®     Cylinders Done Right

It is a good thing that so many vintage bike owners are taking responsibility for their own maintenance and repair. But they will do well to approach those tasks requiring machining skills and understanding with a sober, thoughtful, knowledgable attitude.

Cylinders are special. In my 45 years in the trade, seldom has another engine part posed as much difficulty. One of the major challenges is finding a machine shop that will/can bore a cylinder straight, round, and to a correct clearance. In fact, getting cylinders bored properly is a monumental task. But, no matter how difficult, virtually any amount of hassle is justified when it comes to this part of the engine. For, just as the engine's cylinder head is central to its performance, and painstaking and best practice technique essential to get the best from it, so the cylinders require special attention and an almost obsessive approach for them to function as they should. Anal effort, really, even Herculean at times, because after all, the cylinder is the producer of the engine's pressure and the head is the guardian of that pressure. Nowhere else is effort better spent.

Unfortunately, there is a kind of alternate universe when it comes to these all too important areas of an engine. The serendipity with which most folks approach cylinder heads and cylinders astonishes me. I've often said that instead of porting and big bores, which indeed have their place, spend your resources on getting the cylinders and head to work right, to factory spec - which seldom is the case. I have proven to customers that meticulous attention to standard specification yields benefits rivaling and sometimes excelling that of the more sensational exercises of big cylinder bores and cylinder head porting. Standard performance done right trumps high performance done sloppily every time. Or to say it another way, what is often mistakenly thought mundane and unglamorous cylinder and head rebuild work holds far more performance potential than many realize.

The Automotive/Performance Influence
There are two reasons cylinders are hard to get done right. One is that probably ninety percent of machine shops are part of the automotive trade and thus entrenched in the ethics and tradition of the car world, where no one speaks in terms of tenths of thousandths of an inch and parts simply aren't manufactured to aircraft level standards as powersports parts are. This is significant. The automotive world owns machine shop ethic. The other big problem is when one does find a motorcycle-savvy machine shop it is usually one beholden to and influenced by the racing community, an environment in which long term reliability is seldom considered, forged pistons rule, and the lingua franca is largely Harley-Davidson, a dielect of the auto world. If not this, then drag racing, another universe all its own and one having little relevance to how most motocycles are actually used. Unfortunately, engines assembled nearer the larger end of their maker's recommended clearance range rather than the smaller end are two-thirds worn out already and will exhibit blowby, piston noise, loss of power, and oil consumption way before their time. I don't call that high performance.

Ring End Gaps
In addition to the loss of seal, increased crankcase pressure and increased heat and noise attendant with overly loose piston fitment, there is the issue of ring end gap. This is in my mind the most telling and most disturbing outcome of using some of the aftermarket piston kits. Original equipment pistons have rings that come pre-gapped. If the cylinder clearance is to spec, the ring end gap is correct, and for this reason dealer techs are accustomed to never checking end gap on the major manufacturer's pistons. They don't have to. OEM rings are already "gapped." Perfect every time. Every Honda mechanic knows this, and occasional just-in-case checks always have confirmed it. Aftermarket however, that is a completely different story. Setting end gaps is part of the game in that world. But either way, and the important point is, ring end gap is affected by piston to cylinder clearance, dramatically. A cylinder bored a bit over the correct size exhibits a larger than normal ring end gap. That pi r squared thing, you know. Half a thou too big and the ring end gap grows by more than three times as much. Too large a gap is inevitable when the piston is put in too loose, resulting in poor ring sealing, loss of compression and oil control. And you can't make a too-large gap smaller. Ring end gap also is a good way to tell ring wear. The rings below are both from a big bore kit, one is new, the other has less than 500 miles on it. Same cylinder.

This is a set of Cruzinimage piston rings fresh out of the package. The end gap is a little larger than I like, due to the cylinder not being bored carefully. This, more than anything, is the compelling reason to be careful with cylinder clearance.

Here are the Crusinimage rings after just 470 miles. 470 miles! The end gap is almost double. This says that the ring is wearing out faster than a New Year's resolution! This is the weak point of many eBay big bore kits.


