Technical SBC replacement main bearing caps

I have a bare SBC 4 bolt 350 block without main bearing caps.
I see “replacement” caps on several sites, I ***ume these must come a bit undersized to be align bored/honed
to fit properly?

 

They would have to. I'm pretty sure no one can perfectly match main bearing caps without line boring. At least I wouldn't trust them to be accurate.

 

Why not get a different block? 350’s are very reasonable?

 

Yes, the block will need to be line bored after installing the new caps. This would apply if you buy used OEM or new aftermarket. Be aware that you will find aftermarket caps in OEM configuration (vertical outer bolts) as well as splayed outer bolts that require additional machine work. The machine work is not cheap and you could probably buy a new aftermarket block for the same money.

 

I was thinking of doing the same thing and asked around.....it's pretty pricey. Mine was to make an early vent block ('67 and older) 327 into a 350.

 

Seems like another block would likely be the cheapest route. I have bought several complete 350’s in the last few years for around $100 a piece or so.

 

Gotta be a reason those main caps are missing, you might want to Magnaflux that block before spending any real money on it, just say'n!

 

Just get normal used 4 bolt caps.
The machine shop mills the contact surface about 5 thou so the saddles are smaller
Then they line bore the caps.

If the block is straight , the machinist can set the boring bar to cut the caps only

The usual reason is "the dismantle of death" done by amateurs that throw everything in a box.
Or the block ends up unfinished due to lack of $$$$ at a machine shop [so they retrieve it]

 

Or.......the block is cracked and can't be seen with the naked eye and they saved the caps for future needs.

 

If I was going to use an aftermarket part, I would expect to do so. Stock parts, I'm not so sure it's necessary?

Once again I have to ask, who has real world experience, not anecdotal experience, that it's necessary to do so? Because it sounds to me like I'm hearing anecdotal. Best practice maybe? Told it was necessary from the rebuilder machine shop most likely.

Admittedly, I've never stuck a crank into shells with different caps and did a plastic gauging to check clearances or spun the crank to see if it would spin freely. I doubt many have. And my engines seem to have there caps as amateur as I am in rebuilding but I don't have one to try so I can only ***ume.
I mean really, isn't that what says yea or nay? Besides clearances for oil pressure, it spins easily? lol. I laugh because the spin could be to much clearance.

I'd like to think however, that modern manufacturing tolerances are pretty darn accurate and the reason to align bore would be done initially on manufacture and after that, everything should be close enough to be good for most applications and part switching?

Having said that, and to avoid those who scream blasphemy, to the original question, with aftermarket caps I do agree it would need doing, should be done.

 

A few things.

A fella had a mid 70’s or so Blazer he had this engine out of. I’d guess the “dismantle of death” did kick in.
He lives two doors down from me. When he moved out, I helped the owner clean up what he left, and the block was there, so I snagged it.
It’s standard bore, so I’ve hung onto it over the years.
Why not buy a used one? Well, last time (6/7 years ago) I was looking for one, everything I came across locally was already .030 over. I’d imagine that hasn’t changed much since.
I’d been kicking around the idea of a 383 to convert an old diesel PU I have, it seemed like a good start.

 

I’ve build a few engines over the years with mismatched “ junk box “ find parts and they all ran .

putting it all together , using plastigauge , if you have access to a bore gauge for checking roundness .

I think the big one is are you going racing and beating the engine like a government mule or is the an evening / week end cruiser ?

the end result will determine how you start .

getting it lined bored and knowing it’s absolutely true , round and straight is the correct thing to do .

I’ve also fixed cranks by using a shop rag and valve lapping compound to clean them up, all depends on use and what your expectations are .

