Some little quirks when building engines

[dr_hunt]

All the books that you can read that are published have alot of information, but there's always something lacking that they don't discuss. Here's one thing that I hardly ever run into but have on at least two other occasions, this being the third.

 

When installing the crank, you typically install the center cap, snug it hand tight, rotate the crank and then work your way out from the center each way alternating, while rotating the crank to make sure it spins freely. You do this till you get all 5 caps on and if the crank gets tight you have a problem with the last cap you put on so you check it.

 

Well, Mic'ing the crank and using snap guages in the bearings I determined that I had .0015 clearance on the mains, tight, but it's a street motor and it's within specs. Bearings are C&A mains .010 undersized. When installing the caps everything was great until I got to the rear main, which is the thrust. But since I had mic'd all the clearances I was wondering WTF? So, I installed my dial indicator to measure the thrust clearance and low and behold I had zero!!! It turns, but it's still zero with white grease in place so, it's probably at about .0005 or less. The books don't cover this problem. Option 1 is to have the crank thrust touched up, but you'll have to take the mains to .020 and buy new bearings. In rare instances is a crank grinder going to be able to touch it without having to grind the crank or at least polish it.

 

Let's analayze the situation. The crank is a used piece, stock 400sbc that was running so we know it was ok, same with the block. I had it align honed, caps squared, decked to 9.000 inches, bored and honed with plates and arp studs in the mains. So we know the block is spot on. So, it has to be bearings are alittle thick in the thrust area. So, what we do is get a piece of thick glass. I happened to have some pyrex flat glass out of my fireplace, you put fine sandpaper on the glass and sand the thrust down to get your clearance. I used a good dial caliper to measure the thickness of the thrust as I went keeping it pretty even, with .0005. I took .003 off of the back and just leveled up the front side to where there were no high or low spots. This gave me a measured .003 thrust when I re-installed the crank. Perfect, and now it turns well.

 

So just a thought when your building an engine and you run into this problem. Keep in mind that all bearings are not exactly the same all the time. Keep it in the back of your mind.

[BRAAP]

Yes, crank thrust or end play as it sometimes called, is often overlooked when assembling an engine and I have found just as Doc found, approx 10-20% of the time with domestic V-8’s, the thrust is too tight. Riding on Docs coat tail, I want to elaborate a little more on checking and setting the crank thrust. I use this process for every build I do, V-8, L-series, etc.

 

Crank thrust, what is it and why is it so important? With most manual transmissions and all automatic transmissions, there is force being applied on the back of the crank, pushing it forward. When you push on the clutch pedal, all that force is pushing against the crank thrust bearing. With heavy clutches, you don’t want to hold the clutch in for extended periods with the engine running, not only because it is wear on the throw out bearing and little wearing the clutch, but because it will cause excessive thrust wear! Automatics torque converter is always pushing against the crank. If the thrust is improperly set on and automatic, the thrust will wear to the point the crank moves far enough forward the crank cheeks will contact the block main webs, making loud knocking sound, similar to rod knock. Seen a few examples of that come through our shop from DIY garage builds where the assembler didn’t verify adequate cranks thrust.

 

Why do you need to “set” the thrust when measuring thrust clearance, and during final assembly? We do so to make sure that both halves of the thrust bearing are flush on the side of the thrust bearing that sees most of the thrust loads. For most engines, that load is pushing against the rear of the crank, (trying to push the crank out through the front of the engine). Some applications such as late model manual trans LT1, that load is opposite, pulling on the crank, covered a little further down.

