Just testing the water here, has anybody ever heard of drilling out the spark plug hole in the 346 Cadillac heads to install a later model or better performing spark plug. Seems like it could be do able, but be nice to know without having to learn the hard way what anybody else has done before me? I figure it has probably been done by Ford flathead guys. Any thoughts would be appreciated.
Do Flathead Cads have 14mm plugs? like maybe a J series Champion? Or what? What would you drill them to? Maybe you could use an insert and run 12mm plugs if you wanted to use newer stuff. What's in a Cad stock?
From what I know 14mm is correct although some heads had larger. I am just not an expert in this spark plug area and was casting my line out to see what folks had done to improve flathead efficiency in that department. I have electronic ignition and dizzy and this just seemed to be the area I had not given any thought to and dont have much knowledge in.
All the good plugs are made in 14MM ,poor selection in most other sizes. I was kinda thinking those used 10MM plugs like Chevy in the same era??
I've contemplated this one before too. A few things to consider aside from size, including where the plugs seat and how far the electrode projects into the combustion chamber. My long term aim is a set of Cyclone heads so I dismissed the effort on my Lasalle heads as wasted once I'd found a new set of the correct plugs. That of course brings up the question of the plug size in the Cyclone heads....are they the large or the small diameter? Kev would be able to answer that one.
Some modern motorcycles use 10mm plugs and in high performance plugs they are readly available at Motorcycle shops and spark plug deallers. I believe the best seclection would be in 12mm plugs, again in high preformance/racing plugs. When I made the head for my Plymouth four I used 10mm for that reason. I see some older engines with 18mm plugs. Never run into 12mm in a vintage engine
Well, Kev at Cyclone kindly got back to me with this answer in relation to his heads: "We drill the heads to 14mm. If you have a drill and a tap set up for the smaller plugs, I can leave those blank and just spot face the area." Thanks Kev.
Ok you know the plugs are 14mm. The pretty much standard plug dia for the last 70 years, except for some 18mm Fords. Now you need to know the reach. How long are the threads. They come in .460-.680-.700-7/16-3/8-.445-.500-3/4-and some inbetween sizes. Since he said it was spotfaced I'm guessing it's a gasket seal plug and not a 60 degree taper seal plug like newer hipo stuff. For a cast aluminum head I would use 3/4 reach, but I don't know what his are. Then you need to know what it is you expect these plugs to do. What do you mean "Better performing spark plug"? Is this a blown fuel Cad Flathead? Do you need a colder plug because of preignition? Do you need a hotter plug because of fouling? Could you use a projected tip? I bet if the motor is fresh you want what ever it came with. If it's old and worn you want to go one or two ranges hotter. Depends on how your going to use it. Your not going to pick up HP with plugs but you can keep from losing it.
It is a re-built never run yet flathead with stock heads, a cam, carbs and electronic ignition, no blower, I guess I am trying to do what you said keep all the HP I can get out of it. I have no previous experience with these monsters and saw plugs as an area I had not considered until now. I guess I am after the most efficiency possible, I dont know what bad or good characteristics these engines possess.
They are 10mm, if they are not, then they have been modified already. Yes they can be redrilled and tapped. Cyclones aftermarket heads are 14mm, and I have seen both 10 & 14mm on the Edmunds aftermarket heads (thats not to say the sets I've had that were 14mm weren't drilled out though, I dont know) Hope that helps. Drill 'em, tap 'em, and never look back.
I use 10mm plugs in my Plymouth because of a better heat range choice then with most 14mm plugs. What is the reach for your plugs? You might want to contact Rick Gold at ERC-[email protected]- Spark plugs are his game. He has all kinds of them and knows what works. I guess the Cad came with AC 48 plugs. A very short 10mm plug? You can get NGK and other 1/2 reach 10mms in racing ranges.. Not sure about shorter.
Thanks, I will follow that lead up Rick Gold. I will also go and check out a couple of other pairs of heads in my dads shop to see what plug hole size they have.
