hit them with some PB Blaster you may be able to use a small easy out to remove them without destroying the cover.

WVZR-1

The original screws were M4-.7 and were Torx so if yours aren't Torx they likely weren't M4X.7 either. Someone quite likely has done something other than OE for trim retention.

J H

I use plain ole screws that I install/remove with a flat-head screwdriver

Lager1

Winner Winner Chicken dinner goes to,,,,,,,,,,,,,,,Member S Carter... Just cut a slot on the head of the hex head screw head with a die grinder and used a flat head screw driver to pull them out. No damage done to the alum cover at all, I'm calling this battle a WIN. Thanks again S Carter, I was expecting the worst. Now,, when it goes back together, use the proper screws.

FostersPerformance

Don't forget a tiny bit of grease on the screws when you re install. Next time they will come out like a charm

WVZR-1

You need to ID the thread of what was used in your car by someone else. Generally speaking if you can purchase the same thread with a head type you'd desire you would just 'chase' the female threads and use that thread. M4 X .7 isn't an easy find but if they are in fact M4 X .7 then you could buy perhaps that thread and use the Torx 'button head'. Are they T20 as mentioned or maybe T15?

slammin

WVZR-1 My 91 C4 was missing the distributor cover. I checked the holes in the manifold with a M4x0.7 thread gage. Bingo, a perfect fit. I completed my tool and die apprenticeship program and recieved my journeyman's card the summer of 1971. Since I only needed 3 screws, I bought 8-18 stainless steel button allen screws at the local hardware store. I would have bought torx head if the stocked them. Here is a link to McMaster Carr, listing the metric torx button heads they stock. I didn't need a hundred, so I opted for the allen head screws.
https://www.mcmaster.com/products/sc...d-torx-screws/

s carter

Another source for the screws, I normally put anti seize on the screws and screw down to just touching and just a little snug up
​​​​​​https://corvetteparts.com/item/screw...sion-1985-1991

WVZR-1

I'd avoid these until you actually confirm the female thread you're wanting to fit them to. OP mentioned his weren't Torx headed so it's certainly possible you could have 5$ plus the ship $$ invested and then not be able to use them.