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Stripped Cam cap bolt threads; Wot I did to fix it
Topic Started: Feb 25 2018, 09:33 PM (295 Views)
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This may have been written up elsewhere but I'm tired and thought I'd do this before getting away to my bed.
Oh dear, oh dear. Actually what I said was a bit different but some of you will know that horrible feeling when the tool goes slack just as you had decided 'that's probably tight enough'.....
I was just torqueing up the final cam cap bolt on the CCM 404 after putting in new shims under the valve buckets and it stripped. I'd spent good money on a Teng Tools 1/4" 5Nm upwards torque wrench too. So after I had cried a bit I gave myself a stern talking-to and faced up to the challenge.
Why did it strip?? Here's what I did. It may help someone sometime, although this is simply what I did, not recommending anything here . . .
I dismantled (or, as the American's put it - I tore down) the cam chain guide, cams and extracted all the bolts and the four dowels. Then I cut up strips of breakfast cereal card (I recommend Frosties but perhaps it doesn't matter that much . . .). The strips were about 4mm wide and three inches long. I pushed one down each bolt hole until it bottomed then bent the card over flush with the face of the cylinder head. I then took a 3mm Allen wrench and pushed that down alongside the card, then worked the wrench up and down pressing the card against the hole thus creating an impression of what was down the hole.
I found that each hole had a 8mm non-threaded section of wider lead-in below which was 14mm of thread, so 22mm below the surface face, below which was 4mm of unthreaded hole (see where this is going?).
There are the two deeper holes that are machined to flow oil around the cams so they have a lot more than the 4mm spare depth - I left those alone. More measuring told me that 6 of the bolts were 40mm long while the 2 that hold the cam chain guide down are longer at 45mm. Remember this is a 12 year old bike and those bolt lengths aren't necessarily the correct dimensions . . .
Anyways, I then measured the aluminium cam caps. Well, there's a surprise. The depth of the holes through the caps on the gear-change/stator side are all 21mm. The rearmost on the offside was 21mm next forward (one of the chain guide bolt holes) was 20mm, next forward was 25mm and finally, the one that stripped, nearest the offside radiator, the cap is 26mm thick!!!!! No wonder it stripped - the 40mm bolt was going through 26mm of cap, then 8mm of lead-in leaving only 6mm of thread actually used. The other regular bolts were in contrast enjoying 21mm cap plus 8mm un-threaded section leaving 11mm thread used of the 14 down there.
So, to the fix - I used a M6 x 1.0 plug tap that will cut thread pretty much to the bottom of a hole. I then cut an extra 2mm, or two turns, of thread thus using 2mm of the 4mm spare at the bottom of each of the holes (not those two chain-guide bolt holes - I left those well alone) to give 16mm of thread depth. I used a tiny 5mm spanner on the square end of the tap to wind it down to the bottom - that way I was unlikely to be able to use enough force to strip any more threads. A bit of grease on the end of that 3mm Allen key got the swarf out. I then ordered 8 M6 x 1.0 Allen headed bolts of the internet of 50mm length. With a little patience I cut down each bolt in turn so that it nips up the cap with one spare thread left (don't want the bolt binding at the bottom before it torques up . . .). For five of the original 40mm bolts I made them 45mm long. That's a 1mm washer, then 21mm of cap plus 8mm lead-in plus 15mm thread used - a whole 4 threads more than before. The 2 bolts holding down the chain guide are 45mm rearmost and 50mm forward (1mm washer plus 2mm guide plate plus 25mm cap thickness plus 8mm lead-in plus 14mm thread) and finally, the stripped thread bolt is 50mm (1mm washer plus 26mm cap plus 8mm lead-in plus 6mm mullered thread plus 9mm thread. That's 50% more thread than Suzuki left us with straight from the factory.
I've seen a formula that suggests threads needed = 1.5 times dia. of bolt. So M6 needs 9mm of thread. If that 40mm bolt, engaging in only 6mm of thread, was as-factory then that explains why if you Google search 'DRZ400 stripped head thead' you get a LOT of search results - and I bet they all stripped the same one. :what:
So, to sum up - don't panic, buy a plug tap, cut a couple of extra threads and use a longer bolt.
"Boom!!" as the kids say.
Hope that helps someone.
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Well done.

You've more patience than me.
Just a flunky along for the ride.
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Charlie Big Banana's
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Nice job Keith,
If it aint sharp I can play with it
My Utube Channel, lots of bike vids here. Subscribe if you likehttp://www.youtube.com/user/choppermark?feature=watch

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