1969 CL350 Head Ache!

[K2-K6]

I don't feel its a disaster. From a simple build the engine from parts point of view, then it's logical to go with the opinion of needing another part to replace it. I don't disagree with that.

From a straight engineering and materials point of view its not really a big task and if it were, or is, a rare part then you'd go with a repair.

Toward that view,  I'd put valves and plugs back in it to facilitate a localised blasting of that combustion chamber (maybe you could interest Ash into help with that part) this would need to be done to remove all traces of carbon and contaminants from the damage site. Ideally, you'd heat the head prior to Tig welding for building up the damaged area prior to letting it cool slowly which should avoid distortion. You need to blast it clean to avoid any inclusions in the weld material, and then the inert gas should prevent problems.

Afterwards,  skim it lightly to bring the surface back and finally dress out the combustion chamber shape with abrasive on dremmel to match the squish shape to the good one.

[AshimotoK0]

I am wondering what the actual difference between the 250 & 350 heads are because the valves and guides are certainly the same p/nos. Can you measure the width of the head dome .. I do have a 350 head from my K1 I could measure  but it's buried at the mo'.  'Florence' on here may have 350 head spare .... faint possibly ... worth an ask.

[Tomb]

I would be tempted to sort the valve seats and dress the damage carefully with a dremel. I don't think it will make a jot of difference, you wont be racing it no doubt.  Others may think different but it is your money. 

+1

I've cleaned up and re-used worse heads

[matthewmosse]

On my first big bike, a cb550k3 I found the head to be in similar shape when deciding to sort out a really bad oil leak, on mine the top piston ring was AWOL, on every piston, having exited taking a fair bit of the top of the piston with it. I was due to go on a holiday to Scotland with a mate in a days time so bunged on a decent used set of piston and barrels I had and bolted it all back together after a fairly cursory job of de blurring the chewed up head and grinding the valve seats with 2 grades of valve paste. The bike was up and running with 9 hours to spare before the holiday and ran brilliantly for the week and quite some time after. Eventually a nasty cheap O ring between the head and barrels either melted or swelled in the oil, either way it blocked the oilway and that killed it - never actually broke down, but the rattle was alarming so it got retired, but before that the bike ran fine despite having a head in similar wrecked shape. At least with an imperfect repair you can ride the bike gently, knowing you need to look out for a spare engine or head. On my bike that worked out ok, I found a engine for not very much money that had been by the look of it stored in a canal, bottom end very dead from corrosion, top end very well preserved in oily sludge, beast journal bearings I've seen out of about 15 cb550 top ends.

[florence]

I do have a spare but they are so hard to come by I would have to be very convinced monentarily.

The difference between the 250 and 350 head?  The 350 head has a slightly different combustion chamber shape which corresponds with the different bore size.  I think it would be possible to have someone grind the material away from a 250 head to make it the same. 

How about this, good price I would say:

http://www.ebay.co.uk/itm/Honda-CB350-CB-350-1971-1972-1973-1974-1975-Engine-Cylinder-Head-/311822638588?hash=item489a13ddfc:g:BZsAAOSwol5YxsHb

[florence]

In the eighties I rode one of these things every day, even touring abroad (What was I thinking?).

I once had the choke mechanism go into the bore on my 350.  The metal over time can become brittle so worth a careful inspection each time you have the carbs off.  I have seen some chokes with small amounts missing which probably went in without anyone noticing.  In this case the whole half of the choke came off including the sprung steel bit which lets air in when choke fully closed.  I have also seen carbs with one of the throttle screws missing.  Judging by the damage on yours, I think that may have happened, I can make out a thread mark in on of the collision sites.  Again worth checking when you have them apart.  The vibration from these machines makes everything fall off eventually.  They are great little bikes but need a bit of care.  I even had a silencer fall off.  I was very lucky, it hit the rear tyre and flew over a hedge.

[AshimotoK0]

I probably do have a spare 250 one (or two) and to hang onto it would just be hoarding. Could be tempted to part with it for 50% of what DK are asking but would need to check it out first. Otherwise I would rather hang on to it as Florence rightly says.

Seems from other knowledgeable guys  on here though that fettling of yours is not  out of the question. I can bead blast it for free for you but that would mean post both ways for you.

[florence]

when I had the choke go into the bore I managed to clean up the combustion chamber with hand tools.  It wasn't as bad as yours but the valve surface looks intact which is encouraging.  My one which looked pretty nasty even after tidying up ran absolutely fine and I soon forgot all about it.

[K2-K6]

I know what you experienced Florence, a friend had two of those sluice type choke flaps go through the motor on a 750 K6. One went unnoticed but the other flattened the spark plug gap and the cylinder stopped firing. When the head came off there were barely noticeable marks on piston and head, I was really surprised that there was not more significant visible damage.

The reason I think it's detonation ( obviously I'm looking at the photos) is that the aluminium craters have soft melted edges which just doesn't happen with physical assault but only with heat conditions in the combustion chamber. It appears there are no marks up in the top of the cylinder head either, the damage is confined to the squish area of the head material and this is likely to run hotter than the crown.

It's an interesting case as it appears to have been stopped from running part way through a failure process. The next thing that would have happened is the piston ring lands would start to dissappear, only then are the rings usually released and impact both the piston crown plus head. When that eventuality is taken apart then the rings are often pointed to as the culprit of failure. It's an important point that the chronology is the other way round in that the detonation came first as the route cause, which then consequently trashed the piston rings once they could no longer be held in place.

It's not often that four stroke piston rings fail on their own with all other things being intact first. It's obviously important to ascertain the true cause else you run the risk of a repeat once rebuilt.

