Are clearances actually tighter now?

General engine tech -- Drag Racing to Circle Track

Moderator: Team

Truckedup
Guru
Guru
Posts: 2728
Joined: Mon Jan 21, 2013 2:41 pm
Location: Finger Lakes

Are clearances actually tighter now?

Post by Truckedup »

Compared to the average car engine of 30 years ago,
Do the newest engines have tighter bearing and piston clearances? And If true, is this possible because of more presice machine work?
Motorcycle land speed racing... wearing animal hides and clinging to vibrating oily machines propelled by fire
Walter R. Malik
Guru
Guru
Posts: 6353
Joined: Tue Jul 24, 2012 11:15 am
Location: Roseville, Michigan (just north of Detroit)
Contact:

Re: Are clearances actually tighter now?

Post by Walter R. Malik »

Truckedup wrote: Mon Sep 21, 2020 8:47 am Compared to the average car engine of 30 years ago,
Do the newest engines have tighter bearing and piston clearances? And If true, is this possible because of more presice machine work?
Not necessarily tighter but, probably closer limits.
http://www.rmcompetition.com
Specialty engine building at its finest.
PackardV8
Guru
Guru
Posts: 7619
Joined: Sun Jul 30, 2006 2:03 pm
Location: Spokane, WA

Re: Are clearances actually tighter now?

Post by PackardV8 »

Yes, both tighter and more precise.

We build obsolete engines and the Shop Manual tolerances are on the order of .0005" - .0025".

An example, as late as 1956, the newly constructed Packard V8 machining line couldn't hold exact finished bore diameters, so each cylinder was measured and stamped with a letter code. The piston supplier, Bohnalite, couldn't hold exact skirt diameters, so each piston was measured and placed in a bin which corresponded to a diameter. When engines were being assembled, he read the stamped bore code and reached for a corresponding piston.

Today, most parts are manufactured on CNC lines and verified to be within spec before it leaves the machine. When we measure new connecting rods, they're exactly the same center-to-center length. When we measure a set of new pistons, they're exactly the same skirt diameter and weigh within 1-2 grams of the same.

FWIW, after verifying a set of new powdered metal rods were the same center-to-center, we pulled a used set of last century SBC rods out of the barrel and they varied .005".

jack vines
Jack Vines
Studebaker-Packard V8 Limited
Obsolete Engineering
n2omike
Guru
Guru
Posts: 1067
Joined: Sat Sep 02, 2006 10:56 pm
Location: West Virginia

Re: Are clearances actually tighter now?

Post by n2omike »

Yes. Clearances are tighter. Reasons:

1. More rigid blocks/cranks. They flex less, and can be tighter.

2. More accurate OEM machining. 0.001" clearance is only 0.0005" on each side of the bearing. Most old school machine shops are happy if they can stay within a half thou (0.0005") within tolerance on size... They can also vary in out of round and taper. If the machining is off any AT ALL, there is very little room for error when running tight clearances. Tight clearances with old school machining produces spun bearings. Modern, computerized machining with robots produces ACCURATE OEM parts that can be assembled with tight clearances.

3. Today's thin oils work with these tight clearances. They produce good economy and get pumped to the top end quickly.

If I was building a 'hotrod' engine and using standard machine work, I would err on the side of WIDE clearances. A bit loose is always better than too tight. A high end crank specialist might be able to produce the tight tolerances required for those narrow clearances, but I would not trust that to the majority of crank grinders out there. Going wide on the clearances buys you insurance.
PackardV8
Guru
Guru
Posts: 7619
Joined: Sun Jul 30, 2006 2:03 pm
Location: Spokane, WA

Re: Are clearances actually tighter now?

Post by PackardV8 »

Yes, thinner oils are a part of it. Also worth mentioning is temperature control. My Studebaker and Packard V8s are 700 pounds of cast iron, which takes a loooong time to warm up. There's more coolant in the heater system of an old school engine than in the entire cooling system of a 300 pound current 4-cylinder.

I'm always amazed at the packaging of current design engines. There's no space inside one. Everything fits tightly, where in an old school OHV8, there's room inside to swing a cat by the tail.

The exception to this is current DOHC V6s and V8s. They're huge externally for the displacement. GMs sticking with the pushrod V8 was a genius decision for which they took a beating from the automotive press, but who's smiling now?
Jack Vines
Studebaker-Packard V8 Limited
Obsolete Engineering
peejay
Guru
Guru
Posts: 1946
Joined: Tue Aug 10, 2010 9:16 pm
Location:

Re: Are clearances actually tighter now?

Post by peejay »

Most modern engines have the blocks and cranks and rods marked with letter codes. These codes represent differences as small as one ten thousandth of an inch. Then you choose your bearing shells based on that.

Some engines have piston clearance close to zero. Some older Subarus allowed negative clearance! Everything flexes and shifts around so much that this is not only acceptable, but required to keep the tiny short skirted pistons stable in the bores.
Truckedup
Guru
Guru
Posts: 2728
Joined: Mon Jan 21, 2013 2:41 pm
Location: Finger Lakes

Re: Are clearances actually tighter now?

Post by Truckedup »

I've been told some engines use coated piston skirts fitted with near zero clearance?
Motorcycle land speed racing... wearing animal hides and clinging to vibrating oily machines propelled by fire
RDY4WAR
Expert
Expert
Posts: 516
Joined: Tue Jun 20, 2017 12:58 am
Location:

Re: Are clearances actually tighter now?

