Hello everyone,



while disassembling my L81 engine, I noticed this hole in cylinder number 2.

It is deep, between 0.04 and 0.05".

Given its perfectly round shape, I initially thought it was something that had been manufactured that way, but it's clear that it's a defect! The most surprising thing is that I didn't notice anything when the engine was running.

After analyzing it, I see that a piece of cast iron is missing from the upper crankshaft bearing (photo #2). Perhaps the two are related.

Do you think the block is dead, or would it be financially worthwhile to repair it by installing a liner?

For your information, I am in France, so I don't have a wide availability of parts.

Thank you for your help.

 

Welcome to the Corvette Forum Motobecane!

We are glad you have chose to join us here on the Corvette Forum. This is the right place to be if you have a question or issue with any C3 Corvette.

That is amazing that someone would put a hole there, it is so perfectly round that it must have been put there for some reason. I would also love to hear what that hole was intended for. It doesn't make sense as metal just doesn't fail like that, is there any history of catastrophic failures inside this engine block?

As far as the hole being a problem it appears to be very low on the pistons skirt contact area so maybe using a piston with shorter skirts might help avoid the hole and any interactions it might have. The other option would be to clean it seal it with epoxy and then re-hone the cylinder. The other damage on the engine case could be repaired by a qualified welder who knows how to weld cast iron specifically.

I had a 327 Chevy engine that kept breaking the pistons when the rings were set before perfectly. I ended up just making the piston a bit looser in that hole and the engine was fine after that. With the rings a bit looser the #6 piston stopped breaking and killing the compression.

As people speak up it will be interesting to learn what the reason for the hole is. This forum has so many really experienced Corvette mechanics whose opinions are valued. It is a wonderful place to learn about Corvettes!

Best regards,
Chris

 

Very strange indeed. Does it go all the way through? Sure looks like a drilled hole to me. The crank pad looks weird too but that doesn't look like a drill hole.

 

How much has it already been over-bored ?
What does # 2 Piston look like ? In particular, how do its sides appear ? Especially rear facing side ? And, If oversize, that'll probably be encoded in a head/crown stamp.

WAG: this piece suffered some calamity that made a small crack in cylinder liner; diligent repairman relieved the stress of that crack (in an attempt to prevent crack spreading) and did drill that hole.
That drilled hole appears to be within the sweep of oil ring package. It may (or may not) be stabilized at present, but is not on either minor or major thrust surface.
A rebuild with 6" rods and pistons with shorter compression distance will raise ringpak above hole.

 

This ^
I dont see any cracking around the hole and based on its location, I’d rebuild with 6” rods and shorter pistons skirts.
Would I use that block to go racing? No, but for the typical 2k or 3k miles a year most c3’s see, I’d run it.

 

Thank you for your initial responses, which are quite reassuring.

Regarding the answers:
As for the hole, no, it does not go through the cylinder.
I don't know the history of the block, but it has definitely been disassembled at least once (difference in the engine mounting bolts).
I'm waiting to receive my bore gauge, but measurements with a caliper indicate a 4" bore (to be verified on No. 2).
Piston 2 shows no particular marks and is similar to the others.

If I use 6" connecting rods and pistons with a shorter compression height, the displacement in the cylinder will be the same, right? Unless I convert it to an engine smaller than 350.

Thanks again for your help, guys, you're the best!

 

I would run it like it is with the stock parts. It wasn't a problem before. Just make sure the oil ring gaps don't line up with the hole.

 

*terminology: compression distance or compression height have same meaning.

That (a shorter or taller compression height alone) will Not affect DISPLACEMENT. It's the SWEPT AREA of cylinder that matters most to displacement.
But, depending upon configuration of Piston's CROWN/Valve Relief(s), THAT can affect COMPRESSION RATIO.
* let's assume your old piston has compression height 1.560" and your old rods measure 5.7" center-to-center, then New piston for 6" rod CH should be 1.260"

**we have Not seen your pistons. But, their crowns are probably dished aka "sump head". If so, choosing a flat top piston will raise compression ratio ABOUT 3/4 of full point (if it WAS 9:1 it would become ABOUT 9.75:1).

*** Understand your motor has been disassembled and serviced since it was new. At this point, I cannot KNOW what post-manufacture machining OR parts' swaps has been done to your motor. There's no way to KNOW what you have now without close inspection and measurement and comparisons.