The middle, so-called scraper ring or secondary compression ring is often tapered, as shown in this picture of a CB350F piston. Early Honda top or compression rings were also in many cases. All OEM rings are marked for this reason.

Ring Install Orientation
Piston rings traditionally have had faces that are not square but slightly tapered so they will scrape oil downward away from the combustion chamber. Because of this, the best rings, including all OEM, bear a mark on their upper surface as an aid to proper installation. Many of the recent crop of Taiwanese rings now flooding the market have no marking, leading some to assume these rings are not tapered and thus require no special installation. Unfortunately, good mechanics are finding that some of these rings are indeed tapered, despite not being marked. Watch out for this.

Piston Availability
It is a peculiarity of this industry that it is so hard to get Japanese air-cooled powersports cylinders bored correctly. However, and unfortunately, it is almost as hard to get good pistons. Most early Japanese OEM pistons are available only from old NOS suppliers such as David Silver and some eBay sellers (most of whom sell ones and twos, and usually oversizes not standards). Here are some of the alternatives to OEM:

Wiseco
A long tradition of supplying pistons has made Wiseco the de-facto performance standard. See my article on pistons wherein I describe the pros and cons of forged pistons and the relevancy of each. Wiseco no longer supports early Honda SOHC models, at least the smaller ones. Even the 750 pistons are available hit or miss. I have no issues with Wiseco, unless it's their super-conservative clearance recommendations.

CycleX
The pistons are good, the rings unknown quality, probably Taiwanese same as others. They sell both OEM style cast pistons and Wiseco forged ones. No complaints.

Cruisin Image
This eBay seller's cast pistons are very good, very well made, consistent in size. The Chinese rings less so, though at least they are marked. Close inspection also shows them in many cases to be tapered like OEM and other quality rings. Mitsuo is very good to deal with also, but like many retailers has no technical product knowledge. Forums are divided on whether oil consumption issues are connected with these kits.

Emgo
The pistons seem good, look like OEM copies, but the rings are unmarked and very suspect.

That is a stock Honda piston on the left, aftermarket kit piston on the right. The two are joined by a piston pin for alignment purposes. You're looking at 1mm difference in deck height. Not good. This 3mm overbore piston results in stock cylinder compression. The loss is in the deck.

Honing
Finally, honing. Cylinder honing is yet another nearly lost art. All too often a hone is used incorrectly today. In most folks' minds a hone looks like the one you find on Amazon for brake cylinders. In fact, ones used for cylinders are very different. They are rigid and have micrometer-like sizing adjusters, and the best ones are mounted to the floor for optimum control. There are however hand-held ones that substitute adequately when used carefully and correctly. The technology that goes into a cylinder hone is what it is because of something very important: the cylinder hone is nothing less than the final step in boring the cylinder. That is, the rigid multiblade precision hone is actually part of boring, not used in place of a boring machine but as an extension of the boring process, because it smooths the boring machine's tool marks, imparts the necessary cross-hatch finish that will promote ring seating, and most important of all, takes the final 1-2 thousandths out to reach the finished dimension. It should become clear that this hone is not designed to be and should never be used in any other capacity. It is not used to "freshen a cylinder". It is not used to break the cylinder's glaze. It is not a part of fitting new rings. None of those are legitimate uses of the cylinder hone. There is in fact a special hone for those purposes, one designed to remove almost no metal. It is not like the rigid hone a sizing hone but merely a glaze-breaking hone.

Conclusion
Back in 1978 I disassembled a 50,000-mile 1974 CB750 owned by an inveterate high mileage rider. The pistons were actually crusty, with tons of carbon all built up around their ring lands, on the piston crowns, and inside the oil drain holes. As I cleaned and measured, I fully expected major wear. Guess what? The piston-to-cylinder clearance in that engine was still its as-manufactured 0.0008" even after that kind of mileage. Twenty years later I was building Evolution-based hot-rod Harleys with JE forged pistons and even those huge forged slugs I installed at no more than 0.0015". In that same shop I also built GSX-Rs, ZX-11s, CBXs and Z1s and never did I set up any of them at the clearances folks seem to think necessary today. I have used a lot of different kinds of pistons: ART, JE, Wiseco, Arias, MTC and others. Some were known to expand more than others. But seldom did they have to be installed looser than 0.0015", and the stock air-cooled Japanese stuff I always did at 0.0007"~0.0010". Many folks balk at this. So, here, have a look at the table below made from the data found in official Honda service manuals. Some liquid-cooled engines are included for comparison, and to make the point that engineering, not air vs. liquid, is why Japanese pistons are fitted tight.

