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Cam bearing tower machining marks? Ever seen this?
5 Attachment(s)
So I need to replace the valve stem seals on my NB and I have run into these odd machining marks on the rear of all four exhaust cam bearing towers (or whatever they are called). They are raised enough that I can't get the lifters out past them. It's not a huge deal, going to use a dremel and very carefully get the back flush, but any idea where/how this happened?
Engine is original to the car, early NB made in January of '98 (did they make any NBs in late 97?). 183k miles on it. https://www.miataturbo.net/attachmen...1&d=1640639470 https://www.miataturbo.net/attachmen...1&d=1640639470 https://www.miataturbo.net/attachmen...1&d=1640639470 https://www.miataturbo.net/attachmen...1&d=1640639470 https://www.miataturbo.net/attachmen...1&d=1640639470 |
If I had to hazard a guess I'd say that the buckets were rocking in their bores at some point in time. That's serious enough that I'd be worried. Smoothing them out will help get the buckets out of the head, but I'd be concerned about how that happened.
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All the damage is well above where the buckets can reach with the cams in though, so there would be no way for it to be caused by the buckets. And they are all consistent marks too, like they were made from the same tool. That's why I am so confused.
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The process used in the original machining would’ve required the head to be repositioned after the roughing of the bucket bores to another station where the finish tool would then complete the diameter and roundness. Looks like a fixture clamp didn’t lock causing the location of the 2 processes to be different. Maybe machining chips pushed the locator out of position…? The lifters were assembled into the bores by a mechanical press (or pneumatic) so likely the misalignment wasn’t enough to cause a fault in the process for anyone to notice.
A typical process used today would involve a tool change so the head would not be repositioned. Some of the Japanese mfg still used the inline process though up through the past 10yrs. Old habits are tough to break. |
Originally Posted by x_25
(Post 1614653)
All the damage is well above where the buckets can reach with the cams in though, so there would be no way for it to be caused by the buckets. And they are all consistent marks too, like they were made from the same tool. That's why I am so confused.
Originally Posted by bahurd
(Post 1614675)
The process used in the original machining would’ve required the head to be repositioned after the roughing of the bucket bores to another station where the finish tool would then complete the diameter and roundness. Looks like a fixture clamp didn’t lock causing the location of the 2 processes to be different. Maybe machining chips pushed the locator out of position…? The lifters were assembled into the bores by a mechanical press (or pneumatic) so likely the misalignment wasn’t enough to cause a fault in the process for anyone to notice.
A typical process used today would involve a tool change so the head would not be repositioned. Some of the Japanese mfg still used the inline process though up through the past 10yrs. Old habits are tough to break. |
2 Attachment(s)
Buckets look perfectly fine, but they are steel of some sort and the cam towers are aluminum, buckets are very much gonna win in that fight.
Anyway, a very small stone on a dremel so I don't muck anything else up and all the buckets are freed (and exhaust valve stem seals changed now). https://www.miataturbo.net/attachmen...1&d=1640724828 https://www.miataturbo.net/attachmen...1&d=1640724828 |
Onwards and upwards then!
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