Engine decision

[Oldman74]

yup, pass
suggest also consider cn 638 block --- 350ci, 1 pc rms, ready for (or has) OE roller lifters and step-nose roller cam. Immediately pre-dates 880 block. Most TBI 350s have 638.

That’s the cn of the motor I was looking at but have decided to pass on. In my research on the 638 and 880 blocks, I’ve found a couple of mentions of 638 blocks where the lifter bores had some sort of counter bore which created a problem with using the taller, roller lifters. Something about the oil hole in the lifters being exposed during lift, and then losing pressure. The motor in question is a 4 bolt version, HFT equipped motor that is indeed fully machined for all the OEM roller components.

The other concern is the unknown condition of the bottom end. If I’m going to pay more than $600 for a build candidate, I’d like to be confident I can reuse everything in the bottom end except the bearings, rings, and even an oil pump. The cost of boring, crank, rods, and pistons adds up quickly.

Thoughts?

 

[Oldman74]

Here’s one post from a Chevelle forum I found.

 

[sandlapper]

Bill K is well versed, no doubt.
Begs the question: were they using OE Genuine GM roller lifters ? or Aftermarket ?
Were the naughty motors' lifters' oil holes in same position as OE lifters' oil holes ?

I've not seen a "counterbore" on top of lifter guides; although I've seen plenty of them with a chamfer.

 

[sandlapper]

$5K budget ?
this motor GM 19432779 + a summit cam + new springs = you're under $5500 + It's ALL BRAND NEW

ALL NEW ! Vortec 96-02 350 HD L31 Crate Engine

Check out the deal on ALL NEW ! Vortec 96-02 350 HD L31 Crate Engine at GM Performance Motor

www.gmperformancemotor.com

 

[FS87LT]

As to "new" blocks, I noted in Grumpy Jenkins' book that he said they only used "seasoned" blocks to use in his race cars (in the 1980s era). At that time, I'd always considered "new" to be better than "used", for many reasons. Unfortunately, those reasons did not match for his purposes.

The reason for using "seasoned" blocks is that as they had seen many hot-cold cycles and such, after their break-in period, any machining done to them after that would "be solid" and not move. THAT got my attention, back then. It was explained that as the green block endures its hot-cold cycles, its then-machined dimensions can move/change. I had never considered that, back then, but it made sense.

Seems like he also mentioned that if they had to buy a new block, they'd put it "on the patio" to cure for several months before they'd start to build it for the race car.

Consider that what is termed "break-in period" is really "wear-in period". Which can allow for the block to cure and "move" as it might. As the various moving parts adjust to each other in the process. Which might also relate to the "no wot for the first 500 miles" orientation.

Which might also explain one recommendation of "break-in" of a rebuilt motor (rings and bearings, basically) of to do some "moderately-heavy throttle" driving for the first 100 miles or so, to get the rings seated-in. Throttle and load variations between high and lower vac levels.

Just some thoughts,
FS87LT

 

[Oldman74]

I hadn’t really given the seasoned cast iron view much thought but there is lots of evidence to support it in other applications. I used to restore vintage cast iron woodworking machines. It’s well known that many of the OEMs would age some of cast iron parts, especially those with eventual large flat machined surfaces.

In any case, I want to go through the build process. Maybe I’ll reconsider buying this used 638 engine.

I really appreciate the knowledge and advice from this group.

 

[Patstuff28]

I

As to "new" blocks, I noted in Grumpy Jenkins' book that he said they only used "seasoned" blocks to use in his race cars (in the 1980s era). At that time, I'd always considered "new" to be better than "used", for many reasons. Unfortunately, those reasons did not match for his

As to "new" blocks, I noted in Grumpy Jenkins' book that he said they only used "seasoned" blocks to use in his race cars (in the 1980s era). At that time, I'd always considered "new" to be better than "used", for many reasons. Unfortunately, those reasons did not match for his purposes.

The reason for using "seasoned" blocks is that as they had seen many hot-cold cycles and such, after their break-in period, any machining done to them after that would "be solid" and not move. THAT got my attention, back then. It was explained that as the green block endures its hot-cold cycles, its then-machined dimensions can move/change. I had never considered that, back then, but it made sense.

Seems like he also mentioned that if they had to buy a new block, they'd put it "on the patio" to cure for several months before they'd start to build it for the race car.

Consider that what is termed "break-in period" is really "wear-in period". Which can allow for the block to cure and "move" as it might. As the various moving parts adjust to each other in the process. Which might also relate to the "no wot for the first 500 miles" orientation.

Which might also explain one recommendation of "break-in" of a rebuilt motor (rings and bearings, basically) of to do some "moderately-heavy throttle" driving for the first 100 miles or so, to get the rings seated-in. Throttle and load variations between high and lower vac levels.

Just some thoughts,
FS87LT

I remember reading stories in Hot Rod magazine back in the day. The racers would put the new block in their tow vehicle for a few months to let it season. They would specifically look for old vehicles, just to get the seasoned blocks. This was probably less important when honing torque plates became more common.

 

[sandlapper]

As for bigger cams installed with OE GM roller valvetrain --- there's another limit to lobe lift that's Not in the heads.

OE roller motors use a dogbone (aka lifter guide) to constrain the lifter from turning in bore --- they employ a pair of flats on each lifter that're "keyed" to dogbone. However, with lobe lift greater than about 0.350"; the lower round portion of lifter will bang into the dogbone and begin to dislodge it --- if lifter is no longer constrained, all hell breaks loose. Party over.
FWIW, .350" lobe lift w/ std 1.5:1 RAR = about 0.525" valve lift.

 

[biker]

$5K budget ?
this motor GM 19432779 + a summit cam + new springs = you're under $5500 + It's ALL BRAND NEW

ALL NEW ! Vortec 96-02 350 HD L31 Crate Engine

Check out the deal on ALL NEW ! Vortec 96-02 350 HD L31 Crate Engine at GM Performance Motor

www.gmperformancemotor.com

What a great deal!

 

[Oldman74]

As for bigger cams installed with OE GM roller valvetrain --- there's another limit to lobe lift that's Not in the heads.

OE roller motors use a dogbone (aka lifter guide) to constrain the lifter from turning in bore --- they employ a pair of flats on each lifter that're "keyed" to dogbone. However, with lobe lift greater than about 0.350"; the lower round portion of lifter will bang into the dogbone and begin to dislodge it --- if lifter is no longer constrained, all hell breaks loose. Party over.
FWIW, .350" lobe lift w/ std 1.5:1 RAR = about 0.525" valve lift.

Makes sense to me. Honestly not planning anything over .500” lift. With 170-180cc intake runners, I feel the .480” range is probably more appropriate. Again, I’m only looking for 350-375hp at the crank. Nice reliable street car that’s fun to drive.