CB125 Single (pg 16)

CB175 Twin (pg 29)

CB350 Twin (pg 40)

CB350F SOHC Four (pg 76)

CB400F SOHC Four (pg 106)

Piston 55.97~55.99

Piston 51.96~51.98

Piston 63.97~63.99

Piston 46.47~46.99

Piston 50.97~50.99

Cylinder 56.00~56.01

Cylinder 52.00~52.01

Cylinder 64.01~64.02

Cylinder 47.00~47.01

Cylinder 51.00~51.01

Clearance 0.01~0.04

Clearance 0.02~0.05

Clearance 0.02~0.05

Clearance 0.01~0.04

Clearance 0.01~0.04

Clearance inches 0.00039~0.0016

Clearance inches 0.0008~0.0019

Clearance inches 0.0008~0.0019

Clear inches 0.0004~0.0015

Clear inches 0.0004~0.0015

Clearance average 0.0010

Clearance average 0.0014

Clearance average 0.0014

Clearance average 0.0009

Clearance average 0.0009

Clearance given NA

Clearance given NA

Clearance given NA

Clearance given NA

Clearance given NA

Clearance service limit 0.01181

Clearance service limit NA

Clearance service limit 0.00781

Clearance svc limit 0.00981

Clearance svc limit 0.00981

Ring end gap top 0.0059~0.0138

Ring end gap top NA

Ring end gap top NA

Ring end gap top 0.004~0.012

Ring end gap top 0.0059~0.0138

Ring end gap 2nd 0.0059~0.0138

Ring end gap 2nd NA

Ring end gap 2nd NA

Ring end gap 2nd 0.004~0.012

Ring end gap 2nd 0.0059~0.0138

Ring end gap oil 0.0059~0.0158

Ring end gap all 0.006~0.016

Ring end gap all 0.008~0.016

Ring end gap oil 0.010~0.0352

Ring end gap oil 0.0079~0.0197

End gap svc limit top NA

End gap svc limit top NA

End gap svc limit top NA

End gap svc limit top 0.0276

End gap svc limit top 0.0276

End gap svc limit 2nd NA

End gap svc limit 2nd NA

End gap svc limit 2nd NA

End gap svc limit 2nd 0.0276

End gap svc limit 2nd 0.0276

End gap svc limit all 0.0197

End gap svc limit all 0.0315"

End gap svc limit all 0.0315"

End gap svc limit oil 0.0276

Ring end gap svc limit all 0.035


CB500 SOHC Four (pg 139)

CB550 SOHC Four (pg 141)

CB750 SOHC Four (pg 23)

CB750 DOHC Four (pg 7-1)

CB900 DOHC Four (pg 7-1)

