coating rocker pins & idler gear pins

[jomalkay]

Quick question: I we thinking of coating the rocker arm pins and cam chain idler gear pins with similar coating used on bearings / piston side skirts, more-so to bring them to the high side of the tolerance as they are currently at the low end. I'll use Calico out of NC, the cost is not so much based on what they charge for bearings. Seems like a no-brainer, but...
Just wondering if this is a common thing to do or is there any downside that i am missing? Coating adds total .0008" i would polish and check to make sure i was not oversize.
Also, anyone know where i can get the bushings for the idler arms?
Thanks, John

[Steve@Rennsport]

FWIW,

While I haven't done that yet, when I do, I'll use a DLC coating such as Casidiam or its equivalent. Bearing coatings are very good for engine bearings, however they aren't the best choice for these types of applications.

[jomalkay]

thanks for the input Steve. So it is not a crazy idea! just after posting, i had the brain fart regarding aluminum material vs. hardened steel, this stuff is a science, any metallurgists care to chime in?

[chris_seven]

Coatings based on phosphates, graphite and polymers have all been used to coat the skirts of pistons .

The surface conversion coatings are likley to be the most durable and the screen printed Xylan (PTFE) costings are not very strong.

Initially these coatings were promoted to reduce friction and help break-in but now coatings that provide wear resistance and allow reduced clearances are being promoted.

Just like Snake Oil they get better every year and it is now surprising that pistons without coatings could ever have survived for longer then a few minutes.

I am not sure that any of these coaings would survive on a pin as they are not very hard and not selected for wear resistance.

One manufacturer of Rocker Arm Bushings is Permaglide (Home - Permaglide ® - www.permaglide.de - www.permaglide.com) and if you buy these chose a lead free option as preferable.

DLC Coatings such as Balinite STAR are excellent but quite costly and you would need to superfinish before coating.

If you were planning to use a 906/RSR/935 style solid rocker, which doesn't have a bush then this technique would be very good for avoiding galling between the rocker and the shaft.

The 'Motorsport' Rocker Shafts were tuftrided to avoid this problem.

The downside if using DLC on a Rocker without a bushing is that all of the wear would be limited to the Rocker and the shaft would never wear.

I think shafts are far cheaper than 906 Type Rockers.

DLC is around 2 times harder than a Tuftrided layer.

If you are using a bushed rocker I wouldn't bother to coat the shaft as were will be minimal and friction will be very low due to the presence of the bush.

If you want to bush the Chain Wheel Support you could make a Silicon Bronze Sleeve

With regard to hardened steel running in an Aluminium bore I would think about Hard Anodising and the PTFE impregnating the Aluminium part as this would improve the wear reistance of the Aluminium.

[KTL]

Coating the rocker shafts is not a bad idea to bring them back into spec. However i'd also consider going with more modern bushings as Chris referred to. Talk to cgarr here and he can probably set you up with the bushings he uses to rebush the rockers and the idler arms.

You DEFINITELY want to bush the chain idler support arms. There's a reason Porsche updated these support arms. The older idler support arms without bushings are indeed prone to problems. Over time the support arm material galls and eventually it seizes on the chain housing post.

I recently did a full rebuild on my 3.2SS racecar engine for this very reason. The left idler arm seized on the post and put an extreme amount of tension on the chain. The extreme tension caused the teeth on the I-shaft sprocket to blue from the heat buildup.

However , the damage was not necessarily catastrophic to the rest of the engine. The damage was of course the idler arm and chain sprockets- most notably the sprocket on the intermediate shaft. I replaced the chains as well because that's the accepted practice. I rebuilt the engine on the whole simply because other inspections indicated to me that it was the right time.

[jomalkay]

Chris / Kevin, thanks for the feedback and knowledge. I plan to have ccgar do my rockers with new bushings he uses, was just thinking to get the stock pins at the high side. probaly could also polish them all to a similar size and ask to have the new bushings honed to fit with low tolerance. Should have mentioned in the original post, the motor is an 86 3.2L, idler arms are bushed for the cam housing shaft (need to measure this but suspect little wear), i was more concerened with the idler gear pin. I have decided to go with new idler gears will see the fit when they arrive. Anybody have the spec / wear tolerance for the rocker and idler pins?
I am probaly getting a bit anal on this area, but took the desicion to do it right with the overall build - $$$ - so i am trying to leave no rock unturned.
Kevin, you mentioned you had issue with a 3.2SS, this should have been same idler gear arms (bushed)?? Think i may be missing something. Attached a pic of the 3.2 arms. It's really cool going through the learning curve and actually being able to understand and apply, this is the great thing on these older 911 motors.