For those who might be interested we have some PM's going on in the background, we have reached the following point. This is pretty interesting to me. I never knew old Cads and apparently 216 Chevys had 10mm spark plugs. I thought they were new stuff. When you pull yours out would you do me a favor and email the thread length and what ever plug they are (AC-48 of whatever) Thank you. Rich Fox Well Rich, Ive checked and very interesting the earler heads about 1938 are 14mm and the later ones about 1946 are 10mm approx so I plan to use the later heads which have the 10mm hole it could be 11mm but it was difficult to get an accurate measurement. Well I looked it up on line and they should be 10mm. So you can drill and tap them for 14mm plugs which are much more common, or you can take an old plug to a motorcycle shop and try to find if they have something the same reach. I made some copper spacers to use longer plugs in my head and Champion sells some also. Then you need to consider heat range. Lots of wide open throttle operation you want a colder plug. Lots of stop and start around town a hotter plug or maybe a projected tip plug that will burn clean. In my street driven Chevy/Stude I went one range hotter than stock to keep the plugs clean. My Plymouth went to a cold plug because it's a Bonneville car and runs wide open a lot.With luck the bike shop (if you go with 10mm plugs) will have a cross reference or maybe you can find one on line. Still a good idea is contact Rick and see what he says. He really knows about this stuff. RF. I dont own any plugs as yet so I dont have a plug length. The early heads must have had the holes drilled out to 14mm then. I still need to find out what was the original plug size and especially the length as a start point. Thanks to Rich for his help.
On my Plymouth I used a NGK C8HSA 10mm by about 7/16 long thread. Cheap on the net or at motorcycle shops. CR8HSA is the resister type of the same plug. In Champion I ran Z9Y. for a hotter warm up plug I ran Champion Z6. Without the gasket they measure almost half inch reach. When I made the head I left 1/2 thick over the combustion chamber where the plug went.
so, after reading this infomation on spark.. the 14mm will give not just increase the spark ,but as well hp? just have to redrill and tapped? did it work?
Russ, They are 10mm. I'll measure the thread length and get back to you later today.I would tap them for the better selection of plugs to run that HEI. You WANT to run later heads anyway. The HP was bumped from 135 on early motors, to 150 on later motors, because of the heads. So be running the later heads, you'll have better compression and more HP. I have a good set of '47 heads, if you want to pick them up when you are here.
Russ, just noticed the 'cadillac' on your heads. Are they tank or boat heads? Oops! I see that that question was answered while I was typing.
14mm will not increase hp or much of anything else. But 14mm is a much more common spark plug diameter and for street driving it is hard to find a 10mm plug that isn't to "cold" Motorcycles use plugs with a nice heat range for a race car. Maybe not so good for a street driven car that may idle in traffic.
Russ, I just pulled a plug off of the '47. It has very short threads- 5.39mm. from the plug seat to the outer edge of the electrode is 8.1mm. It is an AC plug, M-8 Hope this helps.
Here is a pic of the plug. I think that if I were to do it, I would add extra material into the spark plug recess, then tap it. That would allow for a better selection of plugs, while maintaining the electrode location. If you picked a plug first, you could work the heads to that series of plug without to much effort.JMHO.
I sourced some Iridium CR8HSA which are model NGK #CR8HIX. I hope to be able to fire the engine up soon. I got my heads in for checking for cracks and flatness. Got plugs coming and next will be period correct engine oil. Thanks to everybody who posted some plug info.
Russ, FYI- I cut a head into, to map the water passages. There is at LEAST a 3/8 boss around the plugs. Plenty of meat if you want to bore the plug holes out to a more common 14mm
Thanks Buddy, spoke with my engine shop guy and that is what we will do next if these small plugs dont work out. Got to say it was a little difficult asking for 8 plugs in a motor bike shop, they only seem to carry 4 in stock.
I get mine online by the box. usually 20 at a time. Don't guess you would be interested in that. In a Champion plug I am runnimg RG4HC plugs. These were reomended by Rick Gold of ERC. Those of you who run at Bonneville know that Rick is very good at tuning and plugs.
Good idea Rich, if these plugs are any good I will buy them in a box online. Seems to be the way to go.
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