 

I've done what you're planning several times.
I've always used OEM caps I've picked up Cheap / Free from a "grenade"
Converting from 2 bolt to 4 bolt is a P.I.T.A because the saddles need milling and seating.
But replacing 4 Bolts with missing caps is easy [or the same with 2 bolts]

I've also converted BBC from 2 to 4 bolt [they have the same saddle width]

Also I have converted 2 bolt early to 2 bolt late in Kent 4 cylinders [Lotus] about a dozen times.
The late bottom end is stronger and the earlier model blocks can go out to 84.5mm bore [+ cl*** legal casting #'s]

We used the method of milling caps and line boring

I have purchased a BBC that was cracked in the main saddle around the oil hole , The seller greased up the bores, inserted old main bearings and torqued them up. [creating the illusion that it was in storage]
But it was su****iously cheap, so I used the main caps to convert a 427 to 4 bolt.

 

In the mid/late 80's I bought a 1966 dated Chevy II/Nova 327 block (rare then, rarer now).
The first thing I asked the guy was if it had been Magnafluxed, said no, so I asked THE IMPORTANT QUESTION, if it ultimately is cracked would he refund my money, he said yes.
It was cracked in a main web, never looked further, he even reimbursed me for the Magnaflux.

 

So those who say you need to line bore them when installing different used main caps. Do you really believe that they line bored every engine they built at the factory? With that said, I would check it with plastigage while ***embling it with used caps just to satisfy myself that it is good enough to use for a daily driver!

 

When I was in high school, I blew the center main bearing cap on the flathead in my '51 Ford. When I pulled the pan, half of the center cap had just disintegrated and was laying in the pan in pieces. Being a broke high school kid, I decided to try and fix it. I went to the local junkyard and got a bunch of center main caps. I checked them all with plati-gage and they all looked acceptable. I put the best looking one one on (using the same main bearing shell!), brazed up the hole in the pan, and put it back together. It ran fine and I drove the car for a couple of more years. When I had to s**** the car (pervasive Minnesota rust), a buddy and I pulled the engine and put it in his car, still going strong. The engine came home to me one more time, but that's another story.

I figure that Ford had gotten their production methods down so well by the time they built that engine, that everything fits together properly. Maybe GM had gotten that good as well.

 

I think we often overthink our engine overhauls. I agree that things like line boring, decking, boring and honing for a perfect fit is great and definitely best practice. However if used parts, dingle ball honing using used main caps etc etc produce a finished engine within the manufacturers specs I see no reason not to do so. Granted if you’re hunting a salt flats record speed or something similar you probably want to spend the time and money getting everything dead nuts. But for most of us doing a stop light to stop light blast or an occasional 1/4 mile blast even bracket racing we just need good enough. For many the cost of precision machine work and parts is cost and time prohibitive and not needed.
That’s my soap box speech for today.
Remember make noise and have fun.
Dan

 

OK, you got me stumped with the "evap block". Don't recall ever hearing the term before.

 

Funny the way people have made the cel phone the center of the universe nowadays but won't show their engine the same love!

 

Sorry, helping a friend with a new car with an evap leak.....I had evap on the brain. I meant early vent(ilation) block, road draft.

 

Keep in mind I wasn’t asking about during a rebuild, but was asking about new “replacement” caps

 

I meant to reply earlier and forgot, but wouldn’t all the blocks from the factories be line bored?
I can’t see a way to cast a block, caps, and not have them correct, otherwise.
Same as lifter bores, cylinder bores also.
In regards to GM, I recall internet chatter (has to be true, right?) that there were only a few machines doing all the finish machining on all the casted blocks.
So if machines “A” machined blocks it could end up a bit different than ones done by machines “B”.

But this truck won’t see a lot of use and surely not extended trips.
Once a week to the feed store, and an occasional haul of a few different trailers. If the the crank spins free and I can get .002-.005 clearance, by mixing up shells, I’ll run it!

 

@Budget36 since the block you have does not have main caps, you can either
1. Buy replacement caps and machine, then build from there.

2. Scrounge used 4 bolt caps then fit and check, hoping they are good enough which is what some are suggesting.
If the engine budget is tight (under 1500) this might be your only choice for this block.
Here are a bunch of 500 dollar kits without cam and lifters. Add in those, plus springs and fluids, starter and water pump, belts and hoses. Might even slide under a grand with careful shopping.
https://www.summitracing.com/search...duct-line/summit-racing-chevy-350-engine-kits
You may end up having to machine anyway. This is the less money, more effort choice. Some thrive on getting to most out of a nickel or have to do that just to play in this hobby.

