1968 L79 Motor Rebuild Cam question

[Jim B.]

I am rebuilding my original owner 1968 L79 (327ci/350HP) and I would like to put a new Camshaft, timing gear and chain in it close to the original.

I looked on the archives and there was mention of the 151 cam from Crane.

When the car was new it had a slight lope and had to be idled at 750 rpm.

My engine is an HT code. In 1968 there were two L79 builds. An HP, for automatics and Airconditioning and the HT for manual transmission cars without air. This HT build was listed as having a special cam. My manual says that the valve springs and lifters are also different.

I have not found any reference to this difference in the archives. Does anyone have any info on what the difference was and what parts match the HT build?? Which build does the "151" cam match?

Any help would be appreciated.

[Joel F.]

Re: 1968 L79 Motor Rebuild Cam question

I do not know a thing about 68's but I'd be very surprised if there was a different valve train for the HP and HT.

[Duke W.]

Re: 1968 L79 Motor Rebuild Cam question

The L-79 camshaft was the same part number from 1965 to 1968 regardless of the option configuration. Different engine codes were used based on the front end accessory confirguration, and there may have been differences in distributor and carburetor configuration due to US or CA emissions.

Also, from 1967 through the late seventies all Corvette SB engine configurations used the same valve spring except for some engines that had exhaust valve rotators, which required a specially designed spring to make room for the rotators. Hydraulic lifters are the same from '57-up for all hydraulic lifter camshafts.

You can buy OE equivalent parts made by Federal Mogul (Sealed Power and Speed Pro brands) from NAPA. They have an online catalog that you can used to find proper OE equivalent replacement parts for your year and engine option and order them on line or from your local store. You can also use the cross reference function to find Sealed Power/Speed Pro part numbers from the OE part numbers like 3863151 for the camshaft and 3911068 for the valve springs.

As far as the timing chain/sprockets are concerned, just buy the OE replacement Sealed Power set. You don't need some aftermarket timing set that costs two or three times as much.

Catalogs from various vendors, including GM, have errors and, unfortunately they are rarely corrected. GM parts books have errors that are decades old. It's always a good idea to verify proper replacement parts using multiple sources, and this can often be done online since most major parts suppliers have online catalogs.

There are some significant differences between the '68 L-79 and earlier verisons - the larger 2.45/2.10" journal diameters and a Quadrajet rather than the earlier Holley carb.

Duke

[Jim B.]

Re: 1968 L79 Motor Rebuild Cam question

The L-79 camshaft was the same part number from 1965 to 1968 regardless of the option configuration. Different engine codes were used based on the front end accessory confirguration, and there may have been differences in distributor and carburetor configuration due to US or CA emissions.

Also, from 1967 through the late seventies all Corvette SB engine configurations used the same valve spring except for some engines that had exhaust valve rotators, which required a specially designed spring to make room for the rotators. Hydraulic lifters are the same from '57-up for all hydraulic lifter camshafts.

You can buy OE equivalent parts made by Federal Mogul (Sealed Power and Speed Pro brands) from NAPA. They have an online catalog that you can used to find proper OE equivalent replacement parts for your year and engine option and order them on line or from your local store. You can also use the cross reference function to find Sealed Power/Speed Pro part numbers from the OE part numbers like 3863151 for the camshaft and 3911068 for the valve springs.

As far as the timing chain/sprockets are concerned, just buy the OE replacement Sealed Power set. You don't need some aftermarket timing set that costs two or three times as much.

Catalogs from various vendors, including GM, have errors and, unfortunately they are rarely corrected. GM parts books have errors that are decades old. It's always a good idea to verify proper replacement parts using multiple sources, and this can often be done online since most major parts suppliers have online catalogs.

There are some significant differences between the '68 L-79 and earlier verisons - the larger 2.45/2.10" journal diameters and a Quadrajet rather than the earlier Holley carb.

Duke

Duke,

Thanks for the info. I am going by my Chassis overhaul manual I got from Chevrolet when I bought the car. It has some "interesting" info in it.

It states that there were two lifters used in Chevy Small blocks which were "exchangeble" in service. It calls them "A" and "B" but does not say what the difference was but the picture shows them of different heights

The Corvette Manual transmission L79 has a unique column of specs applicable to only it.

