Cool bore verses Steel bore

paulkish

old fart
Years ago when we ran boxstock our engine builder told us a well seasoned Cool Bore was a better block to start with then a Steel Bore.

Being it was Dave Turner who told it to us and our cool bore always seemed to run better then the Steel Bore, I always accepted it. I'm sitting here in the morning reading on bob's about engines and fuel and the question about it came back to me.

If a Cool Bore is a better block to start with then a Steel Bore, why is it so?

My thoughts were always it had to do with a more even engine expansion rate of a Cool Bore. Or maybe a seasoned Cool Bore held oil better on the cylinder walls, providing a slight performance advantage. Those are my two best guesses.
 
Paul your observations are valid. In addition the aluminum / silicon cylinder glazes over faster. A good Briggs stock racing engine is refreshened regularly, especially when we had to run the stock rods. Most of the time the engine is honed or at least deglazed. "Grow the Glaze don't break the glaze".
If you'll notice in the Briggs & Stratton service manual they recommend to not deglaze an aluminum cylinder unless it's .005" out, and then bore it.
How many race laps do you think is the equivalent to 1 hour of mowing your lawn ? How about a whole summer ? When we refresh an engine after ,, say? 600 laps that no running at all, even if we do double the RPM, in the whole skeem of that engines design.
So what I'm saying is an aluminum cylinder will glaze over much sooner than a steel one will. And about the time the IC begins to run well because of the glaze,,,, we refreshen it and hone the cylinder.
In my opinion,, a well prepaired, properly maintained used / glazed,1000 lap,even out of round say a half thounsandth, cylinder with new rings prepped properly, seals up and is faster than a fresh properly prepaired cylinder... aluminum or steel. Proven this in many different types of engines.
Most piston rings have a coefficient of conformability of .006". That means they are designed to function in a cylinder that's out of round .006"!! Now we grind them to within tenths of the cylinder size but I hope the point is made. The assembly lube is a big part of the equation for success when building a top notch cool bore. So take good care of your engines, proper oil on the filter, high quality crankcase oil and in 600 plus laps you'll have a better performing engine. Best,WP
 
rebuilt a lot of good cool bores by simply installing new rings and sealing up the valves.. if you are lucky enough to get a glazed cylinder with no scuffing don't ruin it. running the right oil in a cool bore was part of it
 
Questions , I know you need the chrome piston; but do you also need chrome plated rings?
Do standard rings work without eating up the bore?
 
Standard rings will work.We use to preheat our cool bore blocks if they had a fresh bore job.Then after the heat we plunged the block into a bucket of oil.This seemed to help ,at least we thought so.The latest style piston will scuff up a cool bore more so than the long skirt pistons.The best engines we ever had were the cool bore then we later installed a sleeve in.The LA brand sleeve was more consistant than the molded around Briggs sleeve.The sleeve engines seem to be better now than they were in the years past.
 
Standard rings will work.We use to preheat our cool bore blocks if they had a fresh bore job.Then after the heat we plunged the block into a bucket of oil.This seemed to help ,at least we thought so.The latest style piston will scuff up a cool bore more so than the long skirt pistons.The best engines we ever had were the cool bore then we later installed a sleeve in.The LA brand sleeve was more consistant than the molded around Briggs sleeve.The sleeve engines seem to be better now than they were in the years past.

Is it a given the cool bore will scuff more with the new, modern, short skirt piston? If so, is it bad enough to cause builders to stop using cool bores all together? I have read comments from several builders, with good credentials, speak fondly about the cool bore--why then are most builders, when advertising their "national caliber" motor, nearly always listing duel bearing "IC" motors. One would think cool bores would be mentioned for such motors if they are better performers in any way.
 
