Main Cap Problem SBC 1

[BringBackTheCamaro]

Hi everyone,

I'm assembling a 4 bolt 350 SBC kit I bought from Larry's Performance in Montebello, California (TERRIBLE company by the way). I am having problems with the front main cap.

I'm installing ARP studs and it is unusually difficult to install one of the studs with the cap in place. It will screw all the way in but requires a significant amount of torque on an allen wrench to do so, even with lubricant. Without the cap in place the stud will screw in freely by hand just like the others. I noticed a slight resistance above normal for the other stud, but nothing like the stud I just mentioned. I made sure the cap is facing the correct direction. I took out the studs and left the cap in place and looked down the holes with a headlight. The holes in the cap appear to be not perfectly centered with the holes in the block - possibly a cap from from a different block?

Sure I can install the cap and the crank spins freely (with what I think to be a reasonable amount of resistance from friction) but the fact that the stud is stressed such a way worries me. You can actually see how it is deflected to the side of the bolt hole in the cap. I don't think the stud is actually bending, I think its just shifting in its threads.

Could/Should I bother drilling the bolt holes in the cap to a larger size so they dont touch the studs? The registers would still keep the cap centered in the critical direction.

Then again, if that interference is eliminated, might it actually put the bore out of alignment? - assuming the mains were assembled and bored/honed with the interference present.

Thanks ahead for any advice!

 

[EngJW]

If the holes are not in alignment, my shot would be to return the caps to the manufacturer. There is no excuse for that, if that is what is happening. You should not have to be the one to rework a part if it has not been designed correctly.

If the stud is dragging in the cap due to misalignment of the holes, you might see marks in the holes or on the threads from contact. Studs or bolts that are highly stressed in tension should not have a bending stress added to them. It will shorten their fatigue life.

If you open up the hole to clear, you may lose contact area under the nut.

 

[NormPeterson]

Are you absolutely positive that the tapped hole(s) in the block are square?

Norm

 

[Fabrico]

Putting a bind on one or more of the studs would tend to defeat the purpose for using a 4 bolt cap. I would either take it back, check it with another cap, or, if I felt energetic, take the block over there and show them.

 

[nz1Mechanic]

are you sure its the correct cap??? most of these older gm engines are all different and are drilled out individualy so no 2 caps are the same

 

[Fabrico]

The caps are the same. It's the main bearing bore that is not. That's why you must line bore all the mains whenever a cap is changed.

 

[NormPeterson]

I'm still wondering if this problem is in any way associated with a poorly done Helicoil repair in the block, either by the rebuilder or as a backyard repair before the rebuilder ever got that block as a core. BringBack's description of the relative amounts of torque involved with the cap in place vs without keeps bringing me back to a bolt/stud axis that's 'off'. I suppose that it's remotely possible that one or more of the bolt holes for that cap were 'wrong' from the factory, or that only the front cap somehow got mixed up somewhere along the line, or that a tapping operation to clean up the threads somehow wandered off course. In any of those cases, there is an unknown amount of bending of the stud occurring, that increases when the nut is torqued down to spec.

I would not drill out the bolt holes in the cap to fix somebody else's error - regardless of just where the problem lies it should have never left the rebuilder that way, period.

Norm

 

[BringBackTheCamaro]

First off, thanks everyone for your responses so far.

I should note a few things about the engine:
- It is called a 4 bolt block but the first and last caps are only 2 bolt.

- I'm assuming the bores are ok because the shop is supposed to check the alignment of them as part of the block prep machine work AND the crank actually rotates without unusual binding with the caps torqued down. I'm just thinking they thought the interference w/ the bolts for the 1 st cap was not enough to worry about. I talked to the shop and of course they said not to worry about it... but if I have ANY problems I know who to go after.

- I agree that I shouldn't have to be the one to fix this or to even worry about it, but taking the block back is not an option. The company is in California and I'm in Florida. The most I'll do is make the shop pay for any additional machine work I'd have to get done locally.

- The bolt holes in the block and cap appear to be square and original, no repairs. Looking down the 2 bolt holes in the cap with it installed shows that all of the holes are at least parallel, its just that the holes in the caps are offset in the same direction, relative to the holes in the block, in a direction perpendicular to the axis of the crank. In other words, the bolt holes are out of alignment in such a way that it causes the stud to push the cap against the register, on the side of the cap that the stud is on. I'm thinking that I could use my dremel and grind the slightest amount of material out of the bolt hole in the cap, on the side of the interference - just enough to eliminate the interference. I don't think the bolt hole has to be perfectly round. I believe this will work BUT the one thing I am afraid of is that if the interference is ground out, it will cause the cap to distort out of alignment. Is that even possible?

 

[Greenlight]

Whenever you replace the original main cap bolts with studs you should check the main bearing bores. Typically they WILL DISTORT when you change to studs. You should use a dial bore guage to confirm the "roundness" of the bore and have the block honed to correct any problems. Just because the crank rotates freely does not mean the bearing clearance is correct.

 

[waross]

Hi Greenlight;
Did you mean to say line bore instead of "Hone"? My hone will make things smooth but if they're not round it will not make them round.

 

[NormPeterson]

Time to make some careful measurements.

Spacing for the bolts on the two-bolt caps and the inners on the three 4-bolt ones can vary between 3.230" and 3.250" per the 5th Edition of the Chevy Power book. More specifically, that reference has both of those bolt holes being located off the passenger side register of the front cap; min/max values are 0.6195"/0.6295" and 3.8595"/3.8695", with the other register being at 4.486"/4.487". My 7th Edition lists only a 3.24" center distance, with no tolerances given.

