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

[markreimer]

Called my machinist and asked if he can handle a block as small as the 350F. No problem to bore it to 50mm for the Cruizin Image 393cc kit. $190 Canadian for bore/hone. Plus 150 for the kid, and then some gaskets. I think that's what I'll do.

I'm still trying to figure out how that one ring had such a huge gap. Every time I've used Honda rings they haven't needed filing. It doesn't appear that the cylinder was worn at all (it shouldn't have been..) and the ring looks normal. What do you guys think - bad ring that I forgot to measure? Or is there some way a ring could actually wear that much, that quickly?

[innovativems]

With doing the big bore I would add a ttr400 oil cooler setup as it makes the cylinder sleeves pretty thin. 

I would guess the ring was off when out in


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[markreimer]

Cool, I'll look into it.

Ordered the 393 kit, should be here in 1-2 weeks. Then my machinist needs a week to do the overbore, so I'd guess end of the month it should be back together. This is my wife's bike mostly, so she just puts around the city. I'm not too worried about the heat there. But when I ride it that's a different story 

[calj737]

The faster you ride it, the less you need an oiler cooler. And being in Can-A-dah, probably even less so. 

[markreimer]

Hah fair point. It's usually under 77F while riding here, other than a few weekends of the year. Spends more time riding in mid 60's honestly.

So the big bore kit will solve a few of my issues here - the ring gap, piston to bore clearance, etc.

But I still have other issues. Namely, why did those critical oil orifice seals deteriorate so badly. They were OEM Honda. Anyone have an idea? All the other seals from that kit were in new condition still. I coated them in Hondabond, but that shouldn't have hurt it. Copper Coat shouldn't have either.

I wondered if maybe they were crushed and ripped during installation. The head was decked slightly, the block was not. Perhaps it was over compressed as a result? Hmm...

The Cruizin Image kit comes with a head gasket for the 50mm pistons, but I'll still need a base gasket, valve cover o-ring gasket, exhaust gaskets and the oil line seals... I'm thinking I'll order new OEM oil orifice seals if I can find them, but I'd rather not buy them as part of some 40 year old kit again.

The base gasket and valve cover can be whatever brand, NE or something, those are usually pretty easy to keep leak-free.

[calj737]

If the machinist is boring your jugs, have him deck the block too for a frilly flat surface for mating the head gasket. Pay attention to how much is removed from the head and jugs and trim any locating dowels by the same amount. A slight, slight increase in compression should follow too.

[markreimer]

Will do. Last time the head was skimmed by 5 thousandths I think, based on memory. I didn't adjust the dowels at all, they don't appear to have been squished though. I'll dry fit it together and check the clearance first too. Thanks!

[seanbarney41]

Those o-rings probably stuck from heat and sticky gasket stuff, which is exactly what they are supposed to do and got ripped when you pulled the engine apart...stuff like this can happen

[RAFster122s]

Could have been old rubber that fell apart under use.  Use fresh stuff and do not fret over it.
Since you bore it, could the ring have been std size and pistons oversized yielding your issue? Or was it just a hone job the first time.
I hope you machinist is boring with motorcycle tolerances, not auto.  Many a MC block has been destroyed or rendered with short life because the machinist was too loose with the tolerance on his cuts. Ask Mark Paris, HondaMan, he has dealt with this a few times...

Just some out there thinking about it...

David

[markreimer]

Hi David,

Thanks for the input on the seals.

Regarding the cylinders - I didn't bore them actually. I took the cylinders and the pistons to my machinist, who is the local motorcycle machine shop. I've taken a number of SOHC4 cylinders and heads for work there, always had great work done, so i'm not worried about that. I actually brought HondaMan's book with me the first time I went there to have my cb750 bored out, for referencing the tolerances he talks about 

He measured the pistons and the cylinder and indicated there was enough 'meat' left to hone and stay within Honda's spec, though it was getting close to the max. So to recap - I have stock pistons in stock bore, and had it honed and installed new Honda stock-bore rings.

Only thing I can guess is that maybe i measured 2-3 of the rings and they were all identical, so I didn't bother with the last one. Doesn't seem like something I'd do, but hey... I've got a ring in there with massive clearance, and it's not like it wore to that point in 100 miles.

393 kit is in the mail. I'm sourcing a gasket kit and should have the whole thing in my machinists hands in a week or so 

[markreimer]

393cc kit arrive in the mail!

I ordered an NOS gasket kit made by Noboru for cheap, mainly for the base gasket, valve cover gasket, and exhaust gaskets. I put an order in with CMSNL for OEM Honda seals for the oil orifices, drain dowels, cam tensioner ends and cam sprocket bolts. It should all be here in about 7-10 days.

