My Magnum .91fs has at least 100 run hours on it, possibly 150. I'm thinking of changing my bearings. While I'm at it, a new ring too. It runs great and has good compression and conventional thinking is "if it ain't broke, don't fix it".

Do you guys change your rings and bearings on a routine engine hour accumulation or just wait until it's showing signs of wear? I would rather go ahead and change bearings before an issue arises......

If it ain't broke don't fix it.

$13 shipped


Frank Bowman

1211 N Allen Ave. Farmington, N.M. 87401-3568

505-327-0696 Email: [email protected]

I wait until they show signs of wear before changing out any major components. I can't see wasting the money.

Yeah, I agree that a fairly new engine would be worth rebuilding if the parts cost wasn't too high. If the engine is that new, why not send it in under warranty?


Ed Cregger

What makes the Bowman ring better than the stock ring?

I can see where some engines wouldn't be viable to rebuild but here it's different, IMHO.

New Magnum .91fs - $190, $140 if caught on sale.

New ring and bearings - $25 total. If that's all it needs then to me it's definately worth it but I do agree that everyone is different though.

The alloy of steel that Frank uses. I don't believe he uses Iron, I think is Chromoly steel. And, if he custom fits the ring in the engine, the ring gap will be as tight as possible compared to stock rings.

I am not being critical at all, since I plan to buy rings for several of my engines from Frank, but, do his rings cause any more wear in the cylinder bore due to their superior alloys used? TIA

I'm with you Ed. I can see changing out a set of bearings or a ring, but if it's so far gone that I need a piston and liner or a crank it's time to buy the best brand engine of all time.

"Brand New"...

I'm usually tired of the engine buy then and I'm ready for something else anyway. Of course there are exceptions, sentimental value, multi's, etc. but for general sport flying. I’ll give an old engine to some kid at the field and ask him to “see what makes it go�…..My ole man used to buy junk at garage sales and bring it home so I could “operate� on it.

Hands-on experience, it’s one hell of a learning tool.

I've never yet had to change a bearing (all castor ) but I like to check every so often that they feel nice and smooth just in case. With rings I'd only change them when the engine gets harder to hand start. I've only done this once with an Enya 45 that had a couple of hundred hours running and was no longer starting with one flick. Once running it was as good as ever but because it was used for competitions it was necessary to have fast starts. I've never owned a 4 stroke though so I'm guessing it's never hand started which makes that method of recognising worn rings impossible.

I can see changing out a set of bearings or a ring, but if it's so far gone that I need a piston and liner or a crank it's time to buy the best brand engine of all time.

Idunno bout that, it depends on the emotional attachment Ihave to that particular engine.

I have a Saito .82 and, in the process of learning how to tune it (my first 4-stroke), I know I had a few lean runs. It doesn't seem to have quite the power it initally did. Would a new piston and ring be beneficial? I haven't had it apart yet to actually see if there's any problem; just a feeling.

I hand start my four strokes, but you also have other leakage paths to deal with.

You could check the ring gap, but you'd have to know what it should be in the first place. OS seems to follow the common recommendations. The Magnums I've measured had large ring gaps new out of the box.

You should check the valve clearance. If it's a little loose, Saito tend to lose a little power. You probably won't get a lean run on a four stroke, many will let you know they are lean by knocking or throwing the prop.

What a cool dad you had! [8D]

Thanks, I'll check them again. I did check after I'd run between 1/2 gallon and 1 gallon thru it, and they were just a little loose. Might have broken in a little further; will check them again. My biggest problem has been with the carb barrel retaining screw; the original wore and allowed a bunch (like 1/16")of play; put a screw from my RC car supply in, and it wore too, probably because it was just mild steel. I've got the Saito screw set on order; not sure what I'm doing wrong there...

There's supposed to be a spring behind the barrel to keep out the play

That's still there, but there was still quite a bit of play. When I put the new screw in, it was much better, and made my low speed needle seem a lot more consistent. Hope I haven't worn the guide groove; it doesn't appear so, but I'm not sure.

If you wait too long before changing the ring, the bottom ring land will have worn too much, and a new ring will not have a proper seat. A new piston is due in that case.
Better to change the ring before it is worn too much. (max ring gap 1mm, which is about 0.15mm wear on the ring cylinder face)

Check the rigidity of the engine mount. Mounts that aren't rigid enough allow the engine to rotate (vibrate) along the crank axis while running and the throttle barrel bounces around in the bore. This wears or breaks the retaining screw. The barrel is usually full hard so it probably won't wear.

Pe, is 1mm of end gap is acceptable in a <25mm bore engine? An OS FS-91 begins life at about .003", 0.08mm gap. A Magnum FS-91 I measured after 400-500 hours had a gap of .012", 0.3mm, the OEM replacement ring gap was the same! The bore was fine. I happen to have a NIB Magnum 91 at the time and it also measured 0.3mm.

HI the following applies only to two stroke glo engines-as that is all i fly-IF you are running 20 % castor in your fuel you should not need to change your ring or bearings for some time at 100 hrsloss of compression is the sign you need a new ring FRANK BOWMANS rings are great -cheeper have more spring to them also and will bring the engine back to new compression in most cases- do follow FRANKS instructions on ring replacement-bearings will get noisy and rough-that is a sign they need replacing-before they come apartif they come apart they will cause other internal damage metal thru the engine do avoid this-
one thing i do find very odd-i have two engines that have very low compession when cold-yet they both perform as they always did when i run and fly them i will continue to run them this way until they have problems running the plane in the airi am handling glo engines for 26 yearsi do think that the heat produced by the engine running expands the piston and may have the effect of increasing the compression that wayone engine is an abcand one is ringedthe linner on the ringed engine is of such hard material -that i do think the ring will be the one to wear first of the two surfaces-so ring replacement when needed has worked for me many times in the pastcompression often comes up to that of a new engine ENJOY REGARDS TONY "the omega man " think pink ! !

Greg, The value is based on 0.15mm wear on the ring cylinder contact face. Some engines indeed need a long period of running before this value is obtained.

A more sophisticated number would be 15% of local ring cross section (Tring). This accounts for about 5% new condition play with ring ends closed.
Influence on ring gap would then be 2*0.15*Tring*pi for those who like to calculate.
The ships diesels I worked with had 35mm ring sections, 15% of which is 5.25mm. This would be a replace ring gap of 25.7mm. Close enough to the 20mm according to the maintenance books

Pe, Thanks for that. I've not seen that explanation before.

You are welcome.
Most of the engine stuff is universal when size and volume is accounted for. It is the non-universal things that keep designers busy after product release

On full scale engines you check for that wear by placing feeler gages between the ring and cleaned out piston groove. I suppose you could do this with a model engine as well, but I would have no idea what the proper clearance is.

Sport, That is the axial play of the ring you talk about. The dimensional wear I talked about is the radial wear. You can either measure it by ring end gap measurements by placing the ring in a part of the bore that does not wear, OR you can measure the ring cross section directly. For the latter measurement you need to know the dimensions of the ring when it was new (very often not known)