OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk GX160 piston tolerances

I have a GX160 honda engine that may need to be bored over. What are the wear limits for the piston/cylinder? Also, I will need to know what to set the ring gap at and What the spec for piston-cylinder gap should be. Anyone know this? Thanks.

Hi cngodfather, welcome to Outdoorking.

There is a short-form workshop manual for the GX160, which is available for free download here:
http://honda-engines-eu.com/en/images/996.pdf

Unfortunately Page 47, which gives the dimensions you need, has accidentally been duplicated from the GX120 and hence has incorrect dimensions. I do not know of a free download of the full workshop manual, so I will just give the dimensions you have requested, as stated in the full manual.

The service limit for bore diameter without reboring is 68.165 mm, or 2.684". The service limit for the skirt diameter of the piston is 67.845 mm, or 2.671". However you cannot use a minimum sized piston in a maximum sized bore, since the maximum skirt clearance permitted is only 0.005". The permitted skirt clearance for a new or rebored engine is in the range 0.0006" to 0.002".

The ring gaps for new rings in a new bore are specified as 0.008" to 0.014" for the top and second rings, and 0.006" to 0.014" for the oil rings.

If you have any further questions about these dimensions, or about the engine overhaul, please ask them. We will be glad to help.

Thank you! Perfect! That helps me out a lot!

You are more than welcome, cngodfather. I'll leave this thread open in case you have questions as you rebuild the engine.

The largest measurement of bore diameter is 2.679" The cylinder has varying measurements throughout the bore. In your opinion should I get the next size piston or go to the second so the machinist can get enough material out to make sure it is centered perfectly? Thanks.

I knew I was in trouble when I bought a set of stock sized rings and the gap was too large. It is only a few yrs old so I thought I would just re-ring it. Or at least hone then re-ring it. I placed the ring in the cylinder from out of the box and it was .026 ring end gap. . Bummer.

It isn't as bad as you may think, cngodfather.

First, Honda's service limit for GX160 ring gaps (for when you are checking used parts for re-use, or re-ringing without boring oversize and fitting a new piston) is 0.040" for all rings, and you are well within that. I have found with another small Honda engine that oil smoke does not appear until the oil ring gap is rather more than the 0.040" limit.

Second, some replacement rings routinely are made with smaller gaps than original equipment rings, so they will work satisfactorily in fairly worn bores. I was once re-ringing a GXV120 that had 0.007" of bore wear, and I found a set of cheap aftermarket rings from Thailand that gave me gaps ranging from 0.015" to 0.017".

What would you do in my situation? 2.676 average cylinder front to back. 2.670 side to side. . piston needs replaced due to wear around ring area. cylinder is in good visual condition. Thanks

It is normal that the cylinder bore diameter will be largest about 8 mm below the deck height, measured at right angles to the crankshaft axis. The wear will be greatest in the "major thrust direction", with very little in the opposite direction, so nearly all of the wear will be on one side of the bore: it will not be worn symmetrically on both sides. This is because when the crankshaft is just say, 20 degrees past top dead center, the combustible gas in the cylinder has largely burned and the pressure in the combustion chamber is very high, while the angle of the connecting rod causes a large sideways force on the piston, against the cylinder wall. This high rubbing force causes wear on the cylinder bore in that position.

A brand new standard bore should be 2.677" diameter. You report that yours is much smaller than this, along the line of the crankshaft axis (the direction in which there is no thrust and consequently very little wear). It is not possible that this dimension is less than 2.677", so it appears that your measurement is inaccurate. Did you use an inside micrometer? If so, did you check it with a gauge or against a known-accuracy outside micrometer? Are you sure that the screw-on end-cap is not loose? If it were loose, all of your readings would be erratic, not only in absolute terms, but also relative to each other.

Let's posit for a moment that your micrometer has an incorrect zero, and what you have found is that the bore is standard (2.677") along the crankshaft axis, and 0.006" more than this (2.683") at right angles to the crankshaft axis, measured about 8 mm down the bore from the top deck of the cylinder casting. That would mean that you have .006" of bore wear at the most worn point. The bore diameter at right angles to the crankshaft axis usually tapers down from that maximum as you move down the bore, and is usually not measurably worn down at the bottom, below the lowest point the piston rings travel to. The service limit for bore diameter is 2.684", so your wear is only 0.001" less than the service limit. If this is the true situation, it would then be possible to re-ring the engine and put it back to work still barely within Honda service limits. However it would last considerably longer if you rebored it to a larger diameter.