It's not necessarily the the piston design. It's a combination of the piston to cylinder wall clearance and the assembly lube.
The R3 pistons are designed to be run much tighter than the old lawn mower pistons. Excessive clearance will cause scuffing. You must use a chrome plated piston in a cool bore. On a freshly prepaired cool bore be very careful they scratch easly,,from the dial bore gauge to the rings being gapped.
Use regular 30W motor oil for assembly lube. I have a few cases of 1975ish Quaker State. The cylinder won't even see the rings. But if you use a high $ quality synthetic first-as assembly lube the cylinder will be scratched up just from rolling it over to profile the cam. Then run it in the first time with Mobil 1 0W20,on the dyno or for about 50 laps. A good practice session. Then add your favorite kart oil and shes good to go. I've seen cool bores done like this that the cylinder surface looked like it was spray painted silver after hundreds of laps,, like the rings never touched it. I did IC bores this way too.
The myth about regular oil allowing friction to break in isn't true. The old mineral or paraffin base oil burnishes into the fresh metal, something the pure synthetics don't. Mobil 1 will mix with the 30W and alcohol,, when you drain it out she's ready for full synthetic. After the cylinder is glazed you can assemble it with your race oil the next rebuild,new piston and rings. Provided you do not break the glaze. An IC is very fast too,,,,,,, after it's glazed,just takes longer to get there. Best, WP
 
True....I have used the pistons in both type of engines....I remember the dual bearing and single bushing WKA red and black painted engines cool bore .These made some good engines...We would take these right out of the box gas and oil ,start the engine and let it run with no load .After we ran the engine we disassembled ,and began to prep for racing..We had no issue with galling the cylinders and we ran these cool bores from stickers to opens....The stickers we used Power Plus,,our modified we used KL200,.I seen our team mate run a open 8laps with no oil.He checked the cylinder and it was good...Changed the bearing and rod bolts and went racing the next night...The bearing was surprisingly in good shape...However thi isn't typical
 
The way Paul describes is what worked back in the day for sure. Very similar to what we did. It's probably also the reason you don't see builders doing it today. Time = money. As it is, you've got builders dropping the flathead rebuilds altogether in exchange for building two new clones in less time and twice the money. Can't say I blame them, it's just the way it is.
Personally, I don't mess with cool bore blocks anymore. I sleeve them if I know we've got one worth putting the time/money into. We use a hard sleeve that retains it's size, shape, and finish very well over time. With the short skirt pistons and dirt ingestion, the cool bore cylinder doesn't stay pretty nearly as long as the hard sleeve stuff. Some real truth to what Paul (and others) are saying about a glazed cylinder too. This is a big reason that we all plateau finish our cylinders even with the steel sleeve today. Take a look at why WKA disallowed nikasil coating! :)


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Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
27 years of service to the karting industry
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
 
Here we go again .
If i want too check a piston with aluminum polish .
I polish up the skirt . If my rag turns black it is not a chrome pston .
Correct ?
 
I was always told that cool bores had less drag than a steel bore but i could never prove it by pulling a piston /ring with a fish scale (not highly accurate) but always preferred steel bore's they seemed to live longer on methanol.
 
Thanks had too go back to page 27 but i found it .
(One way to tell if an aluminum piston has a chromed coating is to use a bit of polishing compound on a white cloth and rub the piston. If you polish an aluminum surface the rag will turn black as it polishes if it just cleans it and doesn't cause much blackness then there is a surface coating. The chromed coating of briggs piston isn't shiny like you would think chrome would look like.)
Credit to Freezeman :
 
Thanks had too go back to page 27 but i found it .
(One way to tell if an aluminum piston has a chromed coating is to use a bit of polishing compound on a white cloth and rub the piston. If you polish an aluminum surface the rag will turn black as it polishes if it just cleans it and doesn't cause much blackness then there is a surface coating. The chromed coating of briggs piston isn't shiny like you would think chrome would look like.)
Credit to Freezeman :

That is correct.
 
i used to race open class , i liked to use a cool bore because it did not run as hot as a steel bore engine the steel seem to hold in the heat yet many factors involved like keeping the alky carb tuned properly dureing the entire race some cams run hoter than others i just had better luck with the cool bore blocks good luck
 
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