OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk Honda GXV120

Hi guy's Ive been working on a Honda mower with a GXV120 engine..I got this mower a few years ago for free and it run's really well for a mower that's had a hard life and done a lot of work..But curiosity has got the better of me and I like to tinker so Im pulling it down to see how it all work's and give it a good going over and a little paint...Quality is very good with this engine and It all checks out good so far but will know better when I split the crankcase...

Here are a few pics of the head Ive cleaned up,its a well designed setup and easy to work on..

Watch the width of the valve seat contact areas with that engine, and measure the maximum bore diameter. It is very unlikely the bore will wear the 0.010" to Honda's service limit, but after very long use the valve seat contact areas can increase beyond the 2.0 mm maximum. The correct width is only 0.8 mm, and it is easier to maintain a good seal in service if you stay at around 1.2 mm. Once you get an excessive contact width you need a set of valve seat cutters to put it right. Check the valve stem fits in the guides as well: some engines end up with poor fits, especially of the exhaust valve stem. Maximum allowable stem to guide clearance is 0.013" inlet, 0.014" exhaust.

The GXV120 is my favourite small Honda engine. The GXV140 runs at least as well, but has a plastic camshaft, which I don't much care for as a long term proposition. The GXV160 I just don't like. I can't tell from your picture whether you have a single height-lever model (Australian-made base) or a 4 height-lever model (Japanese-made base). I much prefer the 4 lever model due to superior durability, but in normal home use you aren't likely to wear out either type. Personally I don't tolerate bar blades: it isn't that hard to convert to Honda's 4 blade swing-back system which seems much less inclined to damage the base from stone impacts, and gives a much better finish to the lawn.

Thanks grumpy for the service limits,the valve seats are inlet at about 1.3mm and the exhaust at about 1mm I take it the inlet is more from dust over time maybe?...Mower is single lever model but does have 4 blade swing-back setup..

Chris, I must comment, really nice work, as always....can't wait to see it finished, mate.

I don't have a reliable explanation for the greater seat width you have found on your inlet valve. The seat width increase is associated with "valve recession": the valve works its way deeper into the seat over time due to wear on both valve and seat, hence you have to adjust the tappets annually. Generally, because exhaust valves run much hotter than inlet valves, and because they are smaller diameter and therefore have less seating contact area, it is exhaust valves that suffer the worst recession and seating width increase. However engine manufacturers use a harder material for the exhaust valve, and sometimes also for its seat insert, to prevent this from happening. In the end, either valve may recede more, you can't tell until you test it.
Post-edit: The GXV120, and the rest of its engine family, have extremely hard exhaust valve seat inserts. Honda specifies a diamond cutter for re-cutting them. I made an attempt on one with a normal Warren and Brown hardened cutter, and made no impression at all - had to resharpen the cutter, too. The inlet seat in the same cylinder head was very easily recut with the same W&B cutter, it behaved like ordinary cast iron.

Sounds as if your mower is an HRU194, and it hasn't done an enormous amount of work. It may need new piston rings - you will find out when you check the ring gaps.

Thanks Deejay,and thanks grumpy for the great information very helpful....I split the crankcase found some nice black sludge,just have to clean it all and do some checks to see what is needed if any in the way of parts....

If you haven't given it a first-stage clean-out before those pictures, it is remarkably clean. Some pictures of the big end bearing in the connecting rod would be useful, to see how the lubrication has held up, but the most important information is the piston ring gaps. These are measured by removing the rings from the piston, inserting them one at a time in the top of the bore, and pushing them down 1 cm from the top, using the top of the piston to keep them square to the bore (otherwise the gap doesn't mean anything). The gap can normally be measured with feeler gauges.

The service limit for the gaps, for all rings, is 0.039" (1.0 mm). However rings in good condition should be about half that. For new rings in a new bore, the top two rings should have gaps of 0.008- 0.016", and the oil rings, gaps of 0.006- 0.014".

I can't see the oil ring well enough to interpret whether it is a one piece cast iron ring (early production) or two steel rail rings with a separate expander (late production).