There is no column that has the L79 automatic transmission.

Unfortunately very little tech info on the CAM or the Springs, only the lift 0.2981. (which is the higest lift in 1968 other than the Higher end 427's. It also shows a much larger clearance for the pistons and a different diameter piston for the L79 manual transmission which is slightly smaller to allow for the larger clearances.

This manual only shows the 2.44' Journals and an 11:1 compression ratio. The Carburetor is in fact unique to the L79 manual transmission and is a Rochester 7028219.

The valve spring specs appear to be the same for all L79's but my GM overhaul manual says that only the Manual transmission was available with the L79 which I do not believe is the case so I cannot tell if there may be alternate specs. There is no mention of the HP or HT designation but I know them to exist.

The clutch disc I took out appears to be an 11 inch disc, but I have to check it out further. My manual transmission is an M22 going by the markings on the case and what I was told by the dealer.

I had no ridge buildup in the cylinder bores (almost no wear) which I was very happy to see. I maintained the engine very well over the years and it paid off. The Cam timing gear was roughed up and the chain had loosened such that a tooth was probably slipped later in life. The cam looked very good but I haven't checked it out with mic's yet as I have not specs as yet. I thought to replace it just because I was into the engine and the shape of the timing gear.

Anyway, I am having a ball as I haven't done this in years going back to my racing days. Too much time in engineering school and working for a living.

[Gene M.]

Re: 1968 L79 Motor Rebuild Cam question

Jim,
'68 Corvette should be a 10 1/2" clutch with a 4 speed only with 350 HP. Spline is course. 11" would need BB bell housing.
The L79 tranny was a M21. Possible M20 with shorter gears in rear end for top end.

[Jim B.]

Re: 1968 L79 Motor Rebuild Cam question

Jim,
'68 Corvette should be a 10 1/2" clutch with a 4 speed only with 350 HP. Spline is course. 11" would need BB bell housing.
The L79 tranny was a M21. Possible M20 with shorter gears in rear end for top end.

You are right about the clutch. I made an accurate measurment today and it is indeed 10 1/2 in.

The transmission on the other hand is in fact an M22. It was a special order that the dealer put out. It has the "C" designation after the part number on the case and has the bottom drain plug casting although not drilled.

I bought from a copo dealer who specialized in ZL1's, copo camaros and was linked with both Yenko and Baldwin/Motion. The car has a 3:70 posi and when it was new the 2:20 first gear was tough to deal with on launch, but boy did she run.

Anyway, thanks for the reply.

[Jim T.]

Re: 1968 L79 Motor Rebuild Cam question

Jim since you are the original owner of your 68 you know your cars history and with the transmission out it is easy to determine if it is a M22 cose ratio like the dealer told you or a M21 close ratio.
The M22 will not have any rings on the splined input shaft. It will also have a drain plug in the casing as well as a filler plug.
I think you will find you have a M21 close ratio with your L79. The M21 will have one ring on the splined input shaft. It will not have a drain plug in the case.
The M20 Muncie has two rings on the splined input shaft.
My original owner 68 L79 has the M21 and 3:70 rear end.
Have you ever looked behind the radio, or the tach and speedometer for the Corvette order copy? My 68's order copy was/still is behind the radio and shows the M21 transmission.
According to the Black Book there were 80 M22's used in the Corvettes in 1968. There were also only 80 L88 Corvettes in 1968 and all L88's came with the M22 close ratio transmission. The Black Book also lists how many M21 and M20 4-speeds were put in the 68 Vettes. Adding up all the optional transmissions for the 68 Corvette will show that only 326 68's came with the standard equipment 3 speed.

[Larry M.]

Re: 1968 L79 Motor Rebuild Cam question

Jim:

You may want to call and request the "1968 Corvette Restoration Package" from GM while they are still available. Contains the AMA Specs (Automobile Manufacturers Association) which list ALL the details about your car and drivetrain. I did this for my 1967 and was very pleased with the package. Takes about 2-3 weeks mail time to get from GM.

I do not have the phone number handy, but can get it. Perhaps someone else has the number and can post. If not, I will send it to you in the next few days.

