68 327 rebuild questions

[Charles B.]

We are totally rebuilding my 327. The car is numbers matching and I am attempting to maintain originality but am considering practicality. Cylinders will be 30 over. Heads are the original 291 cast, all new valves, 194's. Now the questions. Because of the new unleaded fuel, should I try to maintain the original compression ratio? Should I use domed pistons similar to the originals or flat top? Should I use a replacement cam for the original with comparable specs? Or use a different combination of pistons and cam? I am not experienced in engine rebuilding so please don't "short cut" a lot of terminology. All experienced input will be appreciated.

[Michael M.]

Re: 68 327 rebuild questions

Charles if you want to keep the rebuild practical do it as close to spec's as possible. Go with the same compression pistons to get maximum performance and if you don't have a original camshaft most cam companies have a similar grind. A Thread was posted a couple of weeks ago and the overall decision was that the unleaded fuel of today is OK for high compression engines.

[Timothy B.]

Re: 68 327 rebuild questions

Charles,

Do yourself a favor and check the archives for information that will benefit you greatly in this process.

Don't get a similar cam, get the correct original grind and build the engine stock. Use a stock oil pump/stock pressure, even your original if you take the time to inspect the gears and check cover clearance.

For 327/300 HP you can use cast pistons, 327/350 use forged with domes just make sure compression height is OEM spec. It is very important to check deck clearance before taking the pistons and rods out of engine and rebuild with deck approx .040. This is where head gasket selection is important..

If you are interested in your project, you will spend some time reading/learning but the information is here. Don't let the machine shop remove any material from the heads, (plain the bottom) unless it's really necessary, I think in most cases it's really not!! See if that engine will take a .020 or .010 bore to clean up the cylinders, don't go .030 because the pistons are easier to get. This is the computer age and most everything is a click away..

Original car, original motor, do it right, keep it simple...

[Scott S.]

Re: 68 327 rebuild questions

For 327/300 HP you can use cast pistons, 327/350 use forged with domes just make sure compression height is OEM spec. It is very important to check deck clearance before taking the pistons and rods out of engine and rebuild with deck approx .040. This is where head gasket selection is important..

If you are interested in your project, you will spend some time reading/learning but the information is here.

Hi Timothy,

Could you (or someone) please elaborate on why it is important to check deck clearance before taking the pistons and rods out of the engine?

Is it so that you know if it ran without knocking at a specific measured compression ratio before, you know you can rebuild it to at least the same compression ratio again and run fine, or is it more than that?

[Timothy B.]

Re: 68 327 rebuild questions

Scott,

There are people here that will answer your question better than me. Your assumption is correct, and doing this check also helps in the rebuilding process with regard to installing new parts with the same or DIFFERENT compression height to maximaze compression ratio and quench.

Do a search in archives for some interesting reading about how these things can effect compression and in the end, performance.

[Joe L.]

Re: 68 327 rebuild questions

We are totally rebuilding my 327. The car is numbers matching and I am attempting to maintain originality but am considering practicality. Cylinders will be 30 over. Heads are the original 291 cast, all new valves, 194's. Now the questions. Because of the new unleaded fuel, should I try to maintain the original compression ratio? Should I use domed pistons similar to the originals or flat top? Should I use a replacement cam for the original with comparable specs? Or use a different combination of pistons and cam? I am not experienced in engine rebuilding so please don't "short cut" a lot of terminology. All experienced input will be appreciated.

Charles-----


All 1968 Corvette small blocks, 300 hp and L-79, used the GM #3917291 cylinder heads. In your post, though, you mention "...all new valves, 194s". I presume you mean 1.94" intake valves by this. That size intake valve was only used on the 300 hp engine, so I presume you have a 300 hp engine. If so, that engine did not originally use domed pistons; it used flat top pistons with 4 valve reliefs.

Regardless of whether the engine originally used flat top pistons or whether it's an L-79 and used domed pistons, I would recommend using the flat top pistons. These will give you a compression ratio of about 10:1 and that's as high as I would want it to be. In fact, I'd use thicker-than-stock, composition style head gaskets to reduce the compression about another 1/2 point, or so.

[Duke W.]

Re: 68 327 rebuild questions

With "nominal" .025" deck clearance, the OE flat, notched pistons, and a thin shim-type head gasket the CR is only about 9.8. L-79 was 10.5 at best. Run the numbers though the calculator.

All this has been throughly discussed in the archives.

Duke

[Charles B.]

Re: 68 327 rebuild questions

Joe
It is an L79. However, due to the condition of the engine, sitting since 1975 with both heads cracked and water in 2 cylinders, it was necessary to replace the heads. The engine was totally locked up. I was able to located correct heads casting numbers but had 1.94 valves. Also it is necessary to sleeve the 2 cylinders in order to eliminate the rust. I saw a post you made on June 6, 2006 (I think that was the date) relating to another 327 rebuild, is that info still what you would recommend for the parts?

[Charles B.]

Re: 68 327 rebuild questions

Joe
Actually the date of your response was June 7, 2006 to a thread "1966 327 Rebuild- I have many questions."