Critical planning for such changes includes accurately measuring how far your piston is Below Block's Deck when piston at TDC. When it was new, that ("deck clearance") is ABOUT 0.025". It's critical for best result that you attain a measurement of QUENCH DISTANCE between HEAD surface and Piston Crown of 0.040" +/- 0.005" and that includes the thickness of a FULLY-COMPRESSED head gasket. MANY different head gasket choices (and thicknesses) can help you achieve a properly effective Target Quench.

-add-
often-overlooked phenomenon: the typical crosshatch pattern on cylinder walls does indeed induce piston ring rotation. Hence, why piston-ported Two-stroke engines often have their rings pinned in-place; to prevent their end gaps from hanging-up on edges of ports in cylinder wall.

 

I have been building engines for going on 55 years and have never seen a hole drilled in a cylinder like that before. Hard to tell from the picture but the hole in the main bearing saddle may be from GM drilling for the main cap bolts and came through the casting. I have seen that happen many times. If that is the case, the hole in the bearing saddle will not cause any problems.

The hole in the cylinder can get addressed a few different ways depending on the condition of your parts and your budget and performance goals. Even if you assume the hole was not causing problem, it should be addressed. I would not want rings to span over the holes as it could cause the oil ring to stop rotating which will cause the rings to wear prematurely.

If your pistons are in good shape (no scuffing, proper piston to cylinder wall clearence) then put a sleeve (liner) in that cylinder and finish rebuilding the engine. Fairly inexpensive.

If you need (or want) to replace your pistons, you will have many option to choose from. Going to a 6" rod by itself will not move the piston rings up. To move the ring stack up, you must go to a thinner ring package. The factory rings in a L81 engine were 5/64, 5/64, 3/16. Modern engines today use a much small ring package which allows the rings to be moved closer together. That allows the oil ring to be move much closer to the top of the piston which will put you above the hole in the cylinder. The thinner rings seal better and have much less drag and wear.

Common ring packages today are:
1/16, 1/16, 3/16 probably not enough to move above hole
​​​​1.2mm,1.5mm, 3mm probably not enough to move above hole
.043, .043, 3mm should move above hole
1mm, 1mm, 2mm should move above hole

Aftermarket companies offer pistons in either 5.7 or 6 inch rod pin heights and smaller ring packages depending on whether you want to replace the rods.

Replacing your pistons will also allow you to adjust compression ratio to what you would like for performance.

Keep in mind that if your engine has the original heads on it yet, they are lightweight casting and are limited in performance gains.

The bottom line becomes your budget, availability, and performance goals.

If you let me know what brands you have available to you, (Keith Black, Wiseco, Mahle, JE) I can assist to in choosing parts that will meet your goal.

 

The rings do not rotate.

 

rings absolutely rotate.

 

^^^this^^^

* and any quality modern piston I'd even consider nowadays for a good build will use a ringpak that'll be thin enough to clear that divot; anyway 5.7 or 6.

goodbye tractor rings rearview

 

So this hole will remain a mystery!

I'm going to get some quotes for installing a repair sleeve, but workshops on this continent aren't used to V8s and charge prohibitive prices for these engines...

I do plan to replace the pistons with new ones.

For the rebuild, I had planned to boost the engine a little, and above all to gain torque at low and mid ranges; I already have in stock:
64cc/180cc Flotek cylinder heads (102-505)
a SUM K1222 retrofit roller camshaft, with all the peripherals related to the retrofit);

The initial goal was to use +5cc hypereutectic pistons to achieve a compression ratio close to 10:1

The other modified parts on my engine are:
Edelbrock 2701 intake manifold
AVS2 650cfm carburetor
HEI 8362 ignition
Magnum roller tip 1.52 rocker arm
Long 4-in-1 exhaust

Thank you very much for your information on the pistons. I had no idea about these developments and was foolishly going to go with the original ring sizes.

Pete79L82, I would definitely appreciate your advice on finding the most suitable pistons that would allow me to go above the hole.
I buy my parts from the main international retailers: Summit and Rockauto.

Rebelyell, I'll be receiving the micrometers and bore gauges next week, so I'll be able to measure my block and give you more details on its current condition.

 

If you plan on replacing your pistons, then I see no reason to install a sleeve. The money you save on not putting in a sleeve can go to a better set of pistons with a small ring pack. If you get the rings above the hole, the hole becomes a non-issue. I am not aware of any hypereutectic pistons with a small ring package, so you will need to get a set of forged pistons which are about double the money of a set of hypereutectic pistons. But I my opinion, the end result will be a better quality build.