When you call, you will need your VIN and MAKE, MODEL, and YEAR. Have the order taker repeat what you told them. I did not, and my first package was for a 1967 Corvair. I don't think the girl knew the difference...even though I thought I spoke clearly. Oh well.

Larry

EDIT: The phone number for the GM Restoration Package is 1-800-222-1020 or (810) 696-4800. Request a Customer Service Rep and then ask for the GM Restoration Package for your car.

[Jim B.]

Re: 1968 L79 Motor Rebuild Cam question

Jim since you are the original owner of your 68 you know your cars history and with the transmission out it is easy to determine if it is a M22 cose ratio like the dealer told you or a M21 close ratio.
The M22 will not have any rings on the splined input shaft. It will also have a drain plug in the casing as well as a filler plug.
I think you will find you have a M21 close ratio with your L79. The M21 will have one ring on the splined input shaft. It will not have a drain plug in the case.
The M20 Muncie has two rings on the splined input shaft.
My original owner 68 L79 has the M21 and 3:70 rear end.
Have you ever looked behind the radio, or the tach and speedometer for the Corvette order copy? My 68's order copy was/still is behind the radio and shows the M21 transmission.
According to the Black Book there were 80 M22's in 1968. There were also only 80 L88 Corvettes in 1968 and all L88's came with the M22 close ratio transmission.

I am pretty sure. There are no rings (I am not sure what you mean by "rings") that I can see and it has the drain plug but it is not drilled. There is a blind plug in it. (I heard that this happened with some later M22's)

I bought it from a COPO dealer who specialized in racing cars. A lot of the cars he ordered did not show up on any production lists. My car was ordered for a racing team that broke up before delivery and then I showed up. It has NO options other than the tranmission and the 3:70 posi. The base radio is there and when I asked why it didn't show in the option list I was told that it is always installed unless you get the "radio delete" option. The car was very light and handled very well and was very quick.

It has the dealer's COPO emblem on it. I was going to have them take it off, but it was drilled and bolted into the fiberglas so I left it. Glad I did now.

I have all the original paper work and under options it simply has an order number with the dealer code. This is what they put on the bill of sale as well.

At the time I just wanted a corvette so I didn't care.

The tank sticker was destroyed by a gasoline accident early in the life of the car. I have not taken the dash apart so I don't know if there is anything in there. That will come later.

As far as the production numbers go there were a LOT OF "Mistakes" on that front when the special order cars were concerned. Almost all of the COPO ZL1 camaros had M22's. GM's records were terrible at the best. Also, I think that there are more than 80 L88's around now which is also hard to explain.

It is interesting to go thru this though

[Jim B.]

Re: 1968 L79 Motor Rebuild Cam question

Jim:

You may want to call and request the "1968 Corvette Restoration Package" from GM while they are still available. Contains the AMA Specs (Automobile Manufacturers Association) which list ALL the details about your car and drivetrain. I did this for my 1967 and was very pleased with the package. Takes about 2-3 weeks mail time to get from GM.

I do not have the phone number handy, but can get it. Perhaps someone else has the number and can post. If not, I will send it to you in the next few days.

When you call, you will need your VIN and MAKE, MODEL, and YEAR. Have the order taker repeat what you told them. I did not, and my first package was for a 1967 Corvair. I don't think the girl knew the difference...even though I thought I spoke clearly. Oh well.

Larry

EDIT: The phone number for the GM Restoration Package is 1-800-222-1020 or (810) 696-4800. Request a Customer Service Rep and then ask for the GM Restoration Package for your car.

Larry

I did this about a year ago. The package was a little disappointing because it was mostly references to where you could get parts. They politely told me that there were no records for the 68 model year corvette.

Thanks anyway.

[Duke W.]

Re: 1968 L79 Motor Rebuild Cam question

Duke,

Thanks for the info. I am going by my Chassis overhaul manual I got from Chevrolet when I bought the car. It has some "interesting" info in it.

It states that there were two lifters used in Chevy Small blocks which were "exchangeble" in service. It calls them "A" and "B" but does not say what the difference was but the picture shows them of different heights

The Corvette Manual transmission L79 has a unique column of specs applicable to only it.

