Topic: Diagnosing newly-rebuilt top end / rings / cylinders

[wohali]

Well, it's been 3 years since I posted, but just sitting in my shop, Bessie's not gotten any better. Back then, for the 2011 rally, she was burning about a litre of oil for every fill-up, and I couldn't push her over about 50mph due to the burning. She's been ridden only about 700 mi since the rebuild.

I finally got around to pulling the head on her today and found this ugliness. Leakdown prior to disassembly didn't show the valve seals were at fault. Am I going to have to re-ring her again, just 700 mi after the last re-ringing? Not sure what other things I can look for to determine where the fault lies.

I know I should be thinking something like "Hm, cylinder 1 looks OK, cylinders 2 and 4 show that grey stuff on the valves which is suspicious, but cylinders 2 3 and 4 all show the black gook, and it's on the heads of the pistons, too, along with that golden looking coating." But not being a full-time wrencher this still doesn't help me much...

Full gallery @ http://gallery.atypical.net/lr/cb750/2014-1/

[ekpent]

The far right cylinder looks like it was not firing or getting fuel due to how "clean" it is. Think I remember that you had some rings specially built correct for a f model ?  With that kind of oil consumption on a fresh top end the first thing I would check is to see if the oil scraper ring was oriented correctly and not upside down and the rest are correct also. Was it smoking quite badly ?  I put the scraper rings in upside down once back in the day on an 836 and killed every mosquito in the county    Hopefully if the rings were custom built they were within spec.

[MoMo]

Leakdown would not show leaky valve guide seals  but leaking seats.  Were all piston to wall and ring gaps correct at the first rebuild?  Sorry to see someone have to do double work...Larry

[wohali]

I did indeed have some custom rings made. Unfortunately the manufacturer was not able to have them made in a "stepped" configuration, so they are perfectly symmetrical - no way to put them in upside-down. Perhaps this is the problem and I need to locate some NOS rings...these guys seem to have some: http://www.cb750supply.com/products/4/engine/75/pistons-ring-sets

[wohali]

All the gaps were correct when I put her together but I will obviously check it again. The seats were professionally cut by a local tech (Gord Bush) who raced CB750s in the 70s so they should be OK. I guess I'll try the leakdown anyway but I don't expect a problem.

At the time I had the scraper rings made I was told he couldn't notch them and that they would be uniform / symmetric profile. Perhaps this is the issue and I need "proper" to-spec rings. Hrm.

[Gordon]

I've merged the duplicate topics, so all replies are now in one thread. 

[flybox1]

Since you have an F3, you might need the rings from here...
OEM Ref. #13011-392-004
http://www.vintagecb750.com/products/4/engine/75/pistons-ring-sets

Part #: 05-7300

I used these in my F3 hone/re-ring.  Quality pieces.

[wohali]

Frustrating that 3.5 years ago none of these options were available for standard size F2/F3 rings and I had to spend $1k of my own money to get a tool made up that resulted in rings that were probably insufficient to the task. :/

[ekpent]

Frustrating that 3.5 years ago none of these options were available for standard size F2/F3 rings and I had to spend $1k of my own money to get a tool made up that resulted in rings that were probably insufficient to the task. :/

I remember your thread and goal of having those rings built. Has it really been that long already ? Damn time goes fast when you start getting older !!

[iron_worker]

When you honed the cylinders did you measure them afterwards to make sure they were still within service limit? If you honed them too much it's possible that the cylinder to piston clearance is too large and the rings are not able to seal.

IW

[wohali]

All measurements were taken after honing and showed - within the ability of my mic to check (0.01mm resolution) or the feeler gauges correct spacing. I will be re-checking this shortly as I have to get into the bottom end to deal with missing fifth gear.

While I've got your attention, anyone remember if you can slip the top crank case off over the pistons without removing them?

[iron_worker]

Should be able to take the top case off without taking off the pistons since the cylinder sleeves fit down into the case holes (sleeve OD is quite a bit larger than the piston OD). I would be careful not to scratch up your piston skirts if you do it that way though.

Personally, I would just pull the pistons off.

