Topic: CB350F: two failed top end rebuilds, stock bore and 393cc

[markreimer]

Why bother to clean the inside of the header? Some will burn off and the rest is rust proofing.

Well mainly because I want to know if “top end 4.0” is smoking off the bat. There  is so much oil in it that its dripping out the mid-connection. I know I could burn it out, but at this point I don’t have the patience to spend a few hours burning it out. I want to know if it’s good or bad asap. In a month it’ll be snowing here, so I am on limited time!


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[MRieck]

Thanks MRieck. I'll place an order for Yamiya's guides, which have seals all 'round.

Based on all the heads you've worked on, do you think it's possible that the guides are responsible for the oil pooled on my pistons? Intake ports looked clean. I'm just trying to avoid assuming it's the guides, replacing them, and having the whole thing happen a third time. As much fun as working on bikes is, it's kinda getting old doing the same thing over and over haha! 

That's a good start. Those exhaust guides are throwing a good amount of oil so it can certainly be the main problem. I do not like the 1 piece oil rings that came with your pistons. I do not know how accurate the bore is or the finish but to have that much oil on top of the pistons generally indicates it is coming from above the pistons. Oil would have to get past your oil control ring and the 2 other rings to get to the top of the piston.
 As an aside...I run cast pistons at .001 to .0012.

[Dunk]

Any oil in the intake ports or on the valves? I'm not familiar with the 350f, but is it possible there is a restriction in the oil drainback path, letting oil pool in the head? Even with good guides and seals, you'll get a ton of oil past them if oil is pooling.

[markreimer]

Hmmm, I don’t think so, it’s a pretty clear shot from the head to the pan, but I’ll double check. When I took the valve cover off I didn’t see excessive oil pooled.


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[MRieck]

 I would not worry to much about oil pooling as it has a central cam chain. ZX-11's had crappy oil return as did other engines when right side cam chain drive became popular. It is certainly true that having a big puddle of oil up there can push oil past guides and seals. Not so bad with shim bucket arrangement as the bucket shields the guide.

[RAFster122s]

Could the metallurgy on the rings be less than ideal causing them to not break in well, or cause cylinder wear? Clearly you have a head issue with your guides and possibly the valves. 
Since several have seen problems with smoking after the 393 rebuild/install I wonder if some of the issue is in the kit.  Could the compression be in the 140s to 150 and still have break in issues?
Just wondering if the issues could be in something like this.

Mark, sorry to hear of the issues.  I am sure you can get to the bottom of it. BTW the Cometic head gasket if made from copper is going to leak oil outside the gasket, they are typically used by racers and the leaking is an acceptable thing apparently, it does not affect the compression or head gasket interface to the bore from what I have read.  But, like other things on the internet...with a grain of salt such things should be taken.

David

[calj737]

I think Mike R gave you the root cause 

[disco]

Hi Mark, Very interested to see how this pans out.

I'm just about to install a CI 0.5mm oversize piston kit into my CB750 K1/K2. The cylinder head I'm using is a K0/1 with no valve seals on exhaust (?) side. Mike Rieck's comments have me very concerned. Might have to install new guides with seals.

[markreimer]

RAF122S - I've wondered the same thing as well, but I don't know how to determine that. CI has many many positive reviews for their 836 and 466 kits, though I think they all have 3-piece oil rings. I don't know if these rings were made by the same supplier or not.

I'm heading to the shop to do some measuring and inspection of the valve train and double check the piston-to-bore clearance. By this evening I'll know what my next steps are and what parts to order.

[markreimer]

Measured the guides...drum roll.....

My manual specs them at .0004" of clearance. I measured all 8, and they ranged from .004 to .008. Way more than I should have!!

I measured the piston to bore clearance to verify everything was done properly by the machinist. It was .001, and I had asked for .0008. I've got a couple hundred KM on it since then, so that may account for the discrepancy, or just the accuracy of our tools. Anyway, I think it's clear where the oil is coming from!

Valve stems all measured in-spec. Just the guides are really knackered. I compared it to a 400F 'beater' head I had kicking around, and the max we found on it was .001, so mine are clearly very worn.

Progress!

[markreimer]

Some photos, including Ricks AMAZING CB400F fuel injected, turbo charged project!!







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[jneuf]

Awesome - so glad you were able to find a potential culprit!!

[754]

 OT , but JN , check your PM.

[Dunk]

Worn out guides makes sense. Even if new seals are installed they won't last long if the valve is wobbling all over.

[markreimer]

Totally. It's so wild how tight the tolerances are on these bikes. By hand I figured there was no way these guides were anywhere close to done. But to find out there is basically ten times more play than the minimum spec is crazy! .0004 vs .004.