 

When assembling during the mock up, thrust should be checked dry, no lube. Install the mains, crank and caps, snug down the thrust main, not tight, just barely snug. Now you need to “set” the trust. Using a dead blow hammer, (NOT a metal faced hammer), strike the crank on the snout driving it rearward, then again on the flywheel flange driving it forward this time, but a little harder in this direction as that is the direction of thrust loads on the running engine, (the exception is discussed at the bottom)! Some will use a screw driver to so this, I prefer the dead blow as it is a more positive setting technique. Now tighten down the thrust main cap. With dial indicator against the crank snout or crank cheek to take thrust measurements, use a flat blade screw driver between a crank cheek and main cap/web and “lightly-carefully” pry the crank back and forth. If there is thrust clearance it will make an audible thunk thunk sound as the crank seats against the thrust bearing moving back and forth. take note of the dial indicator reading at each extreme, that measurement is your thrust clearance. When doing this, be very careful not to touch the crank journals with the screw driver, they are soft and mar easily. If the thrust clearance is not adequate, remove the thrust bearings and just as Doc did, remove material from the thrust face. My preference is to only remove material from the side of the thrust bearing that doesn’t see any thrust loading so as to leave the manufacturers coating on the face that does see thrust loads. One technique is to use a fine file to remove the material, another is flat stones, my personal favorites are Doc's approach using a flat surface such as glass and fine sand paper measuring as you go or a milling machine. I’ve used all the above having had to add clearance many a domestic V-8 thrust back in the day. Just be sure to take your time paying close attention to the pressure you are applying so as to remove material equally around the circumference, measuring as you go.

 

As I mentioned earlier, some manual trans such as the later model T-56 for the LT1 have a pull style clutch, as such when you push on the clutch pedal, the clutch is being pulled on so when setting the thrust on an LT1 using a pull style clutch arrangement, make your last hit while setting the crank against the snout setting the thrust halves, and if material needs to be removed from the thrust bearing, remove it from the back side of the thrust bearing.

 

Edited December 12, 2009 by BRAAP

[Inferno 08]

Definitely useful info in this thread.

 

I had to clearance the thrust bearing on the CRX motor I was building since it had absolutely zero endplay. Lots of boost and lots of miles later and that thing is still running strong.

[grumpyvette]

JUST A BIT MORE , CHECKING TRUST BEARING CLEARANCE , PICTURES

 

Edited December 12, 2009 by grumpyvette

[DavyZ]

Good stuff! Very useful information for rebuilding a SBC like mine when the time comes. I will swap a steel crank for my present one when doing a rebuild eventually. Thanks for a great thread, Doc!

 

 

 

Davy

[RTz]

Outside of obvious extreme cases, what is too much thrust, or not enough, and why? In other words, what is an acceptable 'window'? Is this window generally common, engine to engine, or is it application specific?

[BRAAP]

Good question Ron.

It has been my experience that too tight is worse than being too loose.

 

General main thrust clearances range between .003”-.013”. .005-.007” has always been a good safe range for the SBC and L-series. Be sure to always verify acceptable thrust clearance range for the particular engine you're building.

 

Being too loose just allows the oil that much rome opportunity to flow out more freely on the non loaded side thrust side though it is debatable if it has an ill effect once the thrust clearance is greater then .008” or so. Rod bearing clearances on the cranks pin that is fed from the thrust main will have an affect on how much oil goes to the thrust as well.

 

For what it’s worth, the owner of the shop I worked at during the mid to late ‘90’s always had me clearance the Jet Boat V-8’s thrust to the loose side. I am trying to remember exactly why that was, but don’t recall it other than it had something to do with how the impeller could load the thrust bearing as the boat hull/transom flexed under full power/jumping gravel bars, etc. (1100+ HP Twin Turbo Big Block Jet boat that runs UP rivers at 125+ MPH!)

 

 

 

Here is a good read on thrust bearing failures, written by the AERA;

http://www.4secondsflat.com/Thrust_bearing_failures.html

 

 

It would be nice to hear other input/insight on ideal minimum and maximum thrust clearances from the other engine builders here?

[dr_hunt]

Thanks for the cool pictures!! BTW I don't install the crank with dry bearings, your run the risk of galling the bearings IMO. Grease on bearings 1-4 has no impact on measuring thrust and just a whisper on 5 not allowing any excess to get on the thrust is the way I do it whether right or wrong. The crank has to move and dry I can't do, but that is my opinion. If you can't move the crank by hand forward and back then you've got other problems. I use screwdriver to pry forward and back just to make sure full travel is seen both ways.