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

[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.

[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?

[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 

[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. 

[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.

[markreimer]

Rings have been ordered and on the way 


Looks like the rings broke when this piston hit them. There is a very tiny nick at the edge of the skirt on the bottom. No burrs on it. I'm thinking this is fine to use, what do you think?




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[disco]

Smooth it off with fine sandpaper.... then run it. No problem.

[markreimer]

Finally a bit of progress! noboru gasket kit arrived after being held in customs for 9 days. All I need is that replacement set of rings and I can button this back up. The gasket kit is prettt old but all the rubber feels supple and good quality. Funny looking exhaust gaskets though!




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[markreimer]

started measuring ring gap. Straight out of the package, these ring gaps are measuring .006" / .152mm. I checked near the top and near the bottom of the bore. The stock ring gap range is listed as .1-.3mm, so this is just below the halfway point of the recommended range. The oil ring is a touch tighter at .005" / .127mm

Earlier someone suggested aiming for a mid-range figure for the gap given this engine is bored out to 393 now. Think this is close enough? I'm leaning to installing them as-is, assuming every ring checks out the same.

[markreimer]

Update - I see CB400F stock ring gap is .0059". Mine are reading .006" on most of them. Some are showing .005". I think I'll file fit the tighter ones to hit that same spec

[markreimer]

Cylinder is on! Had a couple of brain farts on the way, but it's together now.

I checked the ring gaps and installed the rings, verifying that they all had the 'top' markings facing up, and the order was correct. I had my first very close OOPS when I realized just before sliding the cylinder down that #2 piston had the two upper rings swapped! Glad that didn't happen. So I fixed that up, installed the pistons in the cylinder, then lowered the cylinders (with pistons inside) onto the rods, sliding the middle two wrist pins and circlips in first, then the outer two. This is my first time assembling a motor this way, and it was actually very easy to do by myself. The big bore totally eliminate the chamfer at the bottom of the cylinder, so it'd be super easy to break a ring without a compressor. This made it easy to ensure the rings made it in safely.

Also, not too proud to admit the second major oops I had. I had the cylinder installed and was cleaning up when I realized I had all four pistons in 180 degrees backwards, with the intake side facing the exhaust!!    Thankfully it only took about 30-40 minutes to pull it apart, switch them around and get it all back together. I really like this method of top end rebuilding. Aligning the rod with the wrist pin is a little tricky, but not too bad.

Next up is cleaning up the gasket surface on the head, applying some Hondabond around the main oil supply and return seals, copper coat on the head gasket, and torque the head down. I've got new rubber dampers for the cam chain tensioner, some new camshaft sprocket bolts, and a few other goodies to install, plus a quick carb inspection before buttoning it all up and givin 'er some gas. I'm gone this weekend but hope to fire it up next week at my weekly moto shop night with the guys.

[markreimer]

Cool I'll look into that! I've always just used a razor blade to start, then scotchbrite. That sounds good.

[MetroRedneck]

Second on the stainless brushes.  I find I save time over blades and scotch bright too.

[markreimer]

So something like this, but smaller?

http://www.canadiantire.ca/en/pdp/stainless-steel-wire-brush-0588105p.html

Or these:

https://www.amazon.ca/dp/B01M34Q848

And that doesn't damage the aluminum gasket surface?

[markreimer]

Thank you Cal for the stainless steel brush tip! It was awesome for cleaning up the head gasket surface!

I made big progress today. The engine is basically done but sitting for a few days before I retorque the studs.

I double checked all the oil passages were clean, then applied a thin smear of hondabond around the supply and return lines on both sides of the base gasket.

The head gasket got a few coats of coppercoat in thin stages. I applied hondabond to the outside face of the oil supply seals, very thin and being sure to stay far away from the little holes in the restrictor.

With the head gasket tacky, I installed the cam chain tensioner with new rubber dampers, then the head, and then torqued it down. Next was the cam, and man did I have a time getting the sprocket and chain on. I think the chain wasn't sitting on the lower sprocket properly. A few rotations of the crank and everything fell into place. Assembly lube was applied to the cam bearing journals. Torqued some new cam sprocket bolts down, and my work is done for the day!

Well kinda, couldn't resist opening the carbs and cleaning them up, as well as installing new exhaust gaskets and bolting the pipe on. Now all I have left is cylinder torque, install carbs, valve clearance, add fresh oil and see how it runs.

That'll all happen Thursday.




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