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

[markreimer]

Cool, thanks!

Tonight the engine comes apart and we'll see if something obvious pops up.

Question about measuring valve guide clearance.

In the past I've used a dial indicator, lifted the valve about 1/4" or a bit more from the seat, measured the total "wobble" back and forth, then divided that number by two, and compare that figure to what is listed in the manual. For example, a few years back my cb750 had 0.005" measurable wobble, so when divided by two, it was 0.0025". The max acceptable clearance was listed as 0.003". I called it good and have been riding happy since.

My understanding is dividing it by two determines how much clearance is given on both sides of the valve stem. I based that on some reading here and some engine building tutorial stuff on youtube. Is that correct?

Here's the video I'm referencing - first the clearance is measured with a micrometer of some kind, and then using the wobble method after:

But then I was thinking, the way the manual lists piston-to-bore clearance isn't like that. It just gives a number, like 0.0004-0.0008 for the 350F. That is the difference between the width of the piston and the width of the bore, so the actual clearance around the entire piston would be half that number. It's not like a machinist is going to take that number and double it so I have 0.0004" clearance all the way around the piston, right?

If that's the case, should the number I'm looking for with the valve guide clearance be half what is listed in the manual?

My manual says Valve-to-valve guide clearance:

Intake: 0.0004-0.0012", service limit 0.0118"
Exhaust: 0.0012-0.0020, service limit 0.0118"

[innovativems]

I really have a hard time thinking it's the big bore if it smokes each time the top end was apart. 


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[markreimer]

I hope you are right. But yet it smokes, and smokes for a few others who've also replaced valves and valve guides while using this kit 

I'm not sure it makes sense to say that because it smoked after each time the engine was opened, it must be the head. Reason I say that is each time it was rebuilt, it had a different piston/ring configuration. There is a very real chance it was me who screwed it up each time, or perhaps I screwed it up the first time, and the second time was a bad ring kit.

Just as a reminder, when I bought the bike it didn't smoke horribly, but was enough to warrant a rebuild. Nobody wanted to ride behind me, put it that way. I found #1 and #2 pistons badly scarred and their cylinder walls scratched up. The bike had been crashed and dumped on the left side by the PO 40 years back, then sat. I'm wondering if it was oil starved for a few moments as a result. That or maybe I caused it when I started it up after its long hibernation. I'm just guessing here. I figured I was getting blow-by as a result, so I re-ringed with OEM rings, honed, and piston-to-bore was measured in spec by my machine shop after the hone.

Smoking started about halfway through my first break-in ride. But it was definitely down on power right from the get-go. I couldn't really even maintain 70mph. It started smoking badly about 30 miles in. Burned around 1L of oil in the 150 mile ride. Took it apart and found the ring gaps were massive, 2-3mm on some rings, and figured i was loosing compression and getting crazy blow-by as a result of the gaps. I have no clue how that gap happened, I'm assuming user error, though I have no memory of doing something that stupid. I remember measuring the gaps, and yet there they were...

Rebuilt with 393 - this time ring gaps are absolutely within spec. You know the rest - smoked instantly from start up. Way up on power, but worse for oil consumption.



My plan tonight is to ride it till it's up to temp, then do a compression test for reference. After I'll pull the exhaust and carbs and look for signs of oil making it past the guides.And after that, I'll pull the head and measure the clearance with a dial indicator, then pull the jugs and triple check I put all the rings in right-side-up, and in the right order.


More to come...

[markreimer]

For reference, here is a video of how it was smoking before I left on the break-in ride..

[markreimer]

Regarding the wobble test:

When researching this before doing my guides on my 750 I always found the "divide by 2" procedure to not make any sense, but it sure sounds like a lot of people do it that way. I think the thought behind it is that when you measure valve wobble, the valve head will move back and forth more than if the valve was moved side to side flush with the guide, as per the illustration I've attached. Best way to measure I guess would be with a split-ball inside the guide, but I don't have one of those.

In the end, on my 750 I had new APE guides installed by my machine shop, honed to the clearance spec in Hondaman's book. I can't remember if my 0.005 measurement was before or after that happened though. I would assume before, or else my machinist screwed up!

EDIT:

And here is a guy who's done both tests - first the wobble test, with .003" on his dial indicator, and then comparing it against measuring the stem diameter and the guide bore, which showed a clearance of almost exactly half what the wiggle test showed: 0.0014"....thereby supporting the "divide by 2" method...

https://www.corvetteforum.com/forums/c6-corvette-zr1-and-z06/3168420-new-ls7-valve-guides-measured-results-inside-2.html

[markreimer]

Initial findings:

Compression test shows:

1: 144
2: 140
3: 135
4: 135

Plugs look as follows after riding around town a bit prior to the compression test:



#1 looks a bit wet and also has the highest compression. Taking the headers and carb off now to look in the ports


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[markreimer]

And my beautiful mega soiled exhaust ports 1-4. These were clean when I installed the head!







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[markreimer]

And more photos. Oil on top of all four pistons. I have some scoring on the cylinder walls, though I cannot feel them at all with my finger nail. It’s also on the piston. Perhaps I wasn’t clean enough during instal...?








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[markreimer]

Had a chance to sleep on this after racking my brain last night, and made a plan.

First is the cylinder walls. They aren't damaged, look way worse in the photos than reality. I'm going to bring it back to the shop and get it lightly honed, then clean the snot out of it, as well as the pistons and ring grooves. I really don't think those vertical lines came from the ends of the rings, but rather some grit that wasn't cleaned out. Or perhaps even a casting flash from the piston, not sure if soft aluminum could cause this? What do you think? Anyway, I'll get the jugs and pistons cleaned up. That's easy enough.

As for the head, I didn't measure the valve clearances because it was just so filthy from oil. I ended up spending the evening just cleaning it. I could feel the faintest play in the guide. I'd be pretty amazed if it was past spec. But it needs to be measured, so I'll get the shop to do that for me as well. My dial gauge mount isn't stable enough to give an accurate measurement down to 0.0004".

Speaking in general, based on how much oil was pooled on top of each piston, I'm starting to agree with Innovativems' diagnosis of a head problem. I don't see how that much oil could come past the rings. I had an engine builder come by last night to take a look and he agreed, pointing out that there is loads of oil being pumped up above the pistons in SOHC4 engines, but relatively little being flung up to the underside of the pistons. It was his opinion that it would be very hard to believe how that much oil could make it's way to the underside of the piston, and past seemingly good rings, in that amount of time without something being obviously wrong.

So up next is valve guide measurements, loooots of cleaning (gotta clean the inside of the headers somehow too before the next start-up attempt), and likely ordering new guides and 8 seals from Yamiha.

THE BABY FOUR WILL RIDE AGAIN!

[MRieck]

I wouldn't bother measuring the guides......I'd just replace them. Exhaust guides with no seals are a bad idea......a really bad idea. I have personally redone rebuilt heads that the owners had insisted on non seal valve guides. They all had the exhaust ports covered in thick, baked on oil. One head only had 500 miles on it. It took forever to bead blast the ports.

[markreimer]

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! 

[Jerry Rxman Griffin aka MuthaF'er]

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

[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

[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.