BBC thrust bearing alignment

[TRN]

I am replacing the rear main cap on a BBC. How do you ensure that the thrust surface is perpendicular to the crank CL? I asked my machinist, and I think I got a well disguised "I don't know".

[Mike Rogers]

First i verify the end play with the cap off. I push the crank forward put on the cap and then install the bolts just a little snug. Using a raw hid or smilier type hammer hit the front snout and it will set the cap alignment.

Tq the cap to 25 ft. lbs. and check end play to verify you still have the same with just the bottom bearing . If everything checks out finish tq. up cap. If not repeat.

PS: I just re read your question if you are changeing caps did you check the align hone 99 % of the time you can not just change the caps it will have to be re align honed.

[TRN]

I am fitting the cap to the block prior to align honing. It's my understanding that align honing does nothing regarding the thrust surface. In other words, the thrust surfaces need to be perpendicular to the crank CL prior to align honing.

[TRN]

Am I asking a bonehead question? If so, please enlighten me.

[stock z/28]

Hi,

I dont think its a bone head question, but please bear in mind Im certainly no expert.

I always wondered how to check that as well.

What I came up with is very crude but seems to work ok. Its saved me a couple of times.

I took a junk crankshaft and opened the thrust clearance way up, to about .080". Then I threaded the thrust surface so I could install a set screw with a rounded tip. I put an old set of main brgs in the no 1 and 4 saddle. Install the thrust with no brg. If it has a dowel I generally remove it. I use a scribe on the parting line to check that the cap is aligned and torque the cap.


If you apply a steady pressure (I have used an air cyl device with a ball brg attachment- as Im generally here by myself) to preload the crank in the direction you are checking you can use a dial indicator on the flywheel flange to measure lateral movement. If its off very far it will show up fairy easy.
You may have to realign the cap depending on which surface you are checking.
I ground the flywheel flange flat at the same time I ground the thrust surfaces, so its pretty true. A good cap (or block for that matter) will hardly have any TIR. A cap that is cocked shows up pretty quick.

I use the the air cyl cyl set up and a tenths reading indicator to read the thrust run out on cranks a lot.

On the thust surface of the bearing, any mis match I always install facing the front. I always make sure the last "tap" I give the crank when torquing the mains is to the front of the engine. I have never seen a thrust fail in the front, so I really dont worry about it.

The small block crank that use for this I plunge cut the area with an end mill and used small roller brgs.
I hope I explained this ok. I always though I would make I real neet tool instead of the old crank, but I havent yet.

[Dave Koehler]

You are correct in that the line hone won't do squat for the thrust face. Some line boring machines have accessories for facing the thrust surface but you don't want to do that. Where would you find the bearing with a thicker inside width?

It's a good thing that you are looking at such things but I wonder if you are not getting ahead of yourself and your machinist. Going to the trouble of bolting that cap down will be for naught. The machinist will be fussing and fitting it anyway along with machining the surface where it meets the block.

Unless you are the luckiest guy here and found a cap where the inside diameters actually match up (no obscene offset) your machinist will be doing some serious fussing and fitting followed by line boring and possibly followed up with a line hone.

If you want to check the squareness all you need to do is lay a machinist square on the cap face and thrust face. Note that due to the inset of the thrust face from the casting part you may have to lay a piece of precision round stock in there before using the square.
If all is well, then take a round piece of stock and force the cap towards the back to align it with the block.

[TRN]

Thanks! I had visions of cutting something out of bar stock, levers, pins, etc., etc., a whole day or so, and I have a trashed crank 10 feet away! My main concern is that the cap has no angular misalignment, and this should show that easily.

When I asked my machinist to show me how he ensured that the thrust surface was perpendicular during the cap shaving process, I was not very confident in the response. I recently tore down a SBC that I'd had align honed and the wear pattern on the thrust surface made the misalignment obvious.

[ProPower engines]

When line honing I use a machined ring to align the thrust caps to the machine off the thrust bearing surface.
The thrust surface is machined after the cap is fitted and bolted in place at the factory. Thats why guys have issues.
They forget that the thrust surface is not 90 degrees to the parting face and just use the rough machined area on the end of the cap and pray its straight.
Lay the crank in the block with the front and rear bearings in place. Tap the crank hard to the rear and snug the bolts.
Then check the end play with a dial indicator. If its wrong repeat and bang the crank the other way and recheck.
If the end play is tight then take some feeler blades and find the tight spot.
If you have not had the work done yet be sure to convey the info to who ever is doing the job that way you will not have to address the issue when you assemble.

[modok]

Am I asking a bonehead question? If so, please enlighten me.

maybe, maybe not.

if you have a GOOD cap, then the housing bore is ALREADY 90 degrees to the thrust surface.
All you need to do is fit the cap on there and check the housing bore for taper.
If there is taper re-grind the cap parting surface at a different angle to correct the taper.
If there is taper on the SIDE, (basically it's twisted)....then then the sides of the cap may need to be built-up and re-machined so you fix that.
It may need to be moved side to side anyhow, so you can fix both at the same time if you check first and KNOW which way it needs to go.

If your new cap is machined wrong, or been screwed by the last guy, then we have a more complicated problem.

If the cap is only ROUGH machined, you can machine it to match the old cap.

If the old cap is destroyed, then of course that's out.

[leahymtsps]

13 yr. old post. I would hope it's back together by now

Tom

When line honing I use a machined ring to align the thrust caps to the machine off the thrust bearing surface.
The thrust surface is machined after the cap is fitted and bolted in place at the factory. Thats why guys have issues.
They forget that the thrust surface is not 90 degrees to the parting face and just use the rough machined area on the end of the cap and pray its straight.
Lay the crank in the block with the front and rear bearings in place. Tap the crank hard to the rear and snug the bolts.
Then check the end play with a dial indicator. If its wrong repeat and bang the crank the other way and recheck.
If the end play is tight then take some feeler blades and find the tight spot.
If you have not had the work done yet be sure to convey the info to who ever is doing the job that way you will not have to address the issue when you assemble.

[ProPower engines]

I hope so to
I seen that after just got a notice from back then never check the date.

[MELWAY]

I think it’s good to bring some old posts up from the dead
There is some good info and topics from great minds lurking deep within speedtalk

[modok]

oops

[TRN]

Yes, the motor has been back together for years, and long story short is now bolted to a TH400 instead of a SM465. I appreciate all the suggestions, thank you.

[MadBill]

TRN, you must have been a bit surprised to get a 'reply' notification after over a decade!
Glad you're still with us.