Inliners International Forums General Discussion Hi Performance Use 1/2"bolts for flex plate. Chevy 250

I have always thought about drilling & tapping the crankshaft for 1/2" flex plate bolts because I always had had problems w/the stock ones shaking loose.

Have any of you guys done this on 250 cranks? Outcome?

My old baseball coach had told me many years ago that Kay Sissle used to do that on his 292 cranks.

He also used to tell me to go see Kay because he knew 6 cylinder Chevys & he was good friends w/Kay, never got the chance to meet him, wish I had.

Do the 250 cranks have enough material to install 1/2" bolts.
I never had a 292 crank in my hands to see if there were any differences in size where the flex plates/flywheels mount.

Any help/info.
Thank you.

MBHD

All the 230 and 250 Comp engines we did for guys that was the first thing we checked on the cranks was how much metal was around the crank flange to see if it could be drilled and tapped to 1/2". Most of them can, but there are some that won't because the bolt circle is too close to the OD of the flange. Drilling them to 1/2" won't keep them from backing out if that's what your thinking. That wasn't why we did it.

OK, I'll bite, what is switching to a larger bolt for?

I was hoping it will have better clamping force for the flex plate.

MBHD

1/2 bolts will give more torque but this is not the problem, read my posts on this, you need pins and straps it is covered on my posts. It works at any RPM or power level.

You can have all the clamp force possible, only NO movement between the crank and flywheel will work.

It will provide better clamping force, but as Harry said, the clamping force isn't the issue as to why they come loose. We did it because spinning 9000+RPM with a heavy flywheel and a lot of HP, the 7/16" bolts just didn't seem adequate. If you keep the engine out of the harmonic shake zone they won't come loose. Today, with better dampners it might help keep this problem better under control.

Does your crank have provisions for dowels? That helps prevent flywheel/flexplate movement also.

Hi harry,

I cant seem to find your post talking about the flex plate bolts.

The last thing I did to my flex plate bolts that held in place was, IIRC, I used Moroso fold over tabs & applied JB weld .

That worked & they did not come loose.

A.F.A.I.K., 194, 230-250 cranks never used doweles

MBHD

I found this post from Turbo-6 on Flywheel bolts



From what I have seen don't run an inline at 3000 RPM constant, the problem is harmonics, when I drove my car on the street you could not see out the rear view mirror at 3000, it would vibrate so bad, above and below was fine.

If you keep the flywheel on the next thing to come off is the pin in the flywheel then the harmonic balancer in that order. Be sure that the bolt is in the balancer.

I use a strap as a lock to hold the flywheel bolts and the pin from moving, Two bolts go through the tab and hold the pins the ears on the end of the tab bend over the flats of the bolts.

Harry

problem harmonics 3000 and around 6000 and at 8000 for what I have been told.

Correct Larry. So its best not to plan an engine build that doesn't place the engine in those trouble RPM bands for too long, so try to stay several hundred RPM above or below that.

It's not unique to in-line engines (although a longer crank is more at risk), and it's not the same RPM for all of them.
The harmonic orders (bad RPM spots) will differ among engine designs, chiefly based on crankshaft rigidity.
Stiffer: short OAL, big main bearing journals, big rod bearing journals, short stroke
Weaker: long OAL, small main bearing journals, small rod bearing journals, long stroke.

The number of main bearings matters, but not intuitively. More bearings sounds good, but it also makes the crank longer.

Once the harmonic frequency of the crank is identified (very complex math or very good scientific testing), the orders are simply a math function, where Hz is the frequency, and N identifies which order:
RPM = Hz × 60 ÷ N
Most L6 engines are between 200 and 300 Hz (V8 are higher, V6 still higher).
Using 250 Hz as an example:
Order # RPM
1 15,000
2 7,500
2-1/2 6,000
3 5,000
4 (not relevant)
5 (not relevant)
6 2,500
9 1,667
12 1,250

As you can see, not all occur during useful engine speeds: 1, 9 and 12.
For an L6, the most dangerous are in this order:
worst = 3, 2-1/2, 2, 6, least = 9

Lightening the crank generally raises the RPM somewhat.
The real cure is a SEMA damper tuned to those RPM points.

Here are some effects of using a V8 damper, tuned to a common frequency of 350 Hz.
Order # RPM
1 21,000
2 10,500
2-1/2 8,400
3 7,000
4 (not relevant)
5 (not relevant)
6 3,500
9 2,333
12 1,750

Not too much similarity...

Also Most guys with AT do not run into these same issues as much as US guys who run a STD. Reason being is that the convertor having fluid in it which seems (?) to help to soak up some of the harmonics.

Has anyone used the Stage8 flex plate bolts?

Good, bad? Do those come loose?

MBHD

I used the Stage 8 fasteners on my turbo 292. They worked great, a bit painful to get them all fitted.

Hmm. My Firebird currently runs down the highway between 2,700-3,000 rpm, and when I pulled the engine for the last overhaul I had three of the six flexplate bolts be essentially hand tight. I wrote it off as me possibly be inattentive, but now I wonder. They all got Loctite on them and definitely torqued to spec this time.

I'm re-gearing the car soon with 3.42's because it's what I have with an actual limited slip carrier. That'll make cruise around 2,100 rpm.