It's been a while since I've had my own 4-bolt SBC apart; consequently I don't have any "feel" for how much "room" might exist within the cap holes to accommodate within-tolerance variations in bolt center locations.

Norm

 

[BringBackTheCamaro]

Ok, how about this guys:

Why don't I just stick with the original bolts and don't modify anything? If the block was prepped that way I will only run into uncertainties by changing things around (studs instead of bolts and modifying bolt holes). Also I already have the entire rotating assembly in the motor. I guess I don't really need the strength of ARP studs. Can I trust the stock bolts in a 450 HP motor?

 

[Fabrico]

How about checking the crank/bearing clearence before you change course?

 

[patprimmer]

I have been reading and thinking about this.

I would stay with the studs at 450 HP.

I would have other doubts about a block supplied from someone who mixes and matches caps with such little attention to detail. What else have they missed, like cracks, deck heights etc

I would absolutely NOT bore the bolt holes bigger as that would make the thinest part of the cap even thinner, that is the area between the bolt holes and the front and rear faces of the cap.

I would carefully elongate the holes with a mill, so as to enlarge them to an oval shape with the holes elongated across to the cap to correct the centre to centre dimension re the main journal centreline.

I would then very carefully check the main bearing clearance with long strips of plastigauge going around the journal on the front and back edges. Expect the clearance to open up as you approach the parting line as this is designed into the bearings. How much depends on the bearing type and brand. Try to get a spec from the bearing manufacturer. This will prove if the cap bore is square and centre. If you do this for all five simultaneously, it will prove all are aligned as well as square and centre re the block main bearing bore.

It should spin freely in light oil with all 5 mains torqued down, but no rods installed.

The very small difference in tunnel bore shape caused by the difference in distortion between studs and bolts when both are torqued into place with caps installed should be of no concern as it will mainly effect the bore shape at the parting line where there is more clearance and a few tenths of a thou won't make any real difference. I say this with the proviso that the studs are now free of interference in the bolt holes in the cap, and the nut seating surface on the cap is perpendicular to the stud axis.

This problem might have occurred if the cap was not held square and more metal was taken off one side when the cap was machined to close down the clearance before line boring. If this is the case, the caps may need spot facing around the bolt holes so the nut clamps square on the cap.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evelrod]

I have mixed and matched blocks and caps on occasion...never had a problem with an appropriate line bore or line hone (depending) to square things up. Never had a problem with the bottom end going 'squishy' when studs were installed (that is not to say I have had the same success with studs for the cyl heads). Not sure what I would do in your place...probably take the block to a known quality race related machine shop (if I could not do it myself). I have had VERY tight fitting caps on some engines, e.g. MG's and some Nissan clones. Keep us posted, I am curious.

Pat...With demensions so tight these days, even on my vintage cars, I am surprised that you would reccommend Plasti-Guage as a primary tool, especially since an appropriate bore guage is relatively cheap. Some with multiple mandrels can be used for a variety of bore demensions.
Not knocking PG, just don't like it for layout...okay for checking, though. It's sorta like an educated guess, IMO.

Rod

 

[patprimmer]

Rod

You can measure each bore more accurately with bore gauge or a mic, but plastigauge will be the easiest way to test for square and alignment. Kind of like using plasticine for piston to valve. You can actually see where it clears and where it is close, where a mic only measures where you place it.

I would never recommend plastigauge as the way to obtain real measurements for machining precision parts.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evelrod]

Okay, I'll forgive you this time. ;-)

Rod

 

[BringBackTheCamaro]

Ok, this sounds crazy but what I'm going to do is attempt to remove the crank while leaving the pistons in the bores - since I already have the heads on! It will be a b**** and actually I'm not even completely sure it's possible. I plastigaged the crank bearing clearances before I did a final install of the crank and it checked out, but unfortunately I overlooked the interference until now. Luckily I already happen to have a bore gage, and I wish I took advantage of it during the initial build. If I can get the crank out I'll check the bore against spec with the original bolts - and it better pass because the machine shop was sopposed to have checked this. Then I'll compare it with the studs installed.
One thing that would be amazing is if I'd be able to elongate the bolt holes (oval shape for example, and in a direction parallel to the width of the cap, as in from one bolt hole to the other) and have the bore still within spec, since I can only remove material and not replace it. If the cap goes out of alignment spec after modification I'll have to just give up on the whole project!

I should throw some pictures on here so you all can see what I'm talking about.

 

[waross]

I had a thought that may explain your dificulty.
I understand that the cap was supplied with bolts and you ran into problems when you fitted studs. Have you checked the diameter of the bolts as compared to the studs? Bolts with rolled threads are often thinner than bolts with cut threads. The minor diameter of the threads is usually the limiting factor in the strength of a bolt. The shank does not need to be full diameter to have adequate strength.
You may be able to locate some studs of the same nominal diameter that have a smaller actual diameter.
yours

 

[sbc438]

Here's my $.02... If you are going to install main studs you have to do it before you align hone or bore the main caps!!! Proper alignment is essential from main to main as well as clearance of each bearing and cap distortion. There are many things going on here. With studs you will get more clamp load i.e. more cap distortion, possible alingnment issues (as you are seeing) and many more little details that when combined can make a big differance. I work on alot of race engines from 400hp to 3000hp and would not put studs in unless I'm aligne bore/honing the mains with the studs in.

Now, for the good news... Go buy a new set of bolts and use them, they will be fine. I have a pair of 438 cid small blocks in my boat @ 493 HP that run full load with 2-bolt mains and stock (new) bolts not studs (needed the bolts for the windage tray).

You may be able to get away with using the studs with no problem but why chance it!!!!!!!