There is no column that has the L79 automatic transmission.

Unfortunately very little tech info on the CAM or the Springs, only the lift 0.2981. (which is the higest lift in 1968 other than the Higher end 427's. It also shows a much larger clearance for the pistons and a different diameter piston for the L79 manual transmission which is slightly smaller to allow for the larger clearances.

This manual only shows the 2.44' Journals and an 11:1 compression ratio. The Carburetor is in fact unique to the L79 manual transmission and is a Rochester 7028219.

The valve spring specs appear to be the same for all L79's but my GM overhaul manual says that only the Manual transmission was available with the L79 which I do not believe is the case so I cannot tell if there may be alternate specs. There is no mention of the HP or HT designation but I know them to exist.

The clutch disc I took out appears to be an 11 inch disc, but I have to check it out further. My manual transmission is an M22 going by the markings on the case and what I was told by the dealer.

I had no ridge buildup in the cylinder bores (almost no wear) which I was very happy to see. I maintained the engine very well over the years and it paid off. The Cam timing gear was roughed up and the chain had loosened such that a tooth was probably slipped later in life. The cam looked very good but I haven't checked it out with mic's yet as I have not specs as yet. I thought to replace it just because I was into the engine and the shape of the timing gear.

Anyway, I am having a ball as I haven't done this in years going back to my racing days. Too much time in engineering school and working for a living.

I'm not aware of different height lifters. Maybe someone else can comment.

I don't know why the L-79 transmission has a unique column of specs. Both WR and CR Muncies were available with different standard and optional axle ratios. I don't believe the M-22 CR heavy duty four-speed
was available with L-79. I think it was only availalble with L-88 in '68, but if your car really is a COPO it's possible it could have been built with a M-22, which would be easy to verify with the stamped VIN derivative and trans ID features.

L-79 used a forged piston, and the nominal installation clearance is about .0035 compared to .001" for the cast piston on the base engine.

L-79 was not available with an auto trans in 1968, nor any prior years.

The nominal gross lobe lift of the L-79 camshaft is 0.29807", which is right off the Chevrolet drawing, and both sides use the same lobe. Depending on source, the .050" duration is speced at either 221 or 222 degrees. I use 222, which is what I measured off the drawing data - rounded to the nearest degree.

The NCRS Specification Guide 1968-1982 (a very useful document to have) shows only one L-79 engine code for '68 - HT

Get the "restoration package" because it should include a copy of the AMA specs, which has specs that are not available in other sources.

There was no "radio delete" option in '68 because a radio was not yet base equipment. If it has no radio, it was ordered without a radio option. UL5 "radio delete" was first available in 1972, when a radio was added to the base equipment list.

Chassis/Overhaul manuals are usually frozen prior to start of production and may not include late pre-production or running changes, and even when they go for subsequent printings, they were rarely updated. The AMA specs were technically an "engineering document" and were revised as necessary, but they are not all error free.

Specifications, option combinations, engine codes, etc. can be a slippery slope due to last minute and running changes that are never documented in the publications available to consumers. There are many areas of controversy, some of which can be resolved with a "preponderance of evidence", but some are still subject to debate.

Duke

[Jim B.]

Re: 1968 L79 Motor Rebuild Cam question

I'm not aware of different height lifters. Maybe someone else can comment.

I don't know why the L-79 transmission has a unique column of specs. Both WR and CR Muncies were available with different standard and optional axle ratios. I don't believe the M-22 CR heavy duty four-speed
was available with L-79. I think it was only availalble with L-88 in '68, but if your car really is a COPO it's possible it could have been built with a M-22, which would be easy to verify with the stamped VIN derivative and trans ID features.

L-79 used a forged piston, and the nominal installation clearance is about .0035 compared to .001" for the cast piston on the base engine.

L-79 was not available with an auto trans in 1968, nor any prior years.

The nominal gross lobe lift of the L-79 camshaft is 0.29807", which is right off the Chevrolet drawing, and both sides use the same lobe. Depending on source, the .050" duration is speced at either 221 or 222 degrees. I use 222, which is what I measured off the drawing data - rounded to the nearest degree.