[neildavies67]

Wow, lots of advice for me to mull over! Thanks everyone, feeling much more positive this morning! My current plan is as follows: purchase cheap compressor (hopefully second hand to save cash) and soda/walnut shell blast the crap out of the head to remove carbon. I need to clean up all the other engine cases and various other bits and pieces so I can (kind of) offset the cost against time I would otherwise have spent doing it all by hand. Hopefully get a response from one of my enquiries re: combustion chamber repair, that doesn't cost mega bucks! Otherwise it's Dremel time as suggested!  While I'm spending the kids inheritance, I intend to get the valve seats done, rebore, pistons & rings, cam chain and tensioner. By the time I've got the camshaft and followers done with Boyer Bransden ignition, apart from having spent stupid money which I'll never get back, hopefully whatever caused the problem in the first place wont raise its head again (but my wife may have left me!). Out of interest, the depth of the chamber is 20.3 mm, 64 mm diam with a 30 degree chamfer 4.5 mm wide. All measurements very approximate! How does that compare with the 250 head AshimotoK0? If they could be machined to match the 350 chamber dimensions, I would certainly be interested in taking one off your hands if you're happy to part with one. Thanks for the link Florence, that certainly looks like a good head relative to mine! Does anyone know if the heads are exchangeable across all the models (except the camshaft housing)? It's wierd that the add seems to indicate '71 onwards. I notice the head is stamped M 10, mine is stamped E 2. Is that significant? Once again, many thanks for all the advice and optimism. The body and mind are willing but the wallet is getting rapidly empty!

[AshimotoK0]

Well it seems they are prone to trauma. I dug this 250 spare one out (not one I had reserved for my bikes I am restoring) and some strange marks particularly the almost perfectly semi-spherical indent. I think it came from an engine in bits I bought that had some NOS parts from DS (tensioners etc)  that came with it (K4 engine) that I wanted badly. PS DO NOT buy any cruzinimage tensioners for this engine ...you have been warned !! The genuine main tensioner is mega expensive ..even though the roller in it is exactly the same as the 750 one which is much cheaper. So the temptation is natural to use pattern but DONT . Here it is (t used to be £63plus VAT but actually come down a bit recently) http://www.davidsilverspares.co.uk/CB350K0-SUPER-SPORT-1968---1969/part_2990/

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[K2-K6]

That one above looks definitely as if the cylinder has ingested something,  there's a huge difference between that one and the original subject. Also more pronounced damage up in the centre where something has rattled around.

They are quite a radical spec when you look at the numbers they run. About 110bhp per liter capacity,  that's a lot for a two valve engine even today. That 285 cam would also be quite hefty in a car engine with just marginal tickover stability, reasonable torque spread plus higher rpm flow. Usually known as a rally / mild race grind.

I'd guess from the rpm range,  compression ratio, cam timing etc you've got to be fairly diligent in setting up these twins to really get them running spot on.

My perception in reading other info about them is that cam problems seem to crop up a fair amount,  what would others see?

[AshimotoK0]

That one above looks definitely as if the cylinder has ingested something,  there's a huge difference between that one and the original subject. Also more pronounced damage up in the centre where something has rattled around.

They are quite a radical spec when you look at the numbers they run. About 110bhp per liter capacity,  that's a lot for a two valve engine even today. That 285 cam would also be quite hefty in a car engine with just marginal tickover stability, reasonable torque spread plus higher rpm flow. Usually known as a rally / mild race grind.

I'd guess from the rpm range,  compression ratio, cam timing etc you've got to be fairly diligent in setting up these twins to really get them running spot on.

My perception in reading other info about them is that cam problems seem to crop up a fair amount,  what would others see?

Agree .. I have seen engines particularly from the States that have done miniscule mileages (typically 5k) but the fat cam (286) has pitted lobes. I am sick of people , particularly on the likes of FB,  saying that the '68 250SS (K0) was merely a "learner / ride-to work hack'. Hardy .. for a 4-stroke 250cc to develop a quoted 30 BHP in 1968 was going some in my book. Even comparing 2-strokes The Suzuki Super 6 250 was 29 BHP , later Hustler 33 BHP  and Kawasaki Samurai 250 was 31. BTW ..one for Trigg ... the Samurai had Cdi ignition in 1969 !

I remember my mate at school flogged me his '68 CD175A 'sloper' as he just bought a TR25W (Triumph Trophy 250cc =22 BHP) .. a choice  he kinda regretted forever after.

[K2-K6]

It's hard to find even modern cars that are considered quote radical that don't reach the specific output of these engines,  and they use 4 valve heads along with fully mapped fuel injection and ignition. These old bikes are really quite remarkable in engineering terms.

The camshaft wear problems I feel are mostly related to setting the clearance really accurately, and maintaining it. If you loose the clearance of the rocker to cam base circle then you effectively deprive the whole cam lobe of any sensible lubrication. They are a fairly simple system that dips the cam into oil generally of engines that have the rocker above the cam. The clearance allows the oil to pass as a film onto the rocker pad. If you remove the clearance,  then you effectively squeegee the oil off the cam with the rocker around its entire circumference. What would normally be a resupply before each time the cam peak reaches maximum pressure and loading fails to happen. The cam and the rocker then effectively cold weld parts of each face together which results in pulling both of them apart in a fairly short time frame.

Notice how on that Dutch 450 of Jensen's he is hyper diligent with tappet clearance,  he even, I think I recall, runs them slightly wider during early stages. That's a very important part of his success I believe.

[AshimotoK0]

The head I have that is damaged I would possibly clean up and use if I didn't have others that are much better... centre damage isn't that bad really when you see it in the flesh.

See this old 750 SB ... seems it wasn't just bits in the carbs that found themselves in the engines 

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