Piston 55.97~55.99

Piston 58.47~58.49

Piston 60.965~60.985

Piston 61.95~61.98

Piston 64.46~64.49

Cylinder 56.00~56.01

Cylinder 58.50~58.51

Cylinder 61.01~61.02

Cylinder NA

Cylinder 64.50~64.51

Clearance 0.01~0.04

Clearance 0.01~0.04

Clearance 0.025~0.055

Clearance see 900 spec

Clearance 0.01~0.05

Clear inches 0.0004~0.0015

Clear inches 0.0004~0.0015

Clear inches 0.001~0.002

Clear inches see 900 spec

Clear inches 0.0004~0.0019

Clearance average 0.0009

Clearance average 0.0009

Clearance average 0.0015

Clearance average see 900 spec

Clearance average 0.0012

Clearance given NA

Clearance given NA

Clearance given NA

Clearance given NA

Clearance given 0.0004~0.0020

Clearance svc limit 0.00981

Clearance svc limit 0.00981

Clearance svc limit 0.00981

Clearance svc limit 0.004

Clearance svc limit 0.004

Ring end gap top 0.005~0.013

Ring end gap top 0.005~0.013

Ring end gap top 0.007~0.016

Ring end gap top 0.004~0.012

Ring end gap top 0.006~0.012

Ring end gap 2nd 0.005~0.013

Ring end gap 2nd 0.005~0.013

Ring end gap 2nd 0.007~0.016

Ring end gap 2nd 0.004~0.012

Ring end gap 2nd 0.006~0.012

Ring end gap oil 0.010~0.0352

Ring end gap oil 0.010~0.035

Ring end gap oil 0.010~0.0352

Ring end gap oil 0.012~0.035

Ring end gap oil 0.012~0.035

End gap svc limit top NA

End gap svc limit top 0.027

End gap svc limit top NA

End gap svc limit top 0.020

End gap svc limit top 0.020

End gap svc limit 2nd NA

End gap svc limit 2nd 0.027

End gap svc limit 2nd NA

End gap svc limit 2nd 0.020

End gap svc limit 2nd 0.020

Ring end gap svc limit all 0.027

End gap svc limit oil 0.043

Ring end gap svc limit all 0.028

End gap svc limit oil 0.043

End gap svc limit oil 0.040


CB1100 DOHC Four (pg 10-1)

CBX1000 (79~82 pg 7-1)

VF750C/S 82 (pgs 12-1, 12-5, 25-22)

GL1000 (pg 6-2)

CBR600 F4 (pg 1-8)

Piston 69.970~69.980

Piston 64.47~64.49

Piston 69.960~69.990

Piston 71.945~71.970

Piston 66.965~66.985

Cylinder NA

Cylinder NA

Cylinder NA

Cylinder 72.000~72.150

Cylinder 67.000~67.015

Clearance see 900 spec

Clearance see 900 spec

Clearance NA

Clearance 0.030~0.105

Clearance 0.015~0.050

Clearance inches NA see 900 spec

Clearance inches NA see 900 spec

Clearance inches NA

Clearance inches 0.001~0.004

Clearance inches 0.0006~0.0019

Clearance average see 900 spec

Clearance average see 900 spec

Clearance average 0.0012

Clearance average 0.0025

Clearancce average 0.0013

Clearance given NA

Clearance given NA

Clearance given 0.0004~0.00202

Clearance given 0.0012~0.0027

Clearance given 0.0006~0.0020

Clearance svc limit 0.0039

Clearance svc limit 0.004

Clearance svc limit 0.0042

Clearance svc limit 0.006

Clearance service limit 0.002

Ring end gap top 0.0059~0.0138

Ring end gap top 0.006~0.012

Ring end gap top 0.004~0.012

Ring end gap top 0.010~0.016

Ring end gap top 0.004~0.008

Ring end gap 2nd 0.0059~0.0139

Ring end gap 2nd 0.006~0.012

Ring end gap 2nd 0.004~0.012

Ring end gap 2nd 0.010~0.016

Ring end gap 2nd 0.007~0.012

Ring end gap oil 0.018~0.035

Ring end gap oil 0.012~0.035

Ring end gap oil 0.012~0.035

Ring end gap oil 0.008~0.035

Ring end gap oil 0.010~0.030

End gap svc limit top 0.020

End gap svc limit top 0.020

End gap svc limit top 0.020

End gap svc limit top 0.027

End gap svc limit top 0.020

End gap svc limit 2nd 0.020

End gap svc limit 2nd 0.020

End gap svc limit 2nd 0.020

End gap svc limit 2nd 0.027

End gap svc limit 2nd 0.020

End gap svc limit oil 0.043

End gap svc limit oil 0.040

End gap svc limit oil 0.040

End gap svc limit oil 0.043

End gap svc limit oil 0.040

First three measurements only metric, the rest are Imperial

1 Incredible!

2 Corrected from obvious typo

© Mike Nixon, The Motorcycle Project



See also:
Valve Jobs Done Right
Cylinder Head Porting

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© 1996-2015 Mike Nixon