Curious engine, low compression

[CBX1000chris]

Morning all,
I suspect this post is as much to reassure myself I haven't missed anything as much as asking a question but I'm not too proud to ask!

A friend (yes it is a friend, not me), is building a special using a CBX motor he bought from abroad, Canada I think, we're in the UK. The history of the engine was pretty much unknown and anything we 'think' we know needs to be discounted in the interest of avoiding red herrings.

My friend was unable to get the bike to start so he brought it to me so I could have a look. I have a '79 CBX and have owned bikes for 40 odd years so have encountered many problems in that time and rebuilt more engines than I care to remember. Once again though, I'm not too proud to learn something new.

I went through the usual suspects, fuel, sparks, wiring and found nothing. I then did a compression test and the readings were,

Cylinder
1 - 55psi
2 - 60psi
3 - 70psi
4 - 80psi
5 - 60psi
6 - 65psi

These readings were clearly low, so low that I repeated the test on my FTR223 (brilliant run about bike) to validate the tool and got over 140psi. Pouring a little oil into the cylinder brought the compression up a little but no great improvement. However there was smoke coming out of the exhaust so the oil was being burned off, something was happening in there.

I then looked at valve clearances and although one or two valves were tight none were closed up. The valve timing looked out a little on the exhaust side but not quite a full tooth. Inlet timing was spot on. At this point I spotted the #1 bearing cap was fitted the wrong way around, I later found that one of the #12 bearing cap bolts was finger tight, this doesn't inspire confidence.

Next I knocked up a tool to allow me to perform a leak down test. With the cam lobes not touching the shims I pressurised each cylinder in turn but I couldn't even build up a sensible amount of pressure to establish leak down time. The noise created by air leaking was not in the inlet or exhaust ports, only in the crankcase. Once again I repeated the test on the FTR only to find the pressure was pushing the piston away from TDC however I was satisfied the tool was working as expected. In my mind this test discounted any timing issue as being the source of the problem.

What I had confirmed was that the engine has new pistons, looking through the plug holes they were shiny and new. So, the head and barrel came off and I found the pistons to be marked 4183PS, I google'd this and found them to be a Wiseco part and if I found correct information 0.5mm over size. What I did notice was the rings were not spaced at 120 degrees. I only have a vernier caliper but the pistons measured 64.94mm and the bore 65mm. The pistons have no witness marks to suggest the engine has ever run and there was still grease on the little end eye when I removed the #1 piston. The barrels still have clear honing marks and also show no signs of being run. I've only checked the #1 cylinder but the piston ring gaps in the cylinder are within the specification for a standard engine. Placing the piston in the bore and measuring the gap was interesting. I know pistons aren't round, they are shaped to allow for different expansion once heated up, but the side to side clearance seemed huge at 0.010", front to back clearance was 0.0025" so about what would be expected (a little tight but I was using a feeler gauge). Sorry for the use of both imperial and metric!

I placed the head upside down on a bench and poured in water to cover each of the valves then blew air into each of the inlet or exhaust ports in turn but there was no sign of any bubbles so I believe the valve sealing to be good. This was confirmation of the leak down test.

My conclusions then are that I have compressing leaking past the pistons but I can't say why. I plan to get the barrel and pistons to an engineering firm for their assessment and hopefully they will come up with something. In the meantime can anyone suggest something else to check or point out an error in my testing. Could it be something as silly as the rings?

Thank you in advance for any input.
Chris.

[daves79x]

It would seem that even with what you found, the engine should have more compression than that. Enough to start and seal the rings, you would think. Are you positive the head gasket was correct and sealing?

Dave

[EMS]

There is no doubt in my mind it is the rings, provided all your measurements are accurate. It could be that whoever machined the cylinders for the oversize bore did a lousy job and the bores are not straight.

[CBX1000chris]

Thank you for the replies gents.

Dave - I'm not sure whether the gasket was correct or not, it certainly touched where it was meant to. Only after I'd removed the head did I think the nuts cracked quite easily but at 23 - 25 ft/lbs (10mm) they shouldn't be too much of a struggle to undo anyway. I did look very carefully for any evidence of gasket blow by when I removed the head but couldn't see any. Having put oil in the cylinders I think there would have been evidence if the gasket was leaking. Also, on the leak down test the air noise was definitely in the crank case. Good shout though and I can't eliminate it from the list of possibilities. If I find nothing and the engineering firm draw a blank too I'll rebuild the engine carefully and if it is then OK you may very well be correct and head gasket may have been the culprit.

On the subject of gaskets do you know if I can use a standard head gasket for a 0.5mm over bore?

