Grooved Cam for 65-66 425hp Big Block

[Joe L.]

Re: Grooved Cam for 65-66 425hp Big Block

Jeff --- In Colvin's '65-69 Chev by-the-Numbers, on page 249, there's a reproduction of a GM blueprint for cam # 3856534 [I can't find this in the catalogs] which shows the grooved rear camshaft bearing journal.

The centerline of the oil groove seems not to be in the center of the journal, but 0.51" from the flat face of the rear of the cam. The journal itself is 0.98" wide, but the circumference is chamfered to result in an effective 0.88" width of contact surface with the bearing. The full depth of the groove is 0.10". The width of the goove is 0.18" at depth, with the circumferential edges chamfered at 45 degrees to a depth of 0.02".

Would be best to examine / interpret this page yourself before giving instructions to your cam guy.

Pics below are of the cam I removed from my 396 961 block, but it may be aftermarket (from long ago).








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Wayne------


It's probably aftermarket. However, "CMC"-sourced cores were used by GM to manufacture SERVICE cams after the cams were no longer used in PRODUCTION and, apparently, the demand was low enough that the original specific cores were no longer economically viable.

Usually, the GM cams manufactured from the generic "CMC" cores will have the camshaft part number stamped on one end.

[Michael H.]

Re: Grooved Cam for 65-66 425hp Big Block

All------


Here's what I don't understand about all this: GM supplied the grooved rear journal cams for years (1965 to 1988) in SERVICE for all 65-74 applications. As far as I know, there were no instructions provided with the camshaft requiring that modifications to the rear cam journal were necessary for 67-74 applications.

If there was a problem installing these camshafts in 67-74 engines I would think that would have been LONG AGO discovered and GM would have ceased specifying the grooved camshafts for 67-74 applications..

I agree. I wouldn't worry about it. In the 60's, we installed a LOT of new L88 cams with the groove in new 67-69 Camaro and Chevelle big blocks etc and we never had any issues with reduced oil pressure.

The grooved bearing on a flat cam wouldn't be much different than the oil supply to a crankshaft main bearing. Large oil feed hole and a grooved upper main bearing. Both have about the same clearance. and they don't leak much oil past the bearing.

In the late 60's and early 70's, GM recommended filling the groove in the cam if the cam were to be used in a 67 or later racing engine. A few later, they dropped the recommendation.
I can understand why it may be a slight issue in a racing engine because every little bit of oil leakage means the pump works harder and that's a tiny additional bit of HP but it won't make any difference in a street engine.

If the bearing journal to cam clearance dimension is around .003-.004", just exactly how much oil is going to escape?

I don't like the new facebook thing. Takes forever to load pages here now!

[Clem Z.]

Re: Grooved Cam for 65-66 425hp Big Block

I agree. I wouldn't worry about it. In the 60's, we installed a LOT of new L88 cams with the groove in new 67-69 Camaro and Chevelle big blocks etc and we never had any issues with reduced oil pressure.

The grooved bearing on a flat cam wouldn't be much different than the oil supply to a crankshaft main bearing. Large oil feed hole and a grooved upper main bearing. Both have about the same clearance. and they don't leak much oil past the bearing.

In the late 60's and early 70's, GM recommended filling the groove in the cam if the cam were to be used in a 67 or later racing engine. A few later, they dropped the recommendation.
I can understand why it may be a slight issue in a racing engine because every little bit of oil leakage means the pump works harder and that's a tiny additional bit of HP but it won't make any difference in a street engine.

If the bearing journal to cam clearance dimension is around .003-.004", just exactly how much oil is going to escape?

I don't like the new facebook thing. Takes forever to load pages here now!

with the groove in the bearing journal you are reducing the area of the bearing journal to seal against the bearing surface as the groove is full of pressurized oil and it allows oil to leak out both the front and rear of the journal. every engine i built with a grooved cam in a late model block i used the .062 oil feed hole and never had a problem. was it necessary i don't know but it did not hurt. if i had a street engine already built with a the grooved cam and the large oil feed hole i would not tear it down but if i was still building i would use the .062 hole. . filling the groove with epoxy would be of concern to me as it could possibly come loose. the fact that the .062 hole in the bearing was left out of the later power manual may have been because GM had no more grooved cams left in inventory. JMHO

[Ronald L.]

Re: Grooved Cam for 65-66 425hp Big Block

Wasn't Flint and Tonawanda grinding their own cams? Back then almost all this was in house except casting or forging the pistons.

[Michael H.]

Re: Grooved Cam for 65-66 425hp Big Block

with the groove in the bearing journal you are reducing the area of the bearing journal to seal against the bearing surface as the groove is full of pressurized oil and it allows oil to leak out both the front and rear of the journal. every engine i built with a grooved cam in a late model block i used the .062 oil feed hole and never had a problem. was it necessary i don't know but it did not hurt. if i had a street engine already built with a the grooved cam and the large oil feed hole i would not tear it down but if i was still building i would use the .062 hole. . filling the groove with epoxy would be of concern to me as it could possibly come loose. the fact that the .062 hole in the bearing was left out of the later power manual may have been because GM had no more grooved cams left in inventory. JMHO

[Ronald L.]