[chris_seven]

Originally Posted by jomalkay

and ask to have the new bushings honed to fit with low tolerance. [/img]

I would avoid honing or reaming a DU Style bush they are not really suitable for this type of process.

If you need to adjust the 'fit' I would centreless grind the shaft but I am 100% confident that ccgar knows what he is doing.

[KTL]

I don't know of a spec for the idler sprocket vs. the pin but i'll do some looking.

The sprocket pin gets lubed better than the support post. Reason being is the pin is fed by the cup of oil that is there on the side of the support arm. Whereas the support post/bushing is splash oiled by...... seepage/wicking? That's why Porsche switched from bare bore arms to bushings, I suppose.

cgarr has found that the support post tends to wear out, even with bushed arms. Which is weird because you would think the softer bushing material would wear before the harder steel post material. cgarr replaces the posts with hardened O1 steel shaft material I believe. I recently had him do my rockers and chain boxes. He's a great guy who turns things around quick.


Yep my 3.2SS is based on a '79 3.0L and i'm told it should have been using arms with bushings in them, although not the wide type (those were introduced ~1980). For some reason it had the very old style support arms with no bushings and narrower support/pivot area. Here's some pictures of the damage.



Rather hard to see but if you look closely at the bore you can see some bright areas. That's where the bore is severely galled. I had to pull that support arm off the post with a two jaw gear puller. It didn't take a lot of force to pull it off with the gear puller. But nonetheless I had to resort to the puller because I couldn't muscle it off by hand, nor pry it off with a tool w/out damaging something else.

In the next pics you can see some blueing of the sprocket teeth if you look closely. It's not the flash. The teeth are definitely blued due to excessive heat. Had to get pretty hot, considering the chains and sprockets are fairly well oiled down near the bottom of the engine?

[chris_seven]

The wear regime between the pin and bush is interesting and will depend on the relative hardness of the two parts.

I don't know what the bush in the tensioner arm is made from but if it is a DU type then I agree that pin wear is surprising.

If it were a stronger material such as Silicon Bronze then the hardness can approach that of basic steels.

It is worth hardness testing the pin to see what is going on.

If the pin is a soft steel and the bush is a hard bronze then the problems could have an explanation.

The damaged surface you are looking at isn't nice.

I would describe galling as being caused to too little assembly clearance and this would cause adhesive wear to take place between the two surfaces and it would be difficult to remove the pin even if the assembly hadn't run.

If it was caused by cyclic movement 'in-service' then fretting would be a better description.

The surface damage may appear similar but fretting is more likely to cause long term problems and much more wear leading to an eventual failure.

Both processes will create micro welds that make disassembly a pain.

The blue colour would indicate a temperature of between 550 and 600 degF.

Is this due to chain alignment issues?

[jomalkay]

I think the key question now in my mind is what are the actual spec's / tolerance and are these ideal taking into account higher rpm range is a "bit" more clearance preferred like with crank main / rod bearings.
Do think I'll give Craig a call and get his input.

[KTL]

Thanks for the correction on the terminology. I agree fretting is a better term.

The orginal bushings are what I was referring to with respect to wear on the posts. The factory bronze material must be somewhat hard to impart some significant wear on the post.

My failure was a result of the fretting itself that froze the chain idler arm in an extended position, which kept a substantial amount of tensile load on the chain. I suspect that if the idler arm was in an offset position (keep in mind that this idler arm "floats" back and forth a bit on the post- it is not fixed in place) then I think that would also contribute to the extreme side wear/load seen on that intermediate shaft sprocket.

[chris_seven]

Wear is such a difficult regime to deal with and the 'jargon' that tribologists use is very picky - as I have found out to my cost in the past.

There must be a way to try to control some of the float and it may be worth looking at a DU style bush to replace the bronze.

Working out contact loads from the forces needed to compress the tensioner should be manageable.

Worth a few minutes idle thought