IW

[wohali]

I've decided to get the cylinders checked out by Gord Bush in town (former CB750 racer and tech) and either get them rounded true, or go up a size and go from there. I think I'll order those OEM rings as well once I know what size to get.

Only way to be sure at this point, I think. Harbor Freight gauges and mic just aren't sufficient to get a good reading on roundness at this degree of accuracy

[flybox1]

IIMW, I'd go straight for the (~$120) ebay 836 kit (comes with rings) and have him perfectly bore for each piston, and be done with it.  Keep it simple and shop costs down.

[ekpent]

IIMW, I'd go straight for the (~$120) ebay 836 kit (comes with rings) and have him perfectly bore for each piston, and be done with it.  Keep it simple and shop costs down.

Those cheap kits do not work well compression wise in the later F2 and F3.

[flybox1]

IIMW, I'd go straight for the (~$120) ebay 836 kit (comes with rings) and have him perfectly bore for each piston, and be done with it.  Keep it simple and shop costs down.

Those cheap kits do not work well compression wise in the later F2 and F3.

you're right, sorry. forgot we're working with a F. 

[wohali]

Tomorrow I'll re-measure gaps on the pistons and rings.

Gord wants $350 to hone/bore the cylinders If I have to spend that I might as well go 836 though that's like $800 I think, isn't it?

[wohali]

OK so Gord's saying $60 for honing, $350 for reboring. That seems more in line

The problem is that I think it's past the point of honing working. I'm just about able to slide a 0.003 feeler gauge into the cylinder next to the skirt if I insert the piston from the top of the cylinder. I'm going to have to remeasure everything, and all I have are the Harbor Freight telescopic feeler gauges for inner bore measurements. They often feel to me like they don't have a great tightening screw on them and I worry the measurements are not great.

I guess another other option is to bring them in for $60, have them honed/checked out, and if they measure bad then, I have to order the pistons, wait a few weeks, then get back in line with the tech to have them bored and ringed.

And they're F2/F3 pistons so not a lot of choices. Hrm

[wohali]

Checked out the intake valves today as well. Photos on their way, but I believe they're OK. I used a dial readout and wiggled the valves inside their seals - I'm only seeing 0.004" total deflection (and I'm pushing fairly hard). These are new valve guides and the seals do seem to be "wiping" oil off as I push the valves down. This plus the compression test I believe suggests it's not the valves.

[grcamna2]

wohali,
Try shopping around and discover your best option for replacing the pistons/rings and boring to fit..,even if you need to send them to another area. I think you can get it all done for a lot less than $800. kind of like a package deal,maybe even a forum member here might recommend a good person whom they've used before. just my 2 cents...

[wohali]

Dynoman's piston kit is $500, Gord wants $320 on top of that to bore...that's within the limit you are suggesting.

[grcamna2]

I've got this idea that you should be able to get a wholesale price for the pistons/rings and bore job all for around $500ish. I just don't know where  But I'd try to go for that if at all possible,even if it took a lot of research.I suppose shipping would probably push it over that though...  I just think it's possible,you Know ? !
I suppose I'm assuming Chinese pistons/rings sets though 

I'm not sure if I'm 'with' today's prices so correct me if I'm 'out of it' but,
Bore job: $50 a hole and 4) pistons/rings: $300  ?? that's probably 'way' wholesale,right ? ! 

[calj737]

Isn't bwaller from Toronto? He may have a good recommendation as he's built numerous bikes and motors.

[wohali]

I just sent him a PM giving him a heads up on this thread. Hopefully he'll stop by

[bwaller]

I remember reading that you had rings made. Didn't you organize several members in a bulk-buy, did anyone else have issues? When you installed the rings what else was done, new pistons or just a hone and rings? You mentioned 0.003" piston clearance? Did you measure at the base of the skirt perpendicular to the pin? It sure seems your rings are at fault. Did I read you did a leakdown test before you dismantled the engine. If so what were the readings?

If Bush did the head it won't be worn out in 700 miles. In one picture it looks like a brass guide? Did he replace guides, valves & springs?

I haven't used Bush in years, it was Leitner & Bush then! He has a good reputation. There's a big shop north of Mtl. but they blew a job for me last time. You might ask APE or Millenium if you want options.