[Joe L.]

Re: 68 327 rebuild questions

Joe
It is an L79. However, due to the condition of the engine, sitting since 1975 with both heads cracked and water in 2 cylinders, it was necessary to replace the heads. The engine was totally locked up. I was able to located correct heads casting numbers but had 1.94 valves. Also it is necessary to sleeve the 2 cylinders in order to eliminate the rust. I saw a post you made on June 6, 2006 (I think that was the date) relating to another 327 rebuild, is that info still what you would recommend for the parts?

Charles------


Yes, everything would remain the same now as it was on June 7, 2006. 1968 327 used different rods than 1966 327. However, the 1968 rods, properly serviced and fitted with ARP rod bolts, are fine for your application.

Since the engine is an L-79, I would use a reproduction of the original L-79 '151' camshaft. That will work fine with flat-top pistons and it will work fine with 1.94" intake valve size heads.

[Joe C.]

Re: 68 327 rebuild questions

Joe
It is an L79. However, due to the condition of the engine, sitting since 1975 with both heads cracked and water in 2 cylinders, it was necessary to replace the heads. The engine was totally locked up. I was able to located correct heads casting numbers but had 1.94 valves. Also it is necessary to sleeve the 2 cylinders in order to eliminate the rust. I saw a post you made on June 6, 2006 (I think that was the date) relating to another 327 rebuild, is that info still what you would recommend for the parts?

Have the block sonic tested first if you intend to save it.

Insufficient cylinder wall thickness will allow cylinder flexing in an otherwise "good" running engine. Your engine will display what were considered acceptable operating characteristics forty or fifty years ago, including oil consumption in the vicinity of 1 quart per 1000-2000 miles (with forged pistons) and moderate amounts of blow-by. The proof of the pudding, however, will be lower cylinder pressures/lower power outputs than those developed by a block with more rigid cylinder walls.

"SELECT" engine blocks were used for dyno runs "back in the day", to produce the advertised torque/horsepower numbers that we'd all like to obtain with our vintage engines.

The 327 engine had an inherently unforgiving water jacket core design. Unlike the 283 block, the 327 block is particularly sensitive to core shift, causing thin cylinder wall areas, even in blocks with standard bore. Finding a "select" 327 block as a project engine is a factor RARELY or NEVER considered by most "restorers". If one intends to keep, drive, and enjoy his car for a long time then he should think hard about using a quality "select" block as a foundation!

I would look for minimum finished wall thickness (for a "street" engine) of .180 on the primary thrust surface, .150 on the secondary, and .120 on the non thrust surfaces. This will be hard to achieve with a production block. If you are intent on saving your block, then compromise is allowable, but keep in mind that going very much less than these thicknesses will lead to loss of power, blow-by, and cylinder hot spots. So, if your block can satisfy this guideline, then ask the machinist if a .040 or .060 overbore will thoroughly clean the cylinder walls. Have him hone the bores with NO FINER than 280 grit, and using torque plates.

Cylinder sleeves are more durable than the original gray cast iron cylinder block, but problems arise if they are not installed properly. One common problem is if sleeve is not sitting flush with step on bottom of bore. Sleeve is then cut flush with deck, but creeps upward during use, creating a gap and subsequent leakage. Same situation can arise if sleeve is cut too short for bore, is driven into bore flush with deck, but with gap at bottom. Sleeve creeps downward during use, and causes coolant leakage through head gasket.

[Duke W.]

Re: 68 327 rebuild questions

Joe
It is an L79. However, due to the condition of the engine, sitting since 1975 with both heads cracked and water in 2 cylinders, it was necessary to replace the heads. The engine was totally locked up. I was able to located correct heads casting numbers but had 1.94 valves. Also it is necessary to sleeve the 2 cylinders in order to eliminate the rust. I saw a post you made on June 6, 2006 (I think that was the date) relating to another 327 rebuild, is that info still what you would recommend for the parts?

If you are referring to me, I have made countless posts on engine restoration, and there are many threads that go into great detail - thousands of words on the subject. You need to spend some time researching, but I'll summarize as follows.

1. Use OE equivalent parts - brands like Sealed Power (Federal Mogul) and Clevite (Dana), but KB pistons are fine, and since they offer different volumes than OE pistons, they can be helpful in achieving the target CR range.

2. Learn about the correct, true CR range for various OE engine configurations, how to measure it and how to manage it so you end up in the proper range with the restored engine. This starts with measuring deck clearance BEFORE the block is disassembled.


3. There are internal modifications that can be made, primarly head "massaging" that can yield more power and useable revs if that is an objective.

The difference between the 1.94/1.5 and 2.02/1.6" valves is not great. An average massaged head with the "small" valves will way outflow OE machined heads with the large valves, and I've seen very well massaged small valve heads outflow average massaged big valve heads.

On 300HP rebuilts I do not recommend increasing valve size. The smaller valves have much less tendency to develop a crack between the seats.

Duke

[Charles B.]

Re: 68 327 rebuild questions

Thanks to everyone who has responded to this as well as past threads regarding this subject. The info has been extremely helpful.