View Full Version : honing a cylinder

03-14-2010, 11:33 AM
We've almost finished our Star engine project, but have a small problem: we can't get 'em to run. Although I think most of the problem is the carburation, one issue is blow-by from the piston/ring.

I bored the cylinders in the B'Port with a boring head and the finish leaves something to be desired, it appears almost "mottled". This is probably due to chatter, as the boring bar I used isn't as large as would have been nice. At any rate, a brake cylinder hone doesn't seem to do very much and there's significant blow-by.

Any ideas on how to fix this? The ideal would probably be something like a jig grinder, but my shop doesn't sport one of those. Would a Sunnen-type of hone do the job? A friend has one of those, but I don't know if he has large enough hones, as he's usually doing very small stuff. How about the old split dowel with abrasive cloth?

Here's Debolt's page on the Star: http://deboltmachine.com/_wsn/page3.html

We've previously done the Vaughn and Olds Gearless engines from Debolt (see: http://deboltmachine.com/_wsn/page8.html) and both run great after sorting out the carburation. My Olds has over 850 operating hours on it at present, the Vaughn is over 700.

Tim Clarke
03-14-2010, 11:49 AM
You don't say what the bore size is, but Sunnen hones in one form or another will cover a huge range of sizes. You'll need a micrometer type hone, rather than a spring loaded style, which will follow whatever shape the hole is now. By your description, I'm guessing the hole is somewhat out of round.


03-14-2010, 12:02 PM
A manual horizontal Sunnen hone can comfortably hone up to 2" or so (even though technically Sunnen has mandrels that go up to 6").

If it's much larger than 2", I'd guess that an automotive (vertical) power-stroking Sunnen machine might be a better bet.

Getting accuracy is no problem -- I regularly hold .0001" (size, straight, round) on a ~2" bore that is 6" long on my Sunnen.

03-14-2010, 12:20 PM
Something like this?


03-14-2010, 12:51 PM
The bore is 1.5", leaving out the Northern Auto Parts gadget. I think I'll see if my buddy has a Sunnen mandrel that goes to that size.

Although the bore could be out of round, I don't really see how that's possible with the B'Port boring head. A friend suggested that maybe the casting had hard spots in it from having some mystery metal mixed in the melt.


03-14-2010, 01:42 PM
for that size a sunnen rod resizing hone would do the trick. Peter

03-14-2010, 03:19 PM
Well, you have a problem. I don't know what clearance you bored it to but a bored cylinder on the BridgePort should have been a nice finish in cast iron.

If you bored for .002: clearance it should have worked. If your getting blow by now when you hone it the blow by will not go away, it may be worse. You will probably have to make another piston and rings for it.

The bigger you make the cylinder now the worse things will get.

How much clearance do you have between the piston and cylinder now?

What is the end gap on the rings when they are pushed into the cylinder with the piston so they are straight?

Tim Clarke
03-14-2010, 08:40 PM
Personally, I don't have anything to accurately guage a hole in this size. If it were mine, I'd make a plug guage out of whatever was in the scrap box, just a few tenths under what bore size is suposed to be and see if I had any tight spots. The OP has said his buddy thinks there may be hard spots in the casting. If there are indeed hard spots the boring tool may have jumped over them somewhat. I've machined enough brake drums with hard spots to know all too well how that goes. If there are areas that look funny, which there should be with hard spots, then they should be findable with a snap guage.

In any case, a few licks with a micrometer type hone will show whats going on with this bore.

It really doesn't matter much about the piston clearance. Maybe a bold statement, but it's a lot easier to make a new piston than make a new bore. There's always knurling to take up a little extra clearance, and the knurl will help to hold a little more oil. Piston ring end gap will doubtless increase as the bore becomes larger. Wanna start a big arguement? Get a few engine rebuilders together and state that you think that .004" per inch of cyl bore is a perfect ring end gap.

If it were my engine, I'd hone the cyl to whatever size it cleans up to. Then I'd put it together and see how it runs. If the rings won't seat in or the piston slaps, then I'd take care of those problems. I've driven engines 100,000 miles with theoreticaly too much piston clearance and too much ring end gap.

Anyway, nobody likes to spend what this casting kit costs, and have an engine that won't run. I personally don't think he's far from getting it sorted out.


Tim Clarke
03-14-2010, 08:46 PM
Carl made a very good point about ring end gap......... Are we sure this is the correct ring for this Bore? Does it have tension in the bore? If it's got a ton of end gap, it's probably for a smaller bore.


03-14-2010, 09:00 PM
The ring IS correct for the bore and has tension. The gap is less than .004". As far as piston fit, it's quite good, less than .002" clearance (.001/side). If we have to hone the cylinder out a couple or 3 thousandths and the piston is too loose, it's not that big a deal to make a new one. After all, we made the first one.

