340 Engine Bore

[Steve340]

I stripped my 340 down today and have discovered a really large chamfer at the top of the bores. It is far bigger than anything I have seen before.
It goes out nearly to the 4.180 of the old head gaskets.
I guess the 4.080 Cometic MLS gaskets I bought will not be getting used.
Is it standard or has it been done by a engine re conditioner?

 

[moparleo]

That would have been done by someone other than the factory. Were the pistons in place before you "stripped" it down ? if so, there would be a number stamped onto the top of the pistons stating the over size. The bore should either be straight up to the top of the deck or have a taper worn below the deck.
Mopar Small Block “LA” Engine: Bore & Stroke Chart

 

[Steve340]

Thanks for your thoughts. I had a feeling it was probably not factory.
It was in the car and running but using oil. The bore is 20 over and is pretty fresh. The engine shop fitted KB pistons and I am wondering if they tapered the top of the bore in an attempt to drop the compression. Hard to guess what they may have been thinking. It really is huge compared to anything I have seen before. Maybe they did not have a ring compressor. You almost would not need one.
The motor was done for a previous owner by a "professional shop"
Oh well I will have to make the best of it cannot put metal back. At this stage I can see no way around it just have to live with it I guess.

 

[aussiemark]

I am thinking maybe thermal metal spraying may repair it or have the block sleeved back to standard bore this method will work for sure but would be expensive but 340 blocks are getting more and more expensive and if it is the correct block for the car I would do it to save it. Some people are stupid and shouldn't be let anywhere near something as important as a rare engine block must have been a Chevy mechanic.

 

[AUSTA]

How far down the bore does the chamber extend there should be a wear mark between the top ring & the edge of the chamber you could deck the block then use a thicker gasket or get a larger bore gasket

 

[Steve340]

The engine is the matching numbers block otherwise I might try and get another one.
The upside is it was running and was pretty strong just used oil. It ran smooth and quiet.
That being said I am thinking leave it as the repair might be worse than the problem if you know were I am going. Don't fix what aint broken. The main problem I can see is it will lower the compression a little and may have contributed to the oil consumption.
However it is not a race car I only want to stop it using oil.
A 4.180 bore head gasket will need to be used.
The chamfer extends out about 1 mm from the bore on the deck face and goes down the bore 3.5 mm.
It would be away from the top ring on the KB pistons.

 

[aussiemark]

As long as it is away from the top ring I think it will be ok. Latter on when you go out to .060" it will be less noticeable and then the next rebuild just have all the cylinders sleeved back to std. Sleeves aren't all that bad a lot of the time they don't go to water and the material is a lot better quality for ring seal and porosity this is why 90% of diesel engines use them they run like 20 to 1 comp and you will know the exact thickness of your cylinder walls even sonic testing won't pick up small pockets or imperfections in a parent bore. I once had a full set fitted to a Nissan RB26dett engine because I couldn't find a replacement block when I was building that engine and the machinist said he didn't go to water.

 

[moparleo]

The taper shouldn't be the cause of using oil. Do a leak down test.

 

[AUSTA]

The engine is the matching numbers block otherwise I might try and get another one.
The upside is it was running and was pretty strong just used oil. It ran smooth and quiet.
That being said I am thinking leave it as the repair might be worse than the problem if you know were I am going. Don't fix what aint broken. The main problem I can see is it will lower the compression a little and may have contributed to the oil consumption.
However it is not a race car I only want to stop it using oil.
A 4.180 bore head gasket will need to be used.
The chamfer extends out about 1 mm from the bore on the deck face and goes down the bore 3.5 mm.
It would be away from the top ring on the KB pistons.

Wow thats some chamfer would work out around 4.10 at the top your right as long as the sealing face of the gasket isn't exposed you should be right what are the bores like
Like Leo says do a leak down then determine whether it is the heads or the block causing the oiling

 

[quapman]

I would remove at least one piston and check ring gap. That would be my best guess as to where the oil is going. Who knows? All the gaps might be lined up, too.

 

[moparleo]

You might be correct, he said it was done by a "professional shop". Do the leak down test first before you start to disassemble anything.

 

[Steve340]

I have completely stripped the engine but I know what caused the problem. When I bought the car it was detonating very badly. I thought the distributor may have come loose but no seemed tight. I checked the timing and it was running at 32 degrees advanced at idle and from memory about 55 at 3500 rpm. Reset the timing to 12 degrees and detonation gone.
This has damaged the rings and probably the piston ring lands, since I have owned the car (3.5 years) it has always burned oil as a result of the genius who set the timing there.
I modified the breather to control oil consumption and drove it.
I will not take a chance with those pistons they are in the bin.
The chamfer at the top of the bore is a curve ball for sure but I will just have to work around it.

 

[aussiemark]

If you hone the bores I would use the stone finger type hone rather then the brush type this will keep the bore to chamfer transition sharp, the brush type could smooth this area and make it more funnel like.

 

[Steve340]

Thanks for the tips. I am hoping to not bore the engine so it will go to shop and be checked and honed on their machine if all OK.
It had KB pistons in it and I checked the ring gap it was 0.025" very close to what KB recommend they say 0.026"
The ring ends had not been butting together.
I did a bit of measuring/research on compression height and realised he has machined 0.030" from the top of the pistons I suppose to lower the compression.
I am not sure what he was thinking as I have measured it up and calculated the compression ratio to be 8.8 to 1.
Unsurprisingly it was a bit flat down low but actually went OK.

 

[AUSTA]

Just picked up my new 414 stroker & found the bore have a similar chamfer to the 1 you describe
I asked the machinist & he said it is done at the factory as 2 different machines are used to align the bore to the gasket seal the bore is cut to make the gasket concentric to the bore as due to movement when casting the bore can be slightly out of alignment with the head bolts causing the gasket sealing surface to be off centre
He said if you drop the gasket on the gasket will align perfectly with the top of the bore where it has been cut.
Craig said some of the chamfers on the new GM motors are huge due to core shift when casting
I asked him about the compression & he said it would hardly make a difference