redrill bolt pattern on a manual flywheel

[Bronco73]

So I bought the zoom diaphragm pressure plate which is suppose to fit my bronco. It turns out only three of the bolt holes line up out of the six. Is it okay to drill and tap the other three? My thinking is I will be fine since the bolt holes are evenly spaced out. I have transfer punches to make sure I get the center of the mounting hole of the pressure plate dead on to the flywheel.

I also have room to go to the next size up bolt. Should I go to the next size bolt? My current flywheel bolts have a smaller ID than the OD of the pressure plate mounting hole.

When I bought the pressure plate I even check the BC which measured out correctly but I had no way of confirming the spacing between the pairs of bolt holes. I tried to see if it would line up with any of the other predrilled holes in the flywheel with no luck.

Was hoping to start installing some of this stuff tomorrow in the bronco. Not to big of a hurry because I won't have a new spud shaft until Aug 24th.

 

[addicted]

You cant return it and get the correct one? I mean Im sure you "could" drill and tap it but that seems risky and a lot of work. Not to mention you are removing metal from a balanced flywheel. Probably not going to make a big difference in the weight removed vs the balance but personally I wouldn't risk it.
* edit* Never mind. You weren't talking about the flywheel. Sorry.

 

[Bronco73]

My fear is there is something amuck with part numbers. I can try to call zoom and see what they say. The problem I think is because I want a diaphragm plate and not the standard three finger pressure plate which is what came in the bronco. Yes I plan on redrilling/tapping the three mounting holes on the manual flywheel.

As for the balance it will not change because the bolt pattern will be spread out evenly removing the same amount of material so it should have zero affect on that.

I have retapped existing holes that were already in the flywheel from the factory but never tapped. Can't recall what that was for but it worked out well.

I will more than likely do this tomorrow but just wanted

 

[Rustytruck]

Could you take your old presure plate and bolt it to the new pressure plate through the 3 matching holes and drill the proper holes in the new pressure plate? A pressure plate is cheaper than a new flywheel isnt it? Does the pressure plate use alignment dowels that you need to worry about?

 

[patterdale]

I'd try for the correct parts first. Failing that a good machine shop should be your next step. They can assure accuracy and balance.

 

[addicted]

What size clutch do you have?

 

[broncnaz]

X2 on getting the correct part. The issue lies mainly in the fact that the correct diaphragm pressure pate for a bronco is a conversion part not your standard part. Later on in years when the diaphragm clutch became standard a different bolt pattern waas used. I had the same issue with a Hayes clutch I thought they sent me the wrong one, as the box said it had been repackaged had them send another but was in a undamaged box and same issue. turns out they really just mislisted the application.
Of course if you dont go that route then I'd redrill the pressure plate bolt pattern..

 

[Bronco73]

Redrilling the pressure plate is not an option. That would mess everything up plus I do not think the pressure plate has material where it would be needed and it would have to be done by a machine shop as there is no way to keep things concentric between the two pressure plates.

I have already gone round and round trying to make sure I got the correct part to begin with. Other than redrilling the 3 bolt holes I will just have to buy a later model flywheel that is a pre 82 model to keep the correct balance. Had I known it was going to be this problematic I would have never had the stock flywheel resurfaced and just bought a new one for a couple of bucks more.

I will try to get a picture of the bolt holes I need to drill and tap. The pressure plate has 6 mounting holes. One mounting hole for each pair of two mounting holes can be bolted down now. Hope that makes sense. Its the 2nd bolt of each pair that is off. I can bolt the pressure plate down with only 3 bolts and it will be or is concentric to the flywheel. The BC is 12 3/8 and the flywheel measures out to that.

So I only need to drill 3 of the 6 mounting bolts. I would simple be transferring the location based off of the pressure plate which is located by the other 3 bolts. I have a strong machining back ground and the tools to do this correctly, except a drill press, but I have access to one.

 

[Bronco73]

In this photo I reused or tapped the bolt hole to the next size up which was 3/8. I needed to do this because the shoulder of the bolt locates the pressure, just like the stock one located. Drilling these three stock holes was easy since the small counter bore was already the size I needed(5/16) for the 3/8 tap. This only took a few minutes to do. all three holes. You can see that based on how thew bolt pattern is spaced that drilling three new holes will not affect the balance because they are evenly spaced from each other. I have even measured the outside of the pressure plate to the flywheel and everything is dead on. I should also note that three of the pressure plate mounting holes are smaller than the other three by about .030. The smaller three mounting holes are a perfect fit to the shoulder of the 3/8 mounting bolts. I choose to use the smaller three pressure holes to mount to the existing flywheel bolt holes giving me a nice concentric fit.

 

[Bronco73]

Here is how I transferred the location of the other three pressure plate mounting holes. The transfer punch is a tight enough fit that when I hit it with the hammer it gets slightly stuck in the mounting hole of the pressure plate. This should be dead on and these three holes have about .030 clearance in them. Again not by me but that is how the pressure plate came.



As for getting the right pressure plate. At this point this was suppose to be the correct one. I will say I did not call zoom to see if there really is a diaphragm plate that will fit in place of the stock pressure plate.

I will install it and fire it up and see what happens. If it doesn't work then I will just buy a new flywheel so I don't have this issue again later.

 

[astronot]

Sounds like it's a moot point now, but where are the other 3 holes that don't line up? It sounds like you're going to end up with 9 bolt holes in the flywheel with bolts in six of them. How are the other 3 going to be in balance?

 

[Bronco73]

Sounds like it's a moot point now, but where are the other 3 holes that don't line up? It sounds like you're going to end up with 9 bolt holes in the flywheel with bolts in six of them. How are the other 3 going to be in balance?

If you look there is way more than 9 holes in the flywheel now. Just look at the extra drilled holes in the photo. Those are from the factory but are symmetrical. The balance is not affected when things are evenly spaced meaning the material removed is in a symmetrical pattern. Its like one is at 12 o-clock and the opposite one is at 6 so the cancel each other. The same is true for the 3 a 9 0-clock positions.

 

[bmc69]

I will install it and fire it up and see what happens. If it doesn't work then I will just buy a new flywheel so I don't have this issue again later.

Speaking as both a machinist and engine builder...I see not one thing of concern with the way you added the three holes.

 

[Bronco73]

Thanks for the reassurance bmc69.

After a break of swimming and hanging with the son I just went in the garage and finished the last three holes. All 6 bolts can be put in by hand. No interference between any of them and no movement in the pressure plate.

Tomorrow if the boss allows me the time I hope to install the trans and start working on the hydraulic set up before work. I would work on it tonight but we have to go to bmx practice.

 

[Outlaw Bronco]

It will work just fine, and there is no issue with offsetting the balance of it either.

Great work.

 

[astronot]

If you look there is way more than 9 holes in the flywheel now. Just look at the extra drilled holes in the photo. Those are from the factory but are symmetrical. The balance is not affected when things are evenly spaced meaning the material removed is in a symmetrical pattern. Its like one is at 12 o-clock and the opposite one is at 6 so the cancel each other. The same is true for the 3 a 9 0-clock positions.

Sorry. And I'm sure you're right. I just never saw a photo of the flywheel without the pressure plate obstructing it. And I agree that if everything ends up symmetrical it should still be in balance.