Same Problem...More Info
I recently had the same problem. The O'Donnell instructions are somewhat ambiguous. First, yes the stock screws are too long. The directions say for one particular O.S. engine (not the V-Spec), use the enclosed screws and for all other models use the stock screws. Well the package does not come with "any" screws and you either have to cut off about 1/8" from the stock screws (Dremel tool works well) or get different screws.
Second, if you use the O'Donnell button then you have to use the O'Donnell 99 standard plug (or comparable). On the other hand if you use the stock V-Spec button then you can use the O'Donnell 97T turbo plug (or comparable). The instructions say to use the O'Donnell 97T plug, but it never tells you to use the stock button. Quite confusing until you think about it and remember that the threads are different.
After speaking to more than one person from their Tech Support (because each of them gave a different answer) I finally determined that you can use whatever you like. O’Donnell button with standard plug or stock V-Speck button with turbo plug. If you use the stock button the total diameter is smaller than the cut out in the bottom of the head, but just line up the screw holes and it works fine. Make sure you use the O.S. copper gasket. (I purchased a new one to be safe).
I have a Mugen MBX5R with the O.S. V-Spec that is about 2 years old. I put the new head on with the stock button and it screams down the track with no adjusting. Hope this clears up any confusion. One of O'Donnell's people did say that it appears the instructions have some issues. Hopefully they will revise it.
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MBX5R (2), Savage XL, Associated B4, Airtronics M12