Cylinders Done Right www.motorcycleproject.com

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 more than 50 years in the trade, seldom has another engine part represented as much misunderstanding. 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 they aten't on a fifty year old machine, you can count on that. 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-eight 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 they need to in order to do Honda cylinders properly--there are vintage Hondas that properly assemble at 0.0003 inch. That's a third of a thousandth of an inch. 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, special case 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 Chinese 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).

Ring Availability
But far worse is piston ring availability. Many good choices exist for pistons on the aftermarket, but the rings that come with these kits are invariably Chinese. Not good. This situation has become a huge issue for engine rebuilders.

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 greatly misunderstood task. All too often a hone is used incorrectly. In most folks' minds a hone looks like the one you find on Amazon for brake cylinders. You also see many folks on YouTube promoting these hones. But hones used for engine cylinders are very different. They are rigid and have micrometer-like sizing adjusters. The importance of these tools is that a 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 control cylinder oiling, and most important of all, determines the cylinder's 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. Many on forums advocate using a hone to remove rust from a cylinder. That may seem intuitive, but a rusty cylinder is almost surely worn excessively from use, and if not is unquestionably "worn out" by rust's removal of metal, and then you go in there and remove more? It is the worst thing you can do. A rusty cylinder is one that needs to be bored.

Take a look at the factory Honda specifications for the following models. A special note. Honda has fairly consistently made you work at finding their engines' recommended piston-to-cylinder clearances. Only a few official manuals for vintage models give the clearance as a stand-alone spec. In most cases you must determinde piston clearance by doing the math on the official as-manufactured cylinder and piston dimensions.

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}