New guides are on the way. I ordered 8 guides from Yamiya, with 8 guide seals. Gaskets are on the way as well, and a pair of OEM supply and return oil seals arrived yesterday for sealing between the head and cylinder. Now I'll sit and wait about a week or so for them to arrive. If I'm really lucky, I might even get a ride in on the bike before Nov 1st, which is basically below freezing and snowy by then.

[disco]

Hi Mark,

Don't want to be a party pooper......

4/10,000 of an inch is the absolute lower limit for the intake valve only. The service limit for both valves is 118/10,000 of inch. Which is nearly 12 thou. So your measurement of 8 thou is still within the service limit if these manuals are correct. I've attached the service data which is on this sohc website under manuals etc.

I'm thinking there's a typo in there......the service limit is at least 6 or 10 times the assembly limit, depending on which valve you consider?

However, I think you're on the right track with the likely root cause being the valve guide clearance.

[markreimer]

Hmm I see what you mean. That does seem like an odd service limit...


One other thing I noticed this morning, have a look at the staggered wear marks on my #2 piston second ring. Doesn’t seem to be a pattern to the wear or line up with the gaps in the oil ring. Any idea if this is normal? Keeping in mind there is only 80 miles on it? I’m asking because someone questioned the quality of this set of rings and I don’t know if that black coating is expected to stay put or not...




Here’s how the others are wearing in





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[markreimer]

Not much new to report here other than my last remaining parts have arrived. OEM base gasket, valve cover seal, circlips, CI head gasket, Yamiya valve guides and seals. Heading to the machine shop for the guide install.




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[ekpent]

 Wish someone knowledgeable would have commented on those nice ring wear pictures you took.

[markreimer]

Me too. Pretty weird. I showed it around to some friends of mine who've build a number of motors. The best guess was from dirt/debris that wasn't cleaned up well enough, due to the fact that the piston skirts have matching scratches in most places. It's possible, I had the block and pistons off three times before I got it all settled in, which would have created lots of opportunity for stuff to get stuck to the pistons and me not notice it.

[markreimer]

Oh, I just realized you were referring to the piston ring wear marks. I was looking at photos of my 750 rings before I installed them, and two years later when I had to pull the top end off for unrelated reasons. They were black when new, and polished clean the next time I saw them. I'm pretty sure these rings look this way because they simply haven't been run long enough to fully wear away that coating. At least that's my suspicion.

Kinda re-enforced by this article here, which says the coating is for scuff protection during the early stages of an engine break-in before full oiling can be provided to the rings

https://www.hastingspistonrings.com/tech-tips-faqs/maganese-phosphate-coating-on-compression-rings

[markreimer]

I dropped the head off at the machine shop this morning to have the Yamiya guides pressed in and seats cut. For a moment I thought I'd miss my chance to get this built before winter when I could only find 6 of the 8 little o-rings that go between the guide/head, but found 'em last minute.

I brought the block along too to get his opinion on the vertical scratches. He didn't think there was any reason to hone them out as they were so subtle, but offered to throw in a light pass anyway since I had brought it in. As for the cause, after inspecting the pistons and bore he believes they were the result of small bits of particulate I didn't clean, and possibly from the edge/end of the ring scratching as it broke in. He pointed out one ring in particular, which felt pretty sharp at its end, and suggested I take a fine file or emory and lightly take the edge off.

All my other gaskets have now arrived - using OEM for all the gaskets and seals again, except the 393 head basket of course. The shop is really full at the moment, but I'm hoping to have it by the weekend. If I do, I'll have it put together and fired up before Monday.

[markreimer]

Also check out this HUGE valve they had in the shop! It’s bigger than my whole engine haha. I had to ask if it was actually real, apparently it’s from some big diesel




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[markreimer]

Got my head back with the new yamiya guides and seats cut. Can’t say I’m happy with the machine shop’s job at first glance.

This was a new machinist recommended to me by a reputable builder for motorcycle work.

The head was returned with a bunch of metal shavings in it, the guides are installed but the ports weren’t cleaned before the old ones were pressed out. I haven’t had a chance to measure with a dial indicator yet but the valves don’t feel like they are tight in the guides as per my .0004” clearance instructions. That’s just based on how it feels by hand. And it seems like the seats and faces of some of the valves had a lot of material taken out of them. I didn’t think the faces needed to be touched, just seats cut.
Looking at these photos what do you think?

I’m going to clean the head thoroughly first. The exhaust valves feel good in the guides, but one or two intakes have enough play that I can feel it by hand, which measured as a lot with the old guides. Hopefully it just needs cleaning. Should have stuck with my old machinist. My 750 head came back looking brand new.










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[calj737]

Seats have to be re-cut with new guides. Gotta be centered and perpendicular to the guide