Thats as it was was when I split the crankcase all I did was drain oil before hand...here are some better pics of said parts, looks like something foreign and hard has gone in with the oil and scored rod bearing face..I will get ring gaps next, I will use top of piston to get rings square to bore...

That mechanism on the camshaft/timing gear is the decompressor. It looks good in the pictures. The rod bearing looks as if it has been assembled with dirt/swarf either in the bearing journal or in the oil hole in the rod, but there are other possibilities, such as inadequate oiling of the journal before assembly.

You have the cast iron oil ring that was used on most GXV120s. It is not too badly worn. Here is the oil ring from one of my GXV120s:

That is what an atrociously worn cast iron oil ring looks like. It had a gap of 0.142" or 3.6 mm. It used 3 mm of oil, measured on the dipstick, in about 1 minute from a cold start. By then it had oiled the freshly cleaned spark plug, and stopped running. My whole street was smogged out.

Some contractors get the very last out of their Hondas before they scrap them. Yours does not seem to have done a whole lot of work.

grumpy I can see what you mean looking at the oil ring I have compeard to the very worn one in your pic...ok with mine 1st ring and oil ring are on service limit at about 1mm,2nd ring is over at about 1.5 why its like that I dont have a clue...I will get rings for it and new seal's /head gasket and I should have a good engine that will do many more years work... bore looks good with a shadow of a lip I cant even feel with a fingernail, I was surprised to see what the ring end gaps where especially 2nd ring...

You haven't given a figure for cylinder bore wear so I can't comment on whether your rings lasted as well as I'd expect. It sounds a bit unusual though, Chris. Because the top ring is chromed in Honda's original equipment ring sets, it normally wears a lot less than the other rings. The oil ring wears most. The second ring wears fairly quickly until it loses its taper, then more slowly after that.
Here are the ring gaps from a mildly worn GXV120 (0.002" bore wear) as it was when I received it, smoking just slightly on heavy loads or when increasing engine speed:
Top ring (chromed): 0.029"
Second ring (tapered): 0.057"
Oil ring (cast iron): 0.121"
Compared with the service limit for all ring gaps of 0.039", it is unsurprising that this engine still had perfect compression, but was burning enough oil for my wife to object to the smell and smoke (which was less than a 2 stroke of course, but was noticeable).

Here is Honda's diagram of the ring design:

The taper on the second ring is so that it can act as an oil scraper ring.

You can get two types of rings for it, generically. The cheaper option is the Thai ones which are very like the original ones except the top ring is not chromed. I have used them in a GXV120 with a rather worn bore (0.007" of wear): their advantage was that the ones I got at least, had a smaller ring gap than OEM rings, so I had a ring gap at the high limit for a new engine, despite the worn bore. They worked perfectly in the engine (no oil consumption, and compression figures identical to mean specification in the workshop manual). I only ran that engine for 4 hours though, so I can't comment on longevity. In the case of the engine with only 0.002" bore wear, I used original equipment rings obtained from the vendor instead of from Honda:
http://www.ebay.com.au/itm/OEM-Genu...mowerAccessories&hash=item485700105e

The attraction of the OEM ring set is that it is late production so it has chromed steel rail oil rings, the same design as for a car engine. I expect these rings will last longer than I will. I've only run that engine 2.5 hours so far. It runs like a new one, of course. IMO your rings are too worn to even consider putting back in the engine. If I were you I'd use the NPR rings in your engine, but I'm not you, so obviously you'll make your own decision. Note that the NPR rings are intended for an original bore size, so they will give a larger ring gap than the Thai rings, which are intended for use in a worn engine.

Thanks for your help grumpy, I have nothing to measure bore wear with...but im starting to think this engine has been pulled down before cleaned bore for a good look and in the lower part looks like its been hit with sand paper it dont look factory...not had time to work on it but when I do will do update of where Im at with it....

Factory bores normally have honing marks in them. The honing marks are eventually removed by the piston rings, except at the bottom of the bore, below the lowest point the rings reach. However factory honing normally is very even, and the marks should cross each other at an angle of around 60-90 degrees. Aftermarket honing is seldom even.