So now I can take the block to the machine shop and get it bored. After looking at my manual and a few builds here, sounds like the stock piston-to-bore clearance was quite small, 0.0004", which Hondaman used when building his 350F project a while back. I've also read using a range of 0.0008-0.0010. The Cruizin Image pistons have little micro grooves on the skirts, so I'm going to ask the machinist to get it as close to 0.0004 as he can as I understand these grooves result in a little bit more clearance forming during break in.


I haven't done an overbore this much over stock before. Would anyone here have suggestions on carb jetting changes? Is anything even needed? I have it jetted very close to a 400F already, using the stock air filter/box and a 400F exhaust.

[calj737]

Would anyone here have suggestions on carb jetting changes? Is anything even needed? I have it jetted very close to a 400F already, using the stock air filter/box and a 400F exhaust.

I often see little to no effect in jetting changes with a simple overbore. Changes in compression change vacuum pressure which effects jetting. If the CR is equal to stock, the overbore may actually require smaller jets due to a drop in vacuum pressure.

Just be prepared for some plug chops but don't expect too much change. My opinion.

[markreimer]

Makes sense, thanks for the advice!

[markreimer]

Follow-up question - piston ring end gaps are spec'd as follows according to my manual:

Top, second and one-piece oil ring: 0.1-0.3mm  (0.0039 - 00118")

Service limit: 0.7mm (0.0276")

Should I stick with the same ring gap for the larger piston size? I would assume the bigger bore will generate more heat, and perhaps require a little more expansion room? Any thoughts on that? I guess the same question applies to piston-to-bore clearance of 0.0004" (that's stock clearance), should I stick with the stock numbers on these two specs or go up a little?

[markreimer]

Machinist called me today, and he was rather concerned with the request for 0.0004" of piston-to-bore clearance. 0.0008" was fine in his opinion, but tighter than that would be inching toward a press fit, in a manner of speaking, he said. The guy knows his stuff, he's done two 750's for me and a handful of XRs.

I've read a number of engine build threads here who've used the spec .0008-.001, so I'm going to heed his warning and go to .0008. That is still well below the 'acceptable range', so it should be fine.

My CMSNL order arrived today as well, with new cam sprocket bolts, cam tensioner rubbers, oil restrictor seals and oil drain dowel seals. The rest of the gaskets and seals will be here in a week or so.

Here's a question - anyone know how much material can be removed when decking the head and block before worrying about clearance? My head had 0.005" removed in December, didn't have any issues then. I didn't have the block done. I asked the machinist to measure the block this time, and if it has to be decked, then so be it. If it was also 0.005" for example, would that enough combined material removed to require claying the pistons?

[calj737]

That amount of milling from the mating surfaces should not interfere with stock cam and valves, and even with oversized pistons so long as the domes are pretty stock. From the factory, you probably have a generous amount of piston to deck clearance already, plus a head gasket to add in.

So unless you go for a really tight squish, you should not be at risk.

[markreimer]

Hit a snag. My machinist called and they can't accommodate the tiny 350f cylinders. They have hone's that are small enough, but not boring capability. This is a bummer as they're one of the few motorcycle-friendly shops in town. I've got calls into two other shops that friends of mine have used for their moto projects, but they weren't able to confirm on the phone that they could do it. They're looking into it. If it comes back as a no, I'll have to mail the block out for machining.

Now accepting recommendations for machine shops in Canada 

[calj737]

If you do have to ship it, fire it down to me. My machinist can accommodate it. Also send a piston and all ring sets so they can be gapped and all measure,ends taken before boring and honing.

[markreimer]

Wow, thanks for the offer, I appreciate it!! 

Since posting, I've found a motorcycle-specific machine shop here that has quoted me $75 a cylinder, which isn't much more than I was originally quoted. They do most of the two stroke and nikasil motorsports work here. Two week turn around.

One other shop quoted me at $95 cylinder, one week turn around. They do mostly auto and heavy machine work though, so I'm less inclined to go that way, though they said 0.0008" clearance wasn't an issue.

Waiting on one last machine shop, recommended to me by the prez of the Manitoba antique motorcycle club. He's done a few SOHC4's but none this small.

[markreimer]

Found a machine shop! $50 a hole for bore and hone. Dropping it off tomorrow and I'll have it end of next week. 

[calj737]

Take your rings and pistons to them too

[markreimer]

Absolutely, he requires I bring them 

[markreimer]

Good news and bad news.

I picked up the engine block from the machinist and it looks good. Pistons are a very precise fit. He found one piston a hair undersized compared to the rest, and honed each cylinder to a specific piston. Pretty sweet. That's the good news.

Bad news is my box of piston rings fell on the floor and one set of them shattered. I tried to order some new ones from Cruizin Image but they're blocking shipping to Canada at the moment?? so I've submitted a request for exemption.

So I'm back to sitting and waiting. in the mean time I guess I can get a head start on filing the ring gaps on the remaining three.

[markreimer]

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[ekpent]

2 steps forward and one back - Don't feel alone,think we have all been there.  Hope that strange acting seller soon gets you the ring you need so you can continue on.