EMS - Hopefully the engineering firm will be able to identify any problems with the re-bore.

Thanks again for you replies,
Chris.

[steve murdoch icoa #5322]

Chris, i have nothing of value to add to the mechanical aspect of the problem but i did want to say thank you for the excellent content of your posting.
Concise, complete information laid out in a fashion that even backyard hacks like myself can follow along.

[Jeff Bennetts]

If there's air leaking into the crankcase then as EMS suggests it's either ring or bore related, it's a little disconcerting that the compression is so low that the motor won't even run well enough to seat the rings, the machine shop will verify that for you but it sounds like a bad bore job if you're measurements are correct.

Cylinders aren't terribly expensive though.

Good luck going forward

[CBX1000chris]

Kind words Steve, thank you. 40 years a engineer, 30 of those in R&D, I soon discovered if your specification isn't clear the dolphin you required arrives looking suspiciously like a hedgehog! It's nice to know that having been retired for three I years write can comprehensible still something.....

[NobleHops]

If there's air leaking into the crankcase then as EMS suggests it's either ring or bore related, it's a little disconcerting that the compression is so low that the motor won't even run well enough to seat the rings, the machine shop will verify that for you but it sounds like a bad bore job if you're measurements are correct.

Cylinders aren't terribly expensive though.

Good luck going forward

I agree with this - and you're on the right track for exploration too - those pistons and bores need to be measured properly and ideally should result in piston-to-wall clearance of somewhere around .002", plus the bores should be checked in a few different axis to see of they have ovalled. Wiseco has a formula for the ring end gaps and there is also a 'right way round' with those' the scraper ring has a bevel and the top ring has writing that should be visible facing up. Wiseco is excellent about tech support and of course will sell you rings if it turns out those are needed. Last - are these piston tops a little wedge-shaped with valve cutouts? Typical Wiseco pistons raise the compression to 10.25:1, a bump from stock.

A stock head gasket will work fine.

Great tech here: http://blog.wiseco.com/everything-you-n ... t-ring-gap

[CBX1000chris]

Hi,

Yes, the piston tops are wedge shaped with valve cut outs. I've arranged to take the head, barrel and pistons into a local firm, South Cerney Engineering, for measurement, they can also do a compression test. I asked if they needed a head gasket to be able to do this but the guy said he thought they already had the necessary kit to compression test a CBX having done one before. I'll be interested in how they do this and what they find.

I'll post the results in a few days.

Regards,
Chris.

[Larry Zimmer]

We shall be watching, Chris. Most interesting. Thanks,

[CBX1000chris]

Hello all,
I dropped the head and barrel off today and was asked a question that I didn't know the answer to. Should the top of the cylinder liners be flush with the head? On the bikes I've worked on I don't remember a liner not being flush but the guy said that the liner being slightly proud is not uncommon.
Please could someone who knows the answer for sure let me know please.
Thank you in advance,
Chris.

[EMS]

The top of the liners have to be flush with the block. The combustion chamber in the head is not exactly round and the same shape as the top of the cylinders. If the liners protrude above the deck, the head will not sit flush and it will not work.
Funny that the shop asked that question. looking at the parts should have told him. I wonder how he thinks he will make a head gasket seal...??/

[CBX1000chris]

There was a misunderstanding about the pressure test (probably on my part). Although I'm sure I said it was air cooled he was thinking about pressure testing a water jacket. I have asked him to confirm the bores are truly round, not oval as was suggested by NobleHops, and the bore diameter and piston clearance. The first thing he did when I got there was place a straight edge across the length of the head and there was daylight peeping though in the middle. I'll let the guy know Honda state a maximum of 0.1mm warpage for both the head and barrel. If it is necessary to skim and head and/or the barrel how much squish is there to play with?

Thank you for the information and advice guys.

[daves79x]

I believe you are OK taking off up to .009 inch. Others will weigh in I'm sure.

Dave

[herdygerdy]

It's been quite a while since I'm been in this deep to an engine, but I vaguely recall the compression rings have to be installed with the tiny markings on the ring facing up towards the piston crown.

If the rings are not installed the correct way up, then low compression would result. IIRC, the top of the ring is ever so slightly angled down from the outside edge of the ring to its inside edge. I understand this is done in order to force the ring harder against the cylinder wall and thereby creating a gas-tight seal. And the top of the ring land on the compression rings are also slightly angled if my memory serves me correctly.

Others pls chime in if I am incorrect, but definitely worth checking the rings were installed correctly on the pistons before shelling out big $$$ fixing a problem you may not have...

Watching this one with interest.