The next decision will be if you want to replace the rods or not. That will depend on the condition of the rods and your budget. There are pros and cons either way to keeping your original rods or replacing them with new. Someone is going to chime in and say you have to go to 6" rods for performance. Not true. I have never seen a performance gain between a 5.7 or a 6" rod in a modest performance build (under 400 HP). Your stock rods, at the very least, should have the big and small end bores checked and a new set of bolts. That will be money invested in an old set of rods. But once reconditioned are adequate for your build. Stock rods use a pressed pin so you will need a machine shop install the pistons on the rods which is more costs. A new set of rods can be gotten that use floating pins so you can install them on the piston yourself. Plus you have big end and small end bores that are true round. New rods can be 5.7 or 6" depending on your preference. So as I said, there are pros and cons either way.

I hope I am not dumping to much information on you but experience has taught me that to get an excellent running engine you need to start out with a solid game plan BEFORE you start building. Good luck with your build.

 

Thank you very much for your feedback, I am learning a lot from you.
I do like your plan; let's avoid wasting time and money on the old connecting rods and go with this solution:

New connecting rods in 5.7 ''
New forged pistons with small ring package

 

I would first look at the oil ring that came out of that cylinder and inspect for damage. If none found then I would leave it alone. If it ain't broke don't fix it !

 

Hello again Motobecane,

It looks like you have a workable plan to get your engine working properly again! I would look at the Oil ring on that particular cylinder (#2?) to ensure that it has no damage caused by the hole on the existing pistons you removed.

I find it interesting that nobody knows WHY the hole was put there in the first place. This is a perplexing situation and I would love to figure out why the hole exists in the cylinder bore.

The idea of avoiding a liner for the cylinder should allow you to save some money there. The pistons for a SB are fairly easy to find in the U.S. and there are lots of choices. I would use as high a compression ratio as the gasoline available to you will allow. Compression makes power so boosting it a bit will make the engine really wake up. It all depends on the Octane rating of the gasoline that you have available. If higher than 93 octane then you could have some real fun simply bumping compression a little bit.

It must be fun to have a Corvette where they are few and far between like in France! When I was in the Middle east I told a local about my C3 and they could not believe that a 2 seat car would have a 7 liter engine up front.
They are still used to the Mini-Japanese 10 passenger trucks for driving around. My C3 is longer and wider that their truck but they had a 1 liter engine to propel that truck, passengers and freight. I have 7 liters of fire-breathing engine to get me and my passenger home safely.

I wish you the very best in getting your engine back together and running in your Corvette! We are here to help in any way that we can!

Best regards,
Chris

 

I prefer the longer rod; if for no other reason than it provides benefit of a better (greater) rod-to-stroke ratio, which produces less force-stress-thrust on sides of cylinder wall.
There are credible arguments; both pro & con. Here's a link, which helps to fairly summarize the discussion: https://www.enginebuildermag.com/201...ng-rod-ratios/

Probably, of greater importance is the following matter: Typically, piston and/or rod changes result in differences in mass and bobweight. The newly-modified rotating assembly may require dynamic balancing. However, it seems OP Motobecane may have limited auto machine shop choices-resources.
Can this motor be balanced prior to reassembly ?

 

Please find attached photos of my piston no. 2. I don't see any particular marks on the scraper ring, but I don't have your experience.
The first photo shows the side exposed against the hole.







So, when replacing the pistons, if I can be sure that I won't come into contact with the hole in the future for a reasonable price, that would be even better, to make sure I don't have to come back to it quickly.
But actually, maybe it's acceptable with identical pistons.

FYI, I'm going to have to change the pistons, because I dropped one during disassembly and broke the skirt, so I'll have to change the connecting rod and piston no matter what!

As for dynamic balancing, I don't really have much access to specialized workshops... Is static balancing really limited on a street configuration like the one I'm planning to do?

For the 6" connecting rods, I understand the arguments put forward, particularly that of reducing wear on the cylinder wall, but it's a classic car and it won't be driven more than 2,000 miles a year. So for the moment, I'm leaning towards 5.7 in order to try and keep within budget.

For fuel, I have the choice between 95 and 98, but I always use 98 because it's the only one with 0% ethanol, while 95 usually has 10%. That's why I was aiming for a compression ratio of 10.

As for a Corvette in France, it's always a big attraction when I take it out! Especially since I live in a small town, you can't go unnoticed. However, despite doubts about the car's functionality, the Corvette is universally loved; everyone loves American cars from those years.

I look forward to hearing from you.

 

As in French Motobecane? Circa 1980. Rick Case Discount Cycles, Cuyahoga Falls, Ohio.

I'd vote for a sleeve in that cylinder and never worry again. As long as it’s possible in France. N’est pas?

Steve O.