Re: Grooved Cam for 65-66 425hp Big Block

Curious about Christopher's comment about oiling on the top side.

Does it stop there or if a non grooved cam is used - does the engine get ruined?

[Clem Z.]

Re: Grooved Cam for 65-66 425hp Big Block

Curious about Christopher's comment about oiling on the top side.

Does it stop there or if a non grooved cam is used - does the engine get ruined?

yes you would get very little oil to the valve train in a 65/66 block as the cam bearing has a shallow groove the some oil could pass by. the guys who designed the BBC must not have read the history of the SB which went thru the same design problem.

[Clem Z.]

Re: Grooved Cam for 65-66 425hp Big Block

the bearing clearance is much tighter on the main bearings. i had a souped flat head ford that would rattle the bearing when you got on it from a dead stop and we pressurize the oil system with the pan off and you should have seen the oil that shot out of the mains. there was too much clearance and the rods were being starved causing the rod bearings to rattle. had to pull the engine and regrind the mains to the correct size with the correct under size bearings. i got all kind of hell from my dad because he told me not to buy this car. i was my first car that i bought on my own. it had duel pipes and belond headers and dual carbs and i just had to have it.

[Michael H.]

Re: Grooved Cam for 65-66 425hp Big Block

the bearing clearance is much tighter on the main bearings. i had a souped flat head ford that would rattle the bearing when you got on it from a dead stop and we pressurize the oil system with the pan off and you should have seen the oil that shot out of the mains. there was too much clearance and the rods were being starved causing the bearing to rattle. had to pull the engine and regrind the mains to the correct size with the correct under size bearings. i got all kind of hell from my dad because he told me not to buy this car. i was my first car that i bought on my own. it had duel pipes and belond headers and dual carbs and i just had to have it.

I hope we get to sit down and tell stories when I see you again. You have a VERY interesting past Clem. I haven't heard "Belond headers" for many decades. Not sure if I'll be at Kissimmee this year though.

[Clem Z.]

Re: Grooved Cam for 65-66 425hp Big Block

I hope we get to sit down and tell stories when I see you again. You have a VERY interesting past Clem. I haven't heard "Belond headers" for many decades. Not sure if I'll be at Kissimmee this year though.

i will not be there either but i should make carlisle in 2012. i also had spark pugs in the tail pipes with a model "T" vibrator coil to fire them. mrs clem's dad was a retired Pa. state trooper and he got the word about me and my fast cars form his buddies in the troopers. the cops used to chase me and if i would loose them they would go to my house and wait for me to come home. i had a garage about a 1/4 mile from home so i would put the car away and sneak home. my mother once said to me the neighbors are going to think that i was a common criminal because the cops were always looking for me. i would get a new car and they would stop me just to make sure that they had the right car in their book.

[Clem Z.]

Re: Grooved Cam for 65-66 425hp Big Block

I hope we get to sit down and tell stories when I see you again. You have a VERY interesting past Clem. I haven't heard "Belond headers" for many decades. Not sure if I'll be at Kissimmee this year though.

the guy that made them was "sandy belond"

[Joe L.]

Re: Grooved Cam for 65-66 425hp Big Block

Wasn't Flint and Tonawanda grinding their own cams? Back then almost all this was in house except casting or forging the pistons.

Ronald------


Yes, the engine plants did manufacture the camshafts from cores that I believe were produced at GM foundries. They produced camshafts for both PRODUCTION and SERVICE.

I think that Flint manufactured most camshafts right up until its closure. I don't know if Tonawanda manufactured SERVICE camshafts after 1975 when big block use in passenger cars ended.

[Timothy B.]

Re: Grooved Cam for 65-66 425hp Big Block

Clem,

Could it be possible the new rear camshaft bearings are produced with a smaller .062 hole so no need for the caution note or are they all .250".

You are right about the evolution of the 265 small block and rear camshaft bearing with regard to oiling the lifter galleries. That's a mistake you would think GM would not make again..

Pat, With a higher volume oil pump what is the relief pressure in the spring..

[Jeff P.]

Re: Grooved Cam for 65-66 425hp Big Block

Wayne;
Thanks for taking the time to share this information and pictures on machining the groove into the "65-66" camshaft. If my memory serves me right, the distributor also has a special groove for oil passage.

[Patrick B.]

Re: Grooved Cam for 65-66 425hp Big Block

Clem,

Could it be possible the new rear camshaft bearings are produced with a smaller .062 hole so no need for the caution note or are they all .250".

You are right about the evolution of the 265 small block and rear camshaft bearing with regard to oiling the lifter galleries. That's a mistake you would think GM would not make again..

Pat, With a higher volume oil pump what is the relief pressure in the spring..

Tim: It is the pump out of my ZL-1 engine. I can't remember why I replaced it 30 years ago because it doesn't look worn. I put a hydraulic cam in the ZL-1 before I used it (which maybe gave me the idea to change oil pumps) so I don't know for sure the relief pressure for this pump. It is currently sold as the service pump for solid lifter SHP Mark IV big blocks so I am assuming the relief is about 65 psi.