1969 CL350 Head Ache!

[AshimotoK0]

Well I rest my case your honour.

I posted a question on HondaTwins for you ..Loads of views but no comments...zilch ...then it usually disappears off the radar. Compare the response on here and you can see why this site is so much better than the others. I  have 'bumped' it so that  may jar/shame  someone into action ..... but it's a fairly basic question ..... there must be someone  on there with an opinion/advice. I know the CB250K was not sold in the States but there are lot of members on there from Europe, NZ and Australia, where they did sell the 250 version in large numbers. This isn't a one off either  ....  I have had the same with requests for advice  on my CD125, CB72 & 450K0 bomber twins ..very little or no response. Have to say you do get a fantastic response on the dedicated CB72/77 Facebook group though, as much as I hate Social Media sites.

http://www.hondatwins.net/forums/48-engine-discussion/94745-differences-cb350k-vs-cb250-cylinder-heads.html

[MrDavo]

You may not get an answer on Hondatwins but you certainly get a lot of retail opportunities. Stephen from Glasgow doesn't have to hide his feet anymore thanks to fungal nail treatment, and 1000's of people from my town are taking advantage of a new funeral policy, who knew?

Another reason to donate here, so we don't have to wade through mountains of such crap.

That said, I know nowt about Honda twins, other than the fact that in a straight drag race on a deserted industrial estate, my mate's Candy Gold CB250 easily beat my BSA A7SS.

[AshimotoK0]

I am beginning to suspect that the head in my picture is in fact a 350K one not a 250k..... I am going to my 'stash' tonight to dig out what I know is deffo a CB250UK one from a 5k mile  UK bike.

[K2-K6]

It's conceivable that they use the same head for both,  what is the bore difference between the two?

If for example you put a 350 head onto a 250 size piston at the design stage you'd loose compression ratio as you have less swept volume going into same sized combustion chamber. It's fairly common to adjust compression ratio with a higher or lower piston crown shape, so there's the possibility that they did this. Worth comparing the two types of piston to see what they tell you as well.

Or they could have done it the opposite way with 250 capacity enlarged to 350 then used lower piston crown to get compression back to where they wanted. This would give more squish band to the 350 which wouldn't be a bad thing and may allow the two to run at the same timing setup.

[AshimotoK0]

Well I cocked that one up. My head picture I posted was from a Cb350K1 and I think I know what the cause of the trauma on it was. I got the bike from DK  with 10k miles but with weather seized engine. When I stripped it the engine was in really nice condition (inc cam and followers) but surprisingly had +0.25mm 1st overbore genuine Honda pistons. The engine was not in fact weather seized...I found a large mangled split pin jamming the primary drive gears.

This picture below is deffo a CB250K0 head that was taken from a 1969 Reg. left in a garden to rust with only just less than 6k miles on it that I 'found' end of last year. Note the lack of chamfer compared with the two CB350 pics. of heads. I reckon that the chamfer is the difference between 250 & 350K heads. I did have a K2 with Read Titan 325cc conversion in the 70's though which used the CB250K head and special Hepolite pistons & rings.

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[K2-K6]

They look like the same design from the photos, the 350 M4 just looks like a tidied up iteration of the casting mold.

It makes sense to chamfer the 350 squish area as it would keep the compression in the design range. I guess if they cast the 250 piston with a bit more meat on it and conversely the 350 version a bit less,  then they may even be able to use the same crank and balanced spec for both motors. I've not seen the rest but it just looks like the 350 is really a mildly stretched 250 rather than a motor in its own right.

[neildavies67]

Here's something interesting, an ebay listing for a new CB250 head K0-K4 but notice in the pics that the chamfer is present, unlike AshimotoK0's head - https://www.ebay.co.uk/p/Cylinder-Head-Honda-Cb250-K0-k4-B4-Cl250k-Part-NOS/605341165
I make the diameter on mine (excluding chamfer) about the same as AshimotoK0's 250 head. Have you measured the depth? Statfold engineering have said that they would have a go at making good my cylinder head but I might look out for a good condition 250 head first! It seems a lot easier to just machine a chamfer than make good on a chamber with a resemblance to the lunar surface!

[K2-K6]

Going on what we've looked at with Ash's examples,  that does look like a 350 in those photos.

It would seem difficult to change the volume of the main chamber as the valve seats and their geometry dominate the shape to the point that it would require more fundamental design change to casting to achieve this. They just look like they trimmed volume with that taper and nothing more.

The usual means of measurement is to use a piece of perspex,  grease the gasket surface to get a seal and place the perspex with a hole drilled through it over the combustion chamber. With valves and plugs installed you can run in something like diesel from a burrette to measure the real volume in cc's.

[AshimotoK0]

Here's something interesting, an ebay listing for a new CB250 head K0-K4 but notice in the pics that the chamfer is present, unlike AshimotoK0's head - https://www.ebay.co.uk/p/Cylinder-Head-Honda-Cb250-K0-k4-B4-Cl250k-Part-NOS/605341165
I make the diameter on mine (excluding chamfer) about the same as AshimotoK0's 250 head. Have you measured the depth? Statfold engineering have said that they would have a go at making good my cylinder head but I might look out for a good condition 250 head first! It seems a lot easier to just machine a chamfer than make good on a chamber with a resemblance to the lunar surface!

I do have a 250K4 head so I will check if that has a chamfer but it's at my stash. Having said that I just checked a later CB250G5  head I have here and that does not have a chamfer and measures 56mm too and that engine uses the same piston part no. as the earlier K models.

[K2-K6]

It looks to me that you'd need about 3.9cc larger volume in the 350 combustion chamber to maintain compression,  at a rough calculation. If you spread that between piston crown and chamfer volume then it's not that much material.

[florence]

Ash, your first head picture is 350 and second 250.  I think the engine was designed as a 325 and then sleeved down to a 249.  The heads are the same apart from the squish area.  One of my 250s had a crankcase which fits the 325 barrel, later ones have smaller holes in the crankcase.  Other than the bore and the squish area and conrods, the engines seem to be the same as one another.

This one, https://www.ebay.co.uk/p/Cylinder-Head-Honda-Cb250-K0-k4-B4-Cl250k-Part-NOS/605341165 is a 350 head despite what the advert says.

[AshimotoK0]

It looks to me that you'd need about 3.9cc larger volume in the 350 combustion chamber to maintain compression,  at a rough calculation. If you spread that between piston crown and chamfer volume then it's not that much material.

I will measure the head volumes on both heads and report back ! Although the stroke is the same for 250cc & 350 (325cc) the big ends have more rollers in the 350 model.

[K2-K6]

To extend what I've posted above, the total volume increase would be as stated ( loosely, 75cc extra capacity divided by two for cylinders swept volume,  then divided by the compression ratio of 9.5).

There is also extra volume that can't be measured in the head shape, that is when you increase the bore size then the area above the periphery of the piston and the head plane also increases, so you'd have to subtract that from the total to get the true increase in volume for just the combustion chamber.

.........which would be the difference in volume of the two different bore sized discs with a thickness of the clearance of piston to head plane dimension, if that makes sense. I've got it going round in my head but it seems harder to write than think about!

It's obviously the combination of these two volumes that make the real difference to the engine.

[AshimotoK0]

Worth an ask:-

https://www.gumtree.com/p/replacement-parts/honda-cb250-350-k4-engine-parts.-/1265551797

[AshimotoK0]

I checked another head today from a CB250K4 and it was the same as the other 250 earlier head I pictured....i.e. with no chamfer 56mm dia.