size of the bolts to mount the 425 impco?

dutchlincoln79 · Post by **dutchlincoln79** » Thu Jan 17, 2008 11:24 am

Hello,
Can anyone tell me which size the 4 bolts have to assemble the 425 to the throttle plate?
Just been to a tractor company to get some UNC bolts, but he claims that it ain't UNC. Is this correct, and if so, what IS it?

Thanks!

Stef.

C3H8 · Post by **C3H8** » Thu Jan 17, 2008 1:18 pm

They are satndard SAE grade 5 1/4" bolts with a 7/16" wrench flat. Thread is NC. Should be 20 threads to the inch.

Steptoe · Post by **Steptoe** » Thu Jan 17, 2008 1:22 pm

Im preety sure they are unc...(?) use the holley cross head screws, get some of a old junk carb

Another thing to watch is if using the holley 4 barrel gasket there are 3 or 4 diff gaskets...they are not all suitable.
I ended up making my own.

A smear of oil on the 425 surface, place on to the gasket paper on a flat block of wood, place the 425 on the gasket paper then with a block of wood on top of the 425, gently tap the top.
This creates a clear impression to cut out, and use hole punches.

From memory u leave the center hole blocked off (Franz confirm?)

I use a ffairlyy heavy fuel restant grade gasket paper gauge.

A light smear of oil on gaskets before assembly stops them sticking..usefull if out on the road repairs and need to re use a gasket.

C3H8 · Post by **C3H8** » Thu Jan 17, 2008 1:30 pm

Correction to my previous post. I was thinking of the wrong screw. Screws on the IMPCO 425 base are 12-24 X 5/8" long. Its a strange size. The part number is S1-19 and available from IMPCO distributors. I guess IMPCO chose this one so they could sell screws without too much competition

dutchlincoln79 · Post by **dutchlincoln79** » Thu Jan 17, 2008 4:27 pm

thnx.

Going back to the other shop tomorrow to try them.

regards,

Stef.

franz · Post by **franz** » Thu Jan 17, 2008 6:40 pm

You will need to block off two of the original 6 Holley screw holes. With the throttle plate held up vertically, and looking at either the top or bottom, the ones to block off are at the 12:00 oclock and the 6:00 oclock positions. If you leave them open, you will have a high idle that cannot be adjusted down. Some Holleys have a center hole, and some of these have an orifice in them for a controlled idle bleed. Remove the orifice and plug the port.

You can use a self tap socket head set screw or partially thread the hole and insert the set screw, from the top down. You can also use aluminized tape, the type used on air ducts that has a paper backing that is peeled off. This tape is resistant to heat and is real thin. This way, its easy to return the baseplate back to service if so desired.

Franz

dutchlincoln79 · Post by **dutchlincoln79** » Fri Jan 18, 2008 9:50 am

been to a shop, and 5/8" is a HUGE bolt!!

i don't understand, i can't find the size anywhere...
I think i'll drill em out and thread a normal standard size in it...
Damn bolts....

dutchlincoln79 · Post by **dutchlincoln79** » Fri Jan 18, 2008 10:28 am

Franz,

I just put the 425 onto the base plate, but the 425 covers the 2 holes you mentioned.
It --seems-- that it's not necessarily to put 2 bolts into the base plate.
Correct me if I'm wrong?
Now, I've bolted the 425 onto the base plate, and i see that the butterfly valves just touch the 2 notches in the bore of the 425. Also the butterfly plates (of the secondaries) seem just a bit too large for the 425, so i have to chamfer the edge a little bit.
Is this common?
I heard about the notches, but not of the rest.

Please inform before i ruin my 425

thanks,

Stef.

alehander · Post by **alehander** » Fri Jan 18, 2008 11:06 am

" Screws on the IMPCO 425 base are 12-24 X 5/8" long."

12 = Diameter reference
24 = pitch (tpi)
5/8 = length (in)

You're not looking for a 5/8" diameter screw (which IS huge), but a 12-24 screw which is 5/8" long. Good luck, -A

Mattelderca · Post by **Mattelderca** » Fri Jan 18, 2008 11:12 am

dutchlincoln79 wrote:Franz,

I just put the 425 onto the base plate, but the 425 covers the 2 holes you mentioned.
It --seems-- that it's not necessarily to put 2 bolts into the base plate.
Correct me if I'm wrong?
Now, I've bolted the 425 onto the base plate, and i see that the butterfly valves just touch the 2 notches in the bore of the 425. Also the butterfly plates (of the secondaries) seem just a bit too large for the 425, so i have to chamfer the edge a little bit.
Is this common?
I heard about the notches, but not of the rest.

Please inform before i ruin my 425

thanks,

Stef.

Can you take pictures? Maybe we can offer better advice that way, and we always love to see others peoples stuff!

dutchlincoln79 · Post by **dutchlincoln79** » Fri Jan 18, 2008 12:11 pm

Below some picca's

dutchlincoln79 · Post by **dutchlincoln79** » Fri Jan 18, 2008 12:18 pm

any idea how i can add the ford A/T kickdown lever?

Never looked at it for the tp, and its obvious that it doesn't have one...

franz · Post by **franz** » Fri Jan 18, 2008 12:34 pm

Ok, you are using what looks like an 850 baseplate whose throttle blades are quite a bit larger than what the 425 was originally designed for, the 1974 Ford 391 with the 600 cfm Holley 4bbl.

If the throttle blades just barely touch the 425 casting, you can gently grind away on the 425, but if it need some serious cutting, locate a smaller baseplate.

Also, thanks for the VERY nice photos, it just shows that photos can easily explain what it takes many words to convey!

Franz

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size of the bolts to mount the 425 impco?

size of the bolts to mount the 425 impco?

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