# Flywheel bolt holes don't line up correctly



## Woodsmith (Jun 5, 2008)

Sometimes flywheel holes are not symetrical so it only fits the crank in one orientation for balancing.

Is the flywheel your original part?

Who made the adaptor? And was the flywheel available as a pattern?


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## kennybobby (Aug 10, 2012)

The flywheel holes look okay--the mating piece wasn't machined properly. How was the bolt circle measured or specified for drilling? It looks like the hole pattern was traced on a piece of paper and drilled by hand, which is not accurate enough for rotating machinery.


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## Caps18 (Jun 8, 2008)

The flywheel is the original part from GM. I didn't know that the holes weren't perfectly symmetrical until last night.

I will contact the supplier for the hub to see if it is just a simple misunderstanding and that I received the wrong part. 


I guess the next thing is if I do need to get a new flywheel, how do I know that the holes will line up?


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## kennybobby (Aug 10, 2012)

The 6-hole bolt pattern looks exactly symmetrical. The seventh hole is for the alignment pin on the crankshaft.


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## Caps18 (Jun 8, 2008)

That is what I thought too. But I can rotate the flywheel around and the left hole in the above picture is always the one that is 'off'.


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## hmincr (Jan 20, 2012)

Is there enough thickness to tap and screw a bolt into that misaligned adapter hole, tack weld it, cut it off flush and redrill that hole to match the flywheel?

I have done that a couple of times, to cure that problem. The cut off bolt will be threaded into the adapter, so, can never spin, even if the tack weld is not perfect.


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## EVfun (Mar 14, 2010)

Perhaps the hole in the flywheel can be slotted. It should go into the machine shop to be lightened and rebalanced anyway.


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## evmetro (Apr 9, 2012)

EVfun said:


> Perhaps the hole in the flywheel can be slotted. It should go into the machine shop to be lightened and rebalanced anyway.


Amen. I would at a minimum have it surfaced, lose the ring gear, and have it balanced. Maybe you could bring your hub to the machine shop with you and see what they can do to to mate them together.


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## electro wrks (Mar 5, 2012)

Some OEMs offset some of the mounting bolts and/or use dowels so that the FW can only be mounted in one position. Another concern is that the donor ICE is externally balanced. In which case, your FW may have a cast-in counterweight near the rim on the backside. If it does, without re-balancing, spinning it up on an electric motor will cause all kinds of vibration and mayhem.


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## Old.DSMer (May 18, 2012)

I had this happen. The holes are "normally" NOT symmetrical. Yours would be considered a 6-bolt. That extra hole is for alignment. If your original ICE didn't have a dowel, that particular flywheel might have been used on other models that did (S-10 truck, right? I'll bet that engine platform is used elsewhere in GM's lineup).

The two bolt holes on either side of the alignment hole will be spaced apart at one angle. The remaining 4 holes are probably spaced equally to complete the 360 degree circle. I would suggest carefully tracing the flywheel hole pattern on some stiff bristol board. Then get out the old geometry set and start measuring.

I had to have a new coupler machined because I prepared the original drawing and didn't realize the odd spacing.

If you do any kind of modification on a flywheel, I would agree with the previous recommendations of getting it rebalanced.

Before slotting the holes - inspect the bolts and determine if they use a locating shoulder. Also measure to check the bolt/hole clearance. You could throw the system incredibly out-of-balance by slotting the holes. Especially if they are used for alignment.

My original ICE had an external balance shaft and "presumably" a balanced flywheel. I ended up with an aluminum one (removed the ring gear). Its incredible how much lighter they are!!!


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