SBC pressure plate tightening sequence?

[Mac]

I recently read that 3 of the 6 SBC pressure plate bolt holes are smaller and these should be tightened first or the mounting will be off center resulting in imbalance and vibration.

Anybody ever hear this?

[wayne petty]

that makes sense.. if the cover bolts fit into counter  bored holes like a  starter mounting holes in the block...  so the shank of the bolt.. even though its just slightly longer than the thickness of the cover material.. would center and align the cover with the flywheel and keep the shear point of the bolt interface on the shank instead of the threaded area...

would make it very strong...


back to sleep now...

say... why when i post while i am asleep.. do i make more sense..ZZZZ

[tomslik]

I've only ever seen it on 4.3 chevys for some reason and yeah, the clutch will chatter like a beech....

[Mac]

I don't get chatter. In fact the take up is quite smooth. Would this mean the clutch is centered OK ?

I have been chasing a chronic vibration problem between 1000 to 2000 rpms. So far I ruled out misfire, 5 and 7 plug wire crossfire and the the dampener read 0*TDC spot on with #1 piston using a dial indicator off the deck. It's also not driveline as it's there when standing still.
What's not addressed yet is the rear motor mounts. This is a cast iron bellhousing with side mts. The vib seems more pronounced in the cab than it does while working the throttle in the engine bay. I'm thinking this means the issue is more likely on the back side of the engine ??

[tomslik]

what I haven't heard is what sbc you have, could be a balancer walking, wrong flywheel/flexplate, whatcha got exactly?

[Mac]

This is a roller 305 from a `92 Camaro, that I bought cheap. I put in rings, bearings, timing set and head work. I kept track and all the pistons and rods went back in thier orig. bores. The balancer is the orig.. Also, I swapped in a used LT1 cam, re-using the orig. lifters. The TBI was replaced by an Edel. Perf. intake and an Edel. 500 cfm. Block huggers.

It had an automatic behind it in the `maro so I bought a new flywheel, clutch kit and pilot brg. when I put it in my `63 C-10 with a T5. Not high end Hays and McLeod parts, just generic clutch and f wheel (for 1 pc. rear main 305). I know that "new" doesn't necessarily mean good any more so they could have balance issues.
The budget won't allow throwing parts at the problem so I'm hoping for the best diagnosis.

[tomslik]

well, i'm drawing a blank....new mounts at the bellhousing I suppose?
rubber or urethane?

[wayne petty]

i want to know something..

does this engine have a GM small cap HEI... with electronic spark control..


are you using some kind of fuel injection on it.


why i ask... is small cap HEI distributors from gm with the riveted reluctor assembly on the distributor shaft can crack...

when the engine is above 400 RPMs.. the electronic spark timing is in control... and it uses a count down timer to control timing..

what...  yea..  it looks at the sine wave from the pickup coil in ESC mode..  and counts down to create a spark.    if the computer shortens the count down.. the spark is advanced.. if it lengthens the count down the spark is retarded..

if the magnet in the reluctor cracks.. or the magnet in the pick up coil cracks... it can generate GHOST wave forms that are within the other sine waves... this causes an out of phase ignition spark.. that can drive people crazy..    delco and GM offer replacement distributor shafts .. but they are hard to find and not cheep... but there is a save...  cardone who rebuilds a LOT of stuff.. got conventional powdered metal reluctors on shafts created...this makes the timing control much more stable..  as the sine waves are a lot cleaner..

so... take off the cap and rotor.. look with a bright flashlight at the second layer of the small cap reluctor..  you can see some of it exposed at  the shaft ... and more out at the edges of the reluctor.. if you see cracks...

you might have found your issue..

[Mac]

I don't think poly mounts are available and lately I've been supicious of repop rubber parts. Often they're poor quality Chinese (with that nasty smell, you know).
I have an aluminum bell to update it to trans crossmember mount, but that means I have to mod. the park brake linkage. Maybe, only to find out a new (proper balance) P plate, F wheel or dampener would have fixed it.
What to do, what to do?

Thanks for your interest, Tom

Thanks Wayne for yet another comprehensive posting. I think of you as a research librarian at a great big city library. But alas, I have a big cap HEI.

[Mac]

I dug a little deeper -beyond the classic truck parts suppliers- and found rubber mounts from Anchor and Pioneer/DEA at Summit. At $5-$6 a piece it's well worth a try.

[wayne petty]

big cap HEI's have the issue also.. with the pick up coil...

could you be having a air fuel ratio problem during the idle transition circuit operation..

where its too rich or too lean just off idle..

can you take a video of the engine during the vibrations..  and post it to you tube and the link here..  

try not to have the video camera inline with the fan blade.. or take the fan belts off for a 30 second fan free run...  just motor..  not hot air like i am putting out..

))))))))))))))))))))))))))))))))) ahhhh!!!!

[enjenjo]

Do you have the correct pressure plate bolts? They have a larger shank than regular bolts.

[Mac]

I'll have to have a close look at the reluctor and pickup coil magnets. Would cracks cause a consistent out of phase ignition just in this 1000 rpm range? This dizzy was working fine and the 283, it was in last, had no vib issues.

I have a narrow band A/F gauge hooked up that I used to tune the carb. I haven't seen any radical shift, lean or rich, in the vibrating range. The vibs have been unchanged from first start after rebuild thru all the jet and rod changes. It was a used carb that I cleaned thoroughly and kitted.

I did remove all the drive belts and eliminated the accessories as a source of the problem.

I can't honestly remember what I used for clutch cover bolts. I'll have to pull 1 to see. I'll check for 3 of the 6 bolt holes being smaller also.

[wayne petty]

i have seen big cap HEI pickup coils with big chunks of magnet out.. and the engine could barely run... it had so many odd firing pulses from the coil...

[UGLY OLDS]

i have seen big cap HEI pickup coils with big chunks of magnet out.. and the engine could barely run... it had so many odd firing pulses from the coil...

  I fought this problem on an '87 Suburban I bought " right" because NO ONE could resolve the problem....  It showed up immediately  when we put it on a scope ... The firing line display was a sight to behold ...

I also remember most pressure plate bolts as being a "shoulder" bolt with the holes in the flywheel not threaded for the first 1/4" or so to properly locate the pressure plate cover ....

Kinda like this :

http://static.summitracing.com/global/images/prod/xlarge/arp-130-2201_us_xl.jpg

Bob....