The NCRS Specification Guide 1968-1982 (a very useful document to have) shows only one L-79 engine code for '68 - HT

Get the "restoration package" because it should include a copy of the AMA specs, which has specs that are not available in other sources.

There was no "radio delete" option in '68 because a radio was not yet base equipment. If it has no radio, it was ordered without a radio option. UL5 "radio delete" was first available in 1972, when a radio was added to the base equipment list.

Chassis/Overhaul manuals are usually frozen prior to start of production and may not include late pre-production or running changes, and even when they go for subsequent printings, they were rarely updated. The AMA specs were technically an "engineering document" and were revised as necessary, but they are not all error free.

Specifications, option combinations, engine codes, etc. can be a slippery slope due to last minute and running changes that are never documented in the publications available to consumers. There are many areas of controversy, some of which can be resolved with a "preponderance of evidence", but some are still subject to debate.

Duke

OK, I know that the NCRS judging manual is not perfect but if you look at the 1968/69 third edition, also a very good reference to have, page 38 you will see two L79 engines listed. One has AC and power steering broadcast code HP and the other has only a manual transmission and a spec. cam broadcast code HT. I have seen these on the judging field.

This info is also in the GM documentation. That's why I was asking about the possiblity of different cams as I was told by the dealer. He also told me that in order to not have a radio you had to order a car with no radio, which could be called "radio delete". Otherwise the car came with the base radio. Page 10 of the judging manual refers to this.

The dealer (Scuncio Chevrolet in Rhode Island, Bob Johnson was the high performance manager and signed all my purchase documents...google the dealer name) showed the option to order a race car with no radio, to me in his racing options catalog. He ran a race team and ran a series of cars called Strip Teaser and Strip Teaser 2 in the late sixties and early seventies when I was into racing.

The M22 was not just a corvette transmission. It was available on Camaros and on Chevelles as an option and many were made there. My transmission has my Vin stamped on it and the codes for the M22. As the original owner I can say that until I took the engine and transmission out this month, they had never left the car.

I did get the restoration package.

The column I was talking about was a four page specification table on the L79 in the Overhaul manual.

I do thank you for your help Duke. As always you have a lot more info on these cars than can be gotten almost anywhere else and as you state the specs and info are often conflicting and full of folklore.

[Jim T.]

Re: 1968 L79 Motor Rebuild Cam question

Jim what I am refering to as a ring on the splined input shaft is a circular cut into the splined input shaft that will leave a permanent cut in the splined input shaft. If you have kept your Corvette News magazines there is an article in one that shows pictures of the splined input shaft with the circular cut.

[Cecil L.]

Re: 1968 L79 Motor Rebuild Cam question

Duke,
Although this was well before 68, he may be referring to a short lived change of hydraulic lifters announced in Chevrolet Service News Volume 31, Number 4, dated April, 1959.

"Hydraulic valve lifter assembly, part number 5231475, is currently serviced for use in all 1957-58-59 Chevrolet V-8 engines, except the 322 cu. in. truck engine and those V-8 engines incorporating the high performance camshaft (referenced in the Chevrolet Parts Catalog as "Hi Lift Cam"). The overall length of replacement lifter assembly 5231475 is slightly less than that of hydraulic valve lifters used in production of the above engines. Although this difference in overall length exists, both the production and service replacement lifters have identical lift-action and are completely interchangeable assemblies."

These new lifters entered use in production for the 61 model year and apparently caused some problems due to confusion with adjustment for these "long travel" lifters which called for 2 turns from zero lash as opposed to 1 turn for the old style lifters. Failure to adjust the new lifter 2 turns likely ended with cut rocker studs and mistakenly adjusting the old style lifters 2 turns resulted in the plunger bottoming out preventing the valve from closing.

This was brought out in the January, 61 CSN Volume 33, Number 1, along with a very good description of the Engine Positive Ventilation Equipment option.

The March, 61 Volume 33 Number 3 issue of CSN, GM announced the replacement of all "long travel" lifters in service and production with the old style "short travel" lifters.

[Duke W.]

Re: 1968 L79 Motor Rebuild Cam question

Thanks for posting. I was never aware...

Duke