3. Find a good used complete engine. Price out everything from Choice #2 and this is your cap on a used engine. It will not be as fresh as #2, but hopefully you have heard it run and have a decent idea of it's condition.

The person who has the connections to grab 4 bolt main caps and try them out is going to lean that way. Say a local machine shop that does a lot of circle track work and has had block failures but the caps survived.
If you are going to have to rely on FB or CL to try to track them down and hope the seller is trustworthy, good luck.
Ultimately it's going to be your choice and what options are available to you.

 

I think there may be a wrong impression as to how blocks (and many other items in industry) are machined. Transfer lines are used, a block casting goes in, makes its way from one station to the next and a finished block comes out. There will be secondary operations like finish honing, adding main caps and line boring, etc. but the majority of the work is done on one line. A block gets loaded and some of the early machining would provide locating holes and lugs so it can be shuttled down the line and accurately placed for subsequent operations. Here’s examples of the locators used on a transfer line for blocks.

With a block the main caps are not installed on the line, that requires a secondary operation, as does finish honing but broaching the pan rails, main cap register surface, block decks and china walls and drilling & tapping happen on one line.
A block is loaded, one operator has a control panel and can monitor the line for faults, even a broken drill or tap is caught by sensors and changed by attendants to ensure that too many blocks are not affected.

 

As usual Mark is correct.
I thought maybe a person could find caps that would be close enough My bad. A little research showed that finding one cap to fit is near impossible let alone all of the caps.
Line honing would be the minimum machining with more work possible.
@Budget36 sounds like you’ll need to see the machinist and spend a couple few hundred. How in the world are the main caps lost? I guess some people don’t stamp their parts for orientation and then loosely ***emble said parts for storage.
Dan

 

Yes, they absolutely do align bore every new factory engine, the caps are matched to that engine. It is very unlikely that you will ever find a satisfactory match swapping main caps from one block to another without align boring.

 

If one looks they will often see the machining marks on the caps align with the block from when there bored from the factory .
There not just machineing the block saddles with the caps off.

You can find 350 blocks on Facebook market place and elsewhere fairly cheap and easy .
My last 3 four bolt main blocks standard bore were all under $200 .
Running engines are often in the $500 range still.

The odds of some random caps being correct is slim and even swapping positions on the same block will have issues as mentioned above . Then there is the question of why the caps are missing and such.

Kind of wondering if some guys here giving advice have actually built
engines. ??

Can something be cobbled together sure maybe. Is that what one wants to spend money on when doing a rebuild with a bare block as a start?
If you know the block is good then aftermarket caps and a line bore is possibility if your building a high HP engine with $$$$ involved. Generally guys doing this on a factory block are starting with a 2 bolt main and doing splayed main caps .
The cost of this with the rest of the machine work approaches a aftermarket block that will allow bigger bores , better oiling, blind head bolt heads and other benefits.

Generally if a SBC needs line boring ot even honing its often better to find one that has not shifted or warped.

 

Dad and I were building a 455 for a friend of ours, for some reason dad picked up the 400 rear main seal out of the box instead of the thinner 455 rear seal. He went to torque the rear main cap and snapped it in two. We had a couple other blocks and parts and grabbed a rear cap off of one of them, we checked bearing clearance and installed the correct seal. Engine spun over and all was right in the world. Once in the car it fired right up and ran for about 7 minutes till it locked up.

We pulled it apart to find the rear bearing had seised to the crank. After a trip to the shop for a line bore and crank polish it was back together and running great.

Your results may be better than ours, but I doubt it lasts long. Even though we had correct clearance where we checked it, the cap must have been offset and had tighter clearance on the edges.

All caps are machined to a specific block when built new, if you go swapping them and get it to run, consider yourself very lucky!