Engine bore measurements

Because it's the kind of cool factoid that makes life interesting.

Thanks for the figures and link, guys. Just what I need. It's strange that the original manual doesn't give any figures, but neither does it mention honing when replacing the piston rings.

All the ring gaps are much bigger than those figures. Too big to measure with a feeler gauge. I would estimate that they're in the 1 to 4 mm range (perhaps that's a factor in reducing the compression).

There are two compression rings and one scraper ring (all above the gudgeon pin). Should the end gap be the same for all the rings?

Many thanks,

Chris

It's actually a tenth of that: 0.0035". Hardly any ridge at the top of the cylinder. I showed the bore to a friend today, and like me he was just inclined to replace the rings (he wasn't keen on honing, though).

Chris

On Thu, 18 Mar 2010 00:41:41 +0000, the infamous Christopher Tidy scrawled the following:

If you don't hone, how do you expect the rings to seat? Unless the ridge is non-existent, ream and hone for safety and better life. YOu don't want to be having to do this over within the year, eh?

-- No matter how cynical you are, it is impossible to keep up. --Lily Tomlin

Mabee not HONE (which uses rigid stones) but at least DEGLAZE. The abrassive "bottle brushes" do a pretty decent job.

Sorry, that was what I meant. I have the kind of deglazing hone which has three spring-loaded stones and looks a bit like a camera tripod. I've never used the "bottle brush" kind.

Chris

That was a typo, I ment .0035", right on the edge for a bore job, IMO. Greg

On Fri, 19 Mar 2010 01:03:53 +0000, the infamous Christopher Tidy scrawled the following:

The spring loaded monsters work just fine.

-- Adults are obsolete children. --Dr. Seuss (Theodore Geisel, 1904-1991)

--

Chris,

Not sure what model but it looked much like the one in this youtube video

. Big heavy flywheel on it, may have been due to its use on a stationary crane, the flywheel probably weighed more than a modern 7hp small engine. IIRC the starting handle was a S shaped piece of bent bar, not made from flat like the one in the video.

Chris,

That's a Model 4/3. Smaller and faster running than my Model 5. I had a look at more pictures of these lawnmowers. I have now come to the conclusion that my starting handle is probably original, but that it and the dog in the centre of the flywheel are worn, making the handle a loose fit. It may also be missing a rotating grip.

Chris

You don't state whether you took the measurements with the old used rings, or with a new set of rings (I suspect the old used rings).

The top ring groove clearance is fairly bad, but should improve with the new top ring placed in the groove. If the groove clearance doesn't improve to almost .005" with a new ring, you should probably replace the piston. With excessive groove clearance, the ring is probably more likely to break, resulting in cylinder damage/severe scoring.

The ring end gap will be fairly large when checking with a used ring. It's a situation where the new replacement part becomes a gage. I think it's kinda odd that the ring end gap spec isn't included in an overhaul manual. It's been a long time since I was familiar with ring specs, but looking up ring end gap specs for several engines with a similar bore size should give you a good indication of what size the end gaps should be. When installing a standard size ring in a slightly worn cylinder, I wouldn't expect the ring end gaps to be too close (needing filing), but a comparison to similar engines may provide an appropriate range for the size of the gap.

Previous recommendations from others wrt replacing the valve guides and possibly new valves would likely be something you'd want to follow up on. The extreme heat that the exaust valve endures will essentially guarantee guide wear. For a 50 year old engine, maybe it's likely that the guides have been replaced before, but they're not expensive to have a new set installed at a machine shop.