I hadn't given much thought about the torque converter (unlocked) possibly acting as a fluid dampener. Hmm.

I've observed vibration vs rpms too. My stroker with 4.062 stroke, Crower rods, light JE forged pistons, heavy 168 tooth flywheel vibrates at about 75 mph. Smooth at 70 and smooth at 80. Crank and flywheel were balanced together, altered SBC fluid damper on the front. It's noticible enough to make me drive slower or faster to avoid the sour spot.

Not sure what the rpms are in my case. It's a '72 Nova with Muncie, 3.08 gears, tires similar height as stock.

The tabs are easy because it does not mater where the bolts end up when torqued a flat is always in position to bend the tab against, will try to post a picture of one.

Those locking tabs are kinda' like a band-aid in a way. It's like fixing the plumbing in a house that is a 100 years old, you fix one leak or drip and it causes a leak somewhere else. The flywheel bolts loosening up are a warning sign, and keeping them fixed with these plates doesn't fix what causes them to come loose to begin with, and gives you a false sense of achievement. It will just make something else fail if you keep operating the engine in these harmonic danger zones. The best solutions are to not create your build around these bad harmonic RPM's. Like strokersix mentioned, its easy to tell where these harmonic zones are, so tailor your driving habits or builds to stay away from them.

Even street cars woth std trans the bolt will shake out? The vibration from the bad harmonics even woth red locktite will turn them loose?

I never had a issue on the street with the red Loctite. But I hardly pushed the 8000 rpm shifts on the street only on the track, And is the only time I had some issue/s with the bolts coming undone. And as for using the red If it gets to sit long enough for it to REALLY set-up like it should You'll need Heat OR a Impact to get the bolts Loose. Oh and there is Other ways Other then to use tabs Or straps to keep the Bolts in place.

I had my flywheel come loose using red Loctite.
Really scary sounding

Running 4:10 gears trying to keep up w/traffic on the freeway is when that happened.
3500-4000 RPM, let it rev,,, yeah right! Never again!!!!
Overdrive baby! That's the away to go, keep the RPM's down.

I can't count how many times my flex plate bolts would come loose using red Loctite.
Those damm 4:10 gears trying to keep up w/traffic!

Some guys in Brazil would weld there damper so they don't shake loose I heard.

MBHD

When you start doing things to those extremes Hank, you start having severe engine failures like cranks breaking and blocks breaking in half. Bolts backing out and balancers shaking loose are easier to live with if checked routinely, but 2 pc cranks and blocks are harder to deal with. You just have to try and find a happy medium.

I have had good luck with the Stage 8 1/2" bolts and their balancer bolt kit on my 292.

CNC, The dowel pins are what keeps the flywheel from moving they are in with a press fit the TAB keeps them in and the bolts from moving, the pins only go through the crank flange part way there is no way this can loosen! The crank will break before anything that is why I use a steel crank, Have never broke a steel crank.

It is impossible to race an engine and not go through harmonics.

Been doing this since the sixty's when this engine was first made, and found out what worked and did not work, this was my fix and it has not let me down.

Using stage 8 bolts, are like show cars that tighten all the bolts so they are all clocked in the same direction some too tight and others too loose none just right.

Have to disagree on that one. With the ability to flip the tab for adjustment, the Stage 8 bolts very rarely need to have a bolt moved to have them in the proper spot.

From the pictures tlowe posted looks as if you flipped the tips would touch, they must be TIGHT against something so there is NO way the bolt can move at all, or you lost some torque.

It's just the tabs work so easy and can not fail.

Harry,
where can I buy more fold over tabs?

MBHD

I just have them cut with a water jet or lazer

What thickness?

.040"

Will check it out

I spoke to a guy this past week that is using the Stage 8 fastener system and it isn't solving his problems. The bolts are still backing out and the heads are rounding off inside the locking tabs as they're backing out.

IIRC the "orders" always occur at the same RPM regardless of throttle opening or power level.
This means that (if you have the time) a light throttle test slowly running through your RPM range and feeling for "buzzy-ness" through the steering wheel, shifter, clutch pedal, etc. will tell you when an order is really close. Say it out loud so your riding mechanic/wife/fireman can record it. How try up-shifting and pass through the same range again - results should match. Now, try a coast-down through the range from high to low - results should match.

Operating at a particularly bad order (like 3rd) also raises the oil temperature a bit but it may take minutes to show up.

Tab is .050 thick, 3.200 long, .900 wide

Thank you
Turbo-6!

MBHD

Is there a way to post a scan on a forum?
I know nothing about computers.

Yes, but you still have to use a photo hosting site like imageshack or photobucket to do it.

Turbo 6 -

If you have a file with a scan of the tab, PM one of us with your real email address. To PM. as an example, left "click" on "CNC-Dude #5585" and a drop down list will appear with "Send a PM" on it.

Then we can connect by regular email have you send the file as an attachment.

After that one of us with a little more computer savvy can post it on the forum for you.

Here's the sketch for the flywheel locking tabs. Thanks Harry!

One thing the length of the doll pin depends on the thickness of the flexplate or flywheel just make stick out of it a little.

Harry,

what does the 292 crank come with as far as the factory dowels?

Wonder if it would be a good idea to install some dowels on my 250 crank?

MBHD