Flex-hones are the greatest cylinder honing tools in the world. They look like a bottle brush, with small balls of coarse rock like material attached to the end of each bristle. If you're a motor mechanic or engine enthusiast and into rebuilding engines then this will be off interest to you. I first became interested in flex-hones back in 1979 when I first took over the business. I had a few bad experiences with engines not seating the rings correctly in the past and wanted to learn more about the honing process. I made enquires with the company in the USA and they sent me a lot of literature about the subject of cylinder honing and the plateau finishing of bores. I have included these documents for download for your convenience or you can go directly to their site. There is more literature there that I have not gathered on this site.

After reading everything I was convinced flex-hones were the way to go to give my customers the best cylinder bore finish possible and guarantee that I will have no warranty comebacks. I also liked the idea that I would do the job myself and therefore take full responsibility for the outcome. I purchased my first flex-hone for a V8 302 cu in ford engine I was rebuilding. I was very much impressed with the result, so much so that I started to use flex hones exclusively for any type of honing that I needed to do. This included brake wheel cylinders, 2 stroke cylinder bores, valve guides in cylinder heads, lifter bores, cam tunnels in aluminium heads, the applications were endless. I reconditioned over 70 engines in my time as MBS Tuning and not one failed and I accumulated thousands of dollars worth of flex-hones.

After reading the material you will understand that after a re-bore you must allow the machine shop to use the conventional rigid hone, commonly known as a stick hone. Then a flex-hone can be used to get the correct finish. WHY? It has something to do with the amount of distortion to the metal during the boring process. This is pretty much corrected using the stick hone as the stick hone actually removes material and is used to size the final bore dimension desired. Flex-hones do not remove much material and therefore can't correct the massive destruction done to the cylinder wall by the boring tool. A word of caution though, if you are getting your block bored and honed and intend on flex-honing afterwards, DO NOT tell the machine shop of your intention unless you are sure they offer this service. I don't know what their reaction to flex-hones is these days but back in the 80's most engine machine shops frowned at the flex-hone and will try and convince you that the stick hone is the best. It is not worth wasting your breath discussing it with them. I would hope by now that most engine shops are more educated about the flex-hone and realise they are the best tool to use after a stick hone.

Flex-hones are perfect for honing a worn bore or as some mechanic phrase it as de-glazing. An example is if you just want to re-ring the engine insitu, without a full overhaul. Stick hones are useless in this situation as they cannot hone the very top of the worn cylinder where the top ring reaches, the flex hone can. The only remedy is to use a special tool called a ridge remover to remove the unworn part of the bore. This will then allow the stick hone to make contact with the top ring land section of the bore. Honing a bore insitu with a stick hone is unadvisable because if the hone hits the crankshaft it will damage the hone, unlike a flexhone however no damage is done to the tool because it is flexible. However if the bore is very badly worn and you do not remove the ridge, there is a strong possibility of damaging the new top ring. Of course it goes without saying that if your bore is that worn you should have it measured and closely inspect the piston skirts for damage.

Some of the advantages of using a flex-hone are:

  1. Instant ring seating and very quick run in period. Normally most engine builders will allow approx 1000Km run in period, after which they will change the oil & filter and give the engine a check over and tune. With a flex-hone however, I gave the vehicles back, already run in and tuned and told the customer to drive it anyway they wished. I still needed to do the initial service which I generally did at 500Km. This is always necessary just to check everything is running smoothly and to remove any contaminants from the engine during the initial run-in period for want of a better word.

  2. Rings are not worn away as much during the bedding in and run in period resulting in longer life

  3. Bearings last longer due to less bore material in the oil, during the initial run in period till the first service. Don't be fooled into thinking the oil filter will trap all this debris that is a result of conventional honing. You would be surprised to know how much contamination exists in the oil after filtering.

  4. better compression sealing

  5. less blow-by

  6. less oil consumption

When you have your engine rebuilt for you or you are building the engine yourself, ask for it to be flex-honed if the shop has this option, It will cost you more of course because it is an additional operation.

If your into Go-Kart racing this little story may be of interest you. It is my own personal experience  read more about this....

Here you will find a picture of a recent hone job I did. The result speaks for itself.

I also have another article on more advanced uses of the flex-hone in the engine Section and is called Flex-hone Applications in Engines.

Below are a few books on the subject. Great read for a mechanic, may be a bit boring for a few. The Gold book is the best one to start with.