Post by RDY4WAR »

Indeed they do. I'm planning an 2nd gen LT1 build around the tighter ideology. It's a drag race only engine with stock 3.48 stroke, 4.03 bore, .001-.0015" rod and main clearances, 6.125" rods, 1.125" pistons, 1/1/3mm rings, coated bearings, skirts, and pins, and 0w-8 oil.
shoedoos
Expert
Expert
Posts: 674
Joined: Wed Aug 09, 2006 3:14 am
Location: New Zealand

Re: Are clearances actually tighter now?

Post by shoedoos »

RDY4WAR, what do you intend to achieve by running all those tolerances? Have you tightened up anywhere else in the LT1?
n2omike
Guru
Guru
Posts: 1067
Joined: Sat Sep 02, 2006 10:56 pm
Location: West Virginia

Re: Are clearances actually tighter now?

Post by n2omike »

RDY4WAR wrote: Mon Sep 21, 2020 4:41 pm Indeed they do. I'm planning an 2nd gen LT1 build around the tighter ideology. It's a drag race only engine with stock 3.48 stroke, 4.03 bore, .001-.0015" rod and main clearances, 6.125" rods, 1.125" pistons, 1/1/3mm rings, coated bearings, skirts, and pins, and 0w-8 oil.
For adequate precision, you'll need OEM computerized robots doing the machining... or an expensive, high end crank specialist to come close to making power/rpm with those clearances in a SBC block. Even then, you stand a VERY good chance of spinning a bearing. A LOT more to lose with those clearances than what you're expecting to gain. Pretty much a huge wager with little to nothing to gain, and a lot to lose.

Good Luck
User avatar
af2
Guru
Guru
Posts: 7014
Joined: Sun Jan 28, 2007 4:42 pm
Location: Grass Valley, CA :Northern Foothills

Re: Are clearances actually tighter now?

Post by af2 »

RDY4WAR wrote: Mon Sep 21, 2020 4:41 pm Indeed they do. I'm planning an 2nd gen LT1 build around the tighter ideology. It's a drag race only engine with stock 3.48 stroke, 4.03 bore, .001-.0015" rod and main clearances, 6.125" rods, 1.125" pistons, 1/1/3mm rings, coated bearings, skirts, and pins, and 0w-8 oil.
And spin it 4500?
GURU is only a name.
Adam
User avatar
af2
Guru
Guru
Posts: 7014
Joined: Sun Jan 28, 2007 4:42 pm
Location: Grass Valley, CA :Northern Foothills

Re: Are clearances actually tighter now?

Post by af2 »

I guess a crank never deflex ??
GURU is only a name.
Adam
hoodeng
HotPass
HotPass
Posts: 1092
Joined: Sat Mar 04, 2017 6:53 pm
Location: South Australia

Re: Are clearances actually tighter now?

Post by hoodeng »

A number of things have progressed over years. I would think the main one is quality control, and metrology. Closer tolerances can be adhered to if you make/measure the parts consistently more accurately as others have already pointed out.

Around 85 Harley Davidson tightened down on the crankshafts pinion roller bearing to a minimum of .0002" clearance, it's high end was .0009", Most that i have seen were in the .0005" range, when fitting new ones i would now use the .0002" to.0004" range, reason being, first, using a Sunnen honing machine and key-way mandrels instead of cast iron laps when resizing an existing race [line hone if race is replaced], second, measurement, instead of plug fitting the next highest roller diameter, using Mitutoyo or Mercer bore gauges tell you exactly what you have.

For years a lot of guys would rely on wriggling a shaft in the bearing to determine clearance, I have never understood what they felt, bit like wriggling a valve in a guide and giving an accurate assessment of clearance. Maybe they have a lot better feel than me, but an instruments reading is harder to argue with.
Measuring tools are relatively cheap these days compared to what we paid thirty years ago.

And yes Truck, Wiseco make what they call a black edition piston for Harley, these are fitted size for size piston/bore,it has an abrade-able skirt coating called 'ArmorFit' that clearances itself when the engine has been run.

If you are interested in the decimal points onward march to the left get a copy of Simon Winchester's book 'Exactly'

Cheers.
n2omike
Guru
Guru
Posts: 1067
Joined: Sat Sep 02, 2006 10:56 pm
Location: West Virginia

Re: Are clearances actually tighter now?

Post by n2omike »

RDY4WAR wrote: Mon Sep 21, 2020 4:41 pm Indeed they do. I'm planning an 2nd gen LT1 build around the tighter ideology. It's a drag race only engine with stock 3.48 stroke, 4.03 bore, .001-.0015" rod and main clearances, 6.125" rods, 1.125" pistons, 1/1/3mm rings, coated bearings, skirts, and pins, and 0w-8 oil.
In addition to what I said above about precision machining, the newer OEM blocks, cranks, etc are a LOT stiffer and more rigid than the old stuff. Start spinning it hard at all or make decent power, and it will wipe/spin bearings... even it if does have good machine work.

Good Luck
turbo camino
Expert
Expert
Posts: 674
Joined: Sun Aug 10, 2008 11:51 am
Location:

Re: Are clearances actually tighter now?

Post by turbo camino »

But F1 engines are built like that! I mean, how hard could it be?
DON'T PANIC
Post Reply