Sunnen Hone for Perfect Liner Fit

by Marty Davis

To get a perfect finish on a liner and enhance break-in and liner piston fit, the Sunnen hone worked in stages is the answer.

I have wanted to do a Technical Article on this subject for a long time. Some may not agree or understand my findings and opinions, but they are based on a long period of testing and experience. Usually I do a several part article, but this time I think I will make it a complete Technical Article about the subject.

I want you to visualize the perfect finish on a nitro engine liner. Imagine that the liner is magnified 1 hundred or more times and you can see all the machining lines of finish on the liner. What does it look like? Sorry to say, it looks like a file that you use to smooth metal parts. The lines are machining lines from the original boring and then the application of chrome. Most times the chrome is placed over the rough bored brass liner. Kind of like chroming a file. When you get a new liner, the best thing you can do to make the liner break in perfectly and make a seal that will LAST, LAST, LAST is to hone and corkbond it. It must be done in at lease 2 stages. First stage is to use a honing stone that is very fine and will impart a really smooth finish getting rid of most of the "file lines". Second stage is to use a product called "CorkBond" that looks like a honing stone but is really a rubberized product that has very fine lapping compound impregnated in it. The final finish that corkbond produces is like a mirror and looks like a mirrored surface. Now, when you start the breakin process, it is not necessary to run gallons of fuel to get the perfect breakin. The seal ring that you see for a broken in engine is across the entire liner surface. The breakin line will be hard to spot because all the surface will look like that. I was fortunate to be given the Sunnen Honing set that John Ackerman used to rework njitro engines for many years. I have most all the sizes that fit our current engines and have plenty of both honing stones and corkbond finishing inserts.

I just finished building an engine for myself that I will run in my 67 hydro. The Picco Engine that has been in several of my 67 boats for at least 6-7 years sucked in a piece of metal from someplace and ruined the liner and head button. One of my close friends gave me a used liner and piston to replace the one that I destroyed. It didn't have any gouges but the finish was not at all perfect. About 15 minutes of honing produced a liner that had a PERFECT FINISH. I also used another tool that I got from John Ackerman. It is called a Helical Lap. The Helical Lap is a tool that laps the outer diameter of a new or used piston to make it PERFFECTLY ROUND. Using various grades of lapping compound, you can get any finish that you desire and a perfectly round piston. A perfectly round piston paired with a perfectly round honed liner makes for a PERFECT FITthat will last and last. You would be amazed at how many of the liners and pistons that you recieve with a new engine are not at all round. This is the difference between a good running engine and one that will Super Perform and last until you damage it. Seems that the liner/piston fit never goes away, it just gets ruined by damage. Examples: A Glow Plug Element going through the engine, a stray piece of something going through the engine, detonation, etc. The engine that I replaced the liner/piston/head button had been running super well for at least 6-7 years (original liner/piston).

The process for honing and corkbonding a liner is a quick process and one that is not expensive to have done for you. The outcome is awesome and reduces the breakin period to a couple tankfuls.

To get a perfect finish on a liner and enhance break-in and liner piston fit, the sunnen hone worked in stages is the answer.

Who else uses this process? One of the most successful teather car racers and boaters, one of the builders that imports one of the most popular engines for RC Boating. No wonder their equipment is always super fast and reliable. You would be surprised at the number of people who John Ackerman built engines for who won high level awards (US#1's, Excellence of Performance Award, and many Airplane Competition and Speed Records. I also saw engines he built many years ago that qualified for the A Main at the RC Onroad World Championships in Indianapolis that had been honed and cork bonded. There are Many More Successes for engines using this process. which are too numerous to list.
This is not a pitch for business at all but some information that is typically not shared by the top competitors. Is there anyone that will do this for you and your engine? Not sure of others, but I have heard that Stu Barr the importer of CMB Marine Engines owns a Sunnen Hone like mine. I have also heard that he does this service for some customers. My racing partner, Norm Doerr who I sold most of my equipment to several years ago when I thought I would get out of the hobby has my honing equipment at his shop. I told him that he could use the equipment as he wished. I would bet that he might do some honing and corkbonding for fellow racers. You might check with him on that.

OK, so now you know one of the closely guarded secrets of the top competitors in all areas of nitro rc hobbies.

Until next time......

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