Roller Coaster of Engine building

[Guest ohollmen]

Hi Everyone - I'm your stereotypical lurker with lot of browsing in

the forums by the morning coffee, but no activity by now.

I recently did a Scarab to JTR Conversion (march, 3 weeks) with

my own JTR -dimensioned mounts and built a 2.5 x 2 to 3" Exhaust

with one Cat and one (3") Muffler. Although I had solid idea on how

to do it (also with help of JTR book) I got a lot of tip-of-the-day:s

on specific topics here from the forum - Thanks !

 

My 280 Z 2+2 Daily driver is already pretty peppy with stock-cam

350 (with stock 882 smog -era heads, low comp. and Performer manifold),

but I want to extend my poor-mans supercar to next level with a 383

(with 9.8 comp, AFR 195 heads, Comp Cams 268 HE Cam - all parts

officially smog legal). Even my kids are pretty immune to current quarter

mile times with final speed of 99.7, so gotta get them thrilled somehow

 

What makes building an engine a true roller coaster is finding out that the

crank spins ok freely, all clearances check ok, and then finding there's

metal dust and grit packed in a hidden corner after machine shop final wash

(always do it yourself) and after painting the block inside out finding that

machine shop did not clear the block enough for Scat wider-than-normal rod

big ends (althugh they had all my gear !). Well - I guess that's life.

Have to see what kind of pride the shop carries in their work and what the

warranties are...

 

Anyways this brings me to my latest worry with my engine pre-assembly,

where I noticed one of rod small (upper) ends is sidewise offset towards

one of the pin-bosses about 0.060" (i.e. in relation to piston centerline,

measured as a clearance of rod-end from piston pin bosses). This probably

means bore has an offset from axis where it should have been.

The crank and rods are Brand new Scat stuff (3.75" stroke, &" rod len)

so I'd think their geometry and dimensions should be ok.

Anyways outside the possibility of having the cylinder sleeved there's

nothing to do about the fact. The block is bored 0.040" oversize

(went up from 0.030).

 

My main question is wheather the 0.060 offset is something to worry

about when engine output range is 400-480 (targeting at the lower

portion of range with current cam, but want the bottom end to take more

if needed, for example with an upgrade to roller cam).

If you can lend your guru opinions wheather fact based or just based on

gut feeling, that would help me take the next action. I've got all the

parts to put the engine together, so its just a matter of working these

little loose ends together. I had hard time finding anything on the subject

anywhere (hybridz, google, large variety of chevy books I have). Even with

some discussion on topic (I found in a book) there was no figures given.

Obviously the first place I ask is not the machine shop where I got the

work done, cause by them I'm sure any amount offset would be ok

 

One more question I might add here is - when checking cam lobe interference

with stroker crank - should I use the timing chain to drive the cam like under

real engine operation or is it enough to randomly spin the cam a few turns

when each of the connecting rod big ends are near the lobes (of course need to

move the crank at the same time). Which method do you feel is more positive ?

 

Thanks already beforehand !

 

Olli