Odd rocker shaft wear

[Eurasiaoverland]

15B-T engine, low km (ex-military engine that appears to have sat unused for many years). I'm relatively new to OHV engines and this is the first rocker shaft I have dismantled.

I've been doing an engine 'refresh' which included changing the camshft bearings. I decided to take apart the rocker shaft, partly to clean it as it has been off the engine for a while, and partly to see how it is put together. I was not expecting to find any real wear. The hardened points of the rocker arms where they engage with the pushrods and the valve stem caps are only slightly 'polished', no wear patterns. The internal bores of the rocker arms have mild contact patches which to me look perfectly normal for a used part.

The rocker shaft itself however, has what I would regard as considerable wear where it sits in the cast iron support pieces which bolt to the head. And the internal bore of these support pieces has similar wear patterns. I'm no metallurgist, but to me the pitting would indicate corrosion, but there are only a few very small brown marks on the pitted areas. But could it also be from percussion? There is light surface rust on the rough cast surfaces of the rocker supports, but I had assumed this happened after the engine was parked up, and I don't believe it has run for more than a few minutes since then (I got the engine out of a half-cut which came directly from Japan).

The rocker shaft - this is where it goes into the first (of five) support pieces:

The first support piece (you can just see the pitting in the bore, and get an idea of the surface rust).

Any ideas? Is this normal for a rocker shaft? I'm now going to take apart the rocker shaft from my 3BII to compare. It uses the exact same part as the 15B-T but at 567,000 km has probably ten times the mileage of the 15B-T.

Thanks

EO

 

[Eurasiaoverland]

Here's the 3BII shaft (bottom) for comparison:

Judging by how severely my fingernail catches the edge of the worn area where the shaft passes through each of the supports, the 3BII shaft had more wear at the #1 and #5 supports than the 15B-T, but almost no wear at the #2, #3 and #4 supports. The 15B-T shaft has pretty even wear on the #1, #2, #3 and #4 supports, while the #5 support area has very little. I think this might just be down to manufacturing tolerances.

I was initially worried about oil pressure or oil starvation issues, but the rocker bores in the 3BII are more worn than the in the 15B-T rockers, so I don't think that is the case. The 3BII was well maintained and I think did a lot of highway driving. Maybe the 15B-T spent a long time idling or doing lots of short joureys.

So it looks like the wear is normal. To be clear, the shaft does not rotate in these supports, the rocker arms move on the shaft. So the only movement can be lateral (within the oil clearance in an up-and-down sense in reaction to the pressure acting against valve spring tension), or rotational within the locating-pin to rocker shaft hole tolerance.

Oil comes up from the block via the forward-most head bolt bore, is diverted into a small passage which goes through the front rocker support and into the hollow rocker shaft, where it feeds the eight rockers. The rocker supports are not actively oiled, but have a machined slot (see previous post) to catch a bit of the oil that is flying around.

Way forward: New rocker shaft and hopefully supports. Availability should not be an issue as the 14B is still in production.

 

[bj70bc]

i didnt notice that on my 3b when i took the rocker assembly apart
wonder if i missed it....

 

[Eurasiaoverland]

i didnt notice that on my 3b when i took the rocker assembly apart
wonder if i missed it....

Having quickly searched, the old 3B appears to have aluminium rocker shaft supports and cast iron rockers. The later 3B/14B/15B has it the other way round. So maybe this problem did not exist before the update.

 

[aztoyman]

Looks like fretting wear. Lots to read about if you want to look into it. I could be wrong though as it's been a long time since I wrenched for a living.

 

[Eurasiaoverland]

Looks like fretting wear. Lots to read about if you want to look into it. I could be wrong though as it's been a long time since I wrenched for a living.

Thanks! I've had a bit of a read and think you got it spot on - the rocker shaft would be a prime candidate for what they describe as the causes of fretting. I think it's down to the fact it's not a particularly tight fit between the shaft and the supports, and the fact they are not really oiled. Seems like a poor design, but maybe having a tightly clamped shaft causes some other problems. And probably don't want to divert any oil pressure up at the top of the engine.

Seems like drilling a hole through the top of the support to expose a part of the shaft to catch more oil might help, but I think I'll just run with stock pieces - if I can get new supports.

 

[FJBen]

I reused mine, engine had done 485,000kms.
Best picture I have of wear but I don’t remember how deep the bracket wear was.

Machine shop cleaned it up.

 

[Eurasiaoverland]

I reused mine, engine had done 485,000kms.
Best picture I have of wear but I don’t remember how deep the bracket wear was.


View attachment 3899430


Machine shop cleaned it up.

View attachment 3899432


View attachment 3899431

Thanks for sharing that, interesting to see.

Again your 'blue block' 13B-T seems to be special with its own set of rocker shaft supports. Looks like it has cast iron rocker arms like the old 3B, but uses the later rocker shaft 13901-56030 from the 3BII/14B which can be had for about USD 30 from Japan or a bit more from Emirates.

 

[FJBen]

Thanks for sharing that, interesting to see.

Again your 'blue block' 13B-T seems to be special with its own set of rocker shaft supports. Looks like it has cast iron rocker arms like the old 3B, but uses the later rocker shaft 13901-56030 from the 3BII/14B which can be had for about USD 30 from Japan or a bit more from Emirates.

Yeah the later 13bt are interesting mixes for sure. I’m not sure why they updated them for just a short time.

 

[Eurasiaoverland]

Yeah the later 13bt are interesting mixes for sure. I’m not sure why they updated them for just a short time.

My guess would be that with the 08/1988 launch of the 14B, that made the 13B platform effectively obsolete so, rather than fully update the 13B, they kept casting the 13B blocks but used updated tooling such as the larger cam bore and the updated crank bearings which you have on your 13B, and then kept making those engines for the last 17 months of the 1980s before ceasing production of all 13B models in 12/1989.

 

[FJBen]

My guess would be that with the 08/1988 launch of the 14B, that made the 13B platform effectively obsolete so, rather than fully update the 13B, they kept casting the 13B blocks but used updated tooling such as the larger cam bore and the updated crank bearings which you have on your 13B, and then kept making those engines for the last 17 months of the 1980s before ceasing production of all 13B models in 12/1989.

That makes the most sense by far. Interesting for sure. I wish we had some real info on 13bt power numbers. It appears we have wiki which isnt entirely correct.


I’ll be interested to hear your differences between the 2 engines for sure.