There are two ways a cylinder can be "rebored". The way it is usually done in service centers is to perform a sort of glorified honing operation, with the hone guided by the existing cylinder bore, rather than following the original centerline. This operation is relatively cheap, since it uses only the rudimentary equipment that most service centers have. The bore would only need to be increased by one piston size (0.25 mm, or about 0.010"). On the other hand if a genuine rebore was performed, it would require using a very rigid boring head in a milling machine that was in excellent condition. If the setting up of the boring job was performed very accurately, it would be just possible to clean up the bore if the boring bar reached out at least 0.006" further radially than would be called for by the standard bore dimension. That is, the bore diameter would have to be increased by twice 0.006", or 0.012". This is not a standard rebore dimension, so you would have to go up to 0.5 mm, or 0.020", oversize, so that you could use ordinary oversize piston and ring set. Oversize pistons and rings are available up to 1.0 mm, so this does not seem to present any particular difficulty.

The questions to consider are:
- how important is it to have the engine last a long time? Usually in home service, with regular scheduled oil changes, cylinder wear happens rather slowly. Honda engines only become as worn as yours now is, either through long service as contractors' mowers, or in home service if the oil has not been changed in a long time.
- if you want the engine to last a long time, do you actually need it to last as long again as it has lasted up to now? If you would be satisfied with, say, 60% of the time it has lasted up to now, you could adopt the glorified honing approach rather than a true rebore. This might be quite a bit cheaper than having a competent machinist set up the job accurately in a milling machine.
- if you need this machine to last like a new one, you need at least a true rebore - but it might be cheaper to get a new short block instead.

Ok cool. I will order a 0.020 oversize piston set. I will keep you updated on the progress. Also, I was using a dial caliper and starter telescoping gauges. I did find a .002 problem with the caliper I was using.

I'll look forward to further reports then. Please let us know which kind of "rebore" you choose - honing, which produces an S-shaped cylinder, or boring, which is expensive. Some pictures of the parts, the machine tools, and the boring process would be very useful for our archives.

To be honest , I will probably choose a hone. I am kinda anal when it comes to my jobs. I am kinda easy for the first part, but the end result is what really matters. I will buy a cheap hone and work it slowly until the clearance is what I need. I do understand that it will take hours. Kind of a hobby I guess. I will let you know if I mess up or not. Kind of an experiment for myself. Will try to get good measurements in the end to display. Piston is ordered. May be a few days...

Grumpy, if I can make a statement... You are the reason these forums are a great help to many people out there. It is nice people that can help someone that they do not have to ,that gives me confidence that there is hope for this planet. I try to help people too. It seems like most people are out for themselves. I guess what I am saying is thank you again and keep on keepin' on.

cngodfather, I think most outdoor equipment rebores are done with hones, because it is much more cost-effective, and we can each have the fun of doing it ourselves, which is a major part of why we get involved in the first place.

We all share an interest in the equipment we work on, and we get together in places like Outdoorking because that just extends our opportunities both to learn and to help people. When you think about it, it is a pretty good deal for all of us when we can be useful and enjoy doing it.

I'll look forward to hearing more from you as your work progresses.

Hear!Hear! Totaly .

so far just started to hone out the cylinder and my cordless drill's battery went dead. The new piston measures a few thousandths wider at the base of the skirt. is this normal? It is brand new.

Pistons are normally undersized at the top, near the ring grooves, to leave clearance to accommodate the oil scraped off the cylinder wall. The skirt is usually oval, because it only needs to be a close fit on the two thrust faces, at right angles to the gudgeon pin. By making the skirt smaller than the cylinder diameter directly below the gudgeon pin, there is less friction. More importantly, with careful design the piston's thermal expansion can be made to only vary that diameter, parallel to the gudgeon pin, not the diameter across the thrust faces. Automotive pistons have been oval at the skirt in that way since long before WW2. Round pistons would have to have a lot of clearance across the thrust faces to accommodate thermal expansion, and as a result they would "slap" very noisily on cold start-up. There are round pistons in some crude engines, but it is wheelbarrow engineering.

ok thanks. I was kinda worried. I would assume that I should enlarge the bore to allow the piston/cylinder clearance for the largest part of the piston at the skirt?

Thanks again
Chris

Chris, the objective is to get the cylinder the same diameter all the way down, and with its axis as straight and as vertical as possible. With a hone you have no chance of getting it straight and vertical, but you can try for constant diameter. The piston skirt travels most of the length of the bore, and it should have the prescribed clearance the whole way. That will also give the other parts of the piston the clearance they were designed to have.

Well... The cylinder did not hone very well. after hours and hours of honing, the piston finally enters the cylinder. It slides very well if it is rotated 90 degrees. It still is too tight to slide in the correct way. I just bought a slightly used block of ebay for $30. I am going to reuse the original piston with the new rings from earlier.