I need to drill and tap Vortec 350 missing timing cover bolt holes. Help with size!!

[QUAKE_WARS]

I am doing a aftermarket aluminum timing cover on the Vortec 350. I am missing 2 holes on the timing cover because the plastic
timing cover does not use them. What size drill and tap do I need? I decided to go all out with a complete overhaul of this engine
even when this was not the plan. I was suppose to just drop this in and drive it till the LS swap was ready. Well one thing led to
another as you all know it always happen this way LMMFAO!!!

This is NOT my engine picture but just a example. you can see the missing holes Thanks ALL!!

 

[Twisted_Metal]

I would guess you drill and tap them the same size as the other ones.
Take one of the other bolts to a hardware store and use their thread gauge. (If you don't have one)

 

[sandlapper]

see the passenger side cup plug peeking thru the slot in cam sprocket at about 11 o'clock?
Note how that plug has tiny hole in middle of it? The driver side plug has that hole as well.
GM designed those holes for a purpose; and introduced them with advent of L31 Vortec --- to help air escape oil galleries which helps quiet valvetrain at each startup.

Don't screw da pooch as others have; during reassembly, Do Make Certain those plugs are replaced with plugs with same size holes.

With pan off, Do Note: that L31 Vortec oil pump is NOT traditional M55.
A few years prior to advent of L31 Vortec, GM and Melling re-designed pump to have a larger OD (~ 3/4") pickup tube; the newer design is M155, repeat M155. The larger tube improves pump stability and performance in low-to-mid rpm --- where motor typically spends majority of its service. NOTE: M155 is standard volume & standard pressure; but has larger tube

Use a shop that has "Bake & Blast" capabilities to clean iron block & head castings. Money Well Spent!
Far superior results than any other means I know of.
When I get em back, there ain't even a trace of paint etc; anywhere! And the iron looks brand new!

 

[QUAKE_WARS]

Got it, 1/4-20 with a #7 drill bit

 

[Ratty_81]

Just take your time and use plenty of lube while tapping. When it starts to bind up, reverse direction until starts to turn easy then go forward again and repeat the process.

 

[Twisted_Metal]

Since that tap is tapered (not a bottoming tap) You will need to drill and tap a bit deeper than the bolts are long.
You don't want to run out of full diameter threads and break the bolt while tightening.

 

[sandlapper]

think & plan ahead
if OE GM iron vortec valve guides (Integral w/ head casting) are worn beyond spec, don't replace with new guides --- but, instead, have shop ream then install thin-wall liners (bronze alloy) aka K-Liners. That costs much less, has much less risk of cracking head and does Not create a port obstruction (turbulence) like a replacement guide will. K-Liner is a win-win with No downside I'm aware of.

 

[larrylarry]

I’m using an aluminum timing cover also and it was leaking from the bottom of the cover. I had to double gasket the cover to stop it.

 

[QUAKE_WARS]

I’m using an aluminum timing cover also and it was leaking from the bottom of the cover. I had to double gasket the cover to stop it.

Who did you buy it from?

I got mine from HOLLEY. I know I could use it without the 2 missing bolts, But I cant stand to look at it with
the bolts missing. LOL ....The 21-150 is for the carb conversions.

 

[Ratty_81]

Can’t you just use a run of the mill timing cover? What’s the purpose of using the very specific looking Holley unit? Is the motor staying in the truck?