I still think the main problem with the engine not running is carburation, but the few things I've already tried haven't worked.

03-14-2010, 10:39 PM
From what your saying I don't think you have a blow by problem. I have used a squirt can with gas in it to start an engine and keep it running until I could get the carb adjusted. I don't recommend using either to do that, just gas in a squirt bottle.

You could also use and electric motor on the output pulley to run the engine while your trying to get it started. Just use a pulley the size that would only turn the engine at about 100 rpm.

03-15-2010, 12:22 AM
The engine on my chipper is hard to start. I would nearly kill myself pulling the start cord. I took the recoil housing off, now I use a cordless drill with an adapter and 3/8 drive socket on the crankshaft nut. It still takes a while to start but gets going a lot easier by turning continuously. Also I'm not having a heart attack by the time it gets going.

03-15-2010, 05:13 AM
Your piston ring ends should not be aligned.
For example, the top ring ends should be positioned at 12 o'clock, the second ring ends at 3 o' clock, and so forth, positioning the ends 90 degrees from the one(s) above it.
Aligned ring ends will certainly blow by compression.

An auto manual from the fifties recommends spooning small amounts borax into the carburetor of a running 6 cylinder engine to aid the seating of the rings in a troublesome motor, producing blow by.


03-15-2010, 07:33 AM
The Star engine has only one ring, so gap alignment isn't a problem.

I've heard of using Borax or Bon Ami to seat rings, and may try that.

We have "motored" the engine by belting it to the mini-lathe. It's possible to get it firing, but it doesn't want to continue to run. Yesterday, I managed to get it running and "on the governor" a couple of times, but just for a minute or so.

Will keep you posted.

03-15-2010, 08:26 AM
Carburetion is a good place to look. These engines will run with pretty low compression as long as they don't have to do any work. Once running, the rings seat and the cylinder will carbon un and compression will increase.

I have the same problem with my John Deere and think I am going to have to bush the venturi section of the carburetor to increase velocity. This is what I did on the Fairbanks Eclipse and it made a big difference. This is an area that does not necessarily scale well.

The Fixer
03-15-2010, 09:37 AM
Do u have compression...first thing to check. Blow-by is NOT caused by alignment of ring end gaps it's caused by excess ring wear/ lack of ring tension. I've seen literally hundreds of running engines torn down for 'other' reasons that have had the rings end gaps aligned as they do turn over time and not cause any blowby issues. I try to stagger them when I assemble but it's just habit from when the mind set was different. (the old days! )
Did I mention I'm an automotive machinist? lol

are u sure about your spark timing? and yes it could likely be carb issue if u can run on sprayed fuel.

03-15-2010, 02:44 PM
Years ago when I worked at a Cummins distributor the Cummins factory did a test on the ring gaps in line theory because they were having a major oil consumption issue. They pined the rings with the gaps in line and in various positions and found that the alignment of the ring gaps had no effect on blow by or oil consumption so you can forget that idea.

What they did find out was Chevron oil company changed the chemical composition of the Delo 400 without telling Cummins about it. At that time Cummins and Chevron had an agreement about the composition of the Delo 400 and Cummins recommended using Delo 400 exclusively.

When Cummins discovered the change in the oil Chevron got sued because for several years Cummins suddenly experienced bad oil consumption and they tried everything you can imagine to cure the problem not even thinking oil was the issue. I would guess the chemical engineer that changed the compound was fired. You would not believe the things Cummins did to try to fix the problem.

Cummins no longer recommends Delo 400.

03-15-2010, 08:22 PM
I've made several mods to the carburation, all without any improvement in the operation. You're absolutely correct that the carb doesn't scale properly and changes will often have to be made. Of course, Debolt should have figured it out, and apparently DID, at least for their engine(s).

We built 3 of the Olds engines (one to sell), and had an awful time initially: two of 'em ran after a bunch of fussing around, but didn't start reliably, nor easily. The 3rd wouldn't run at all. I spent a bunch of time visualizing the air and fuel flow in the carburator before deciding that a venturi of sorts was called for. The machining and installation was trivial, and the idea was a 100% success with all 3 engines. Not long ago, I received an email from a fella who had built an Olds like ours and had trouble getting it to run. A few days after telling him what we had done, I got a reply with "Thank You!" all over it.

Yet, Debolt's show model seems to run pretty well, presumably built per the drawings.

FWIW: We have a road trip scheduled to Zanesville, OH to see what Debolt can suggest. They've always been very helpful; they want their customers to have running engines. That's one big reason we keep returning to them. We love their models and the way they run, as you know, we run 'em all day, every day at the shows.

Fixer: the spark timing is right at TDC, about where it should be. Plus or minus 5 degrees or so makes little difference on these engine, advanced will give a bit more power, retarded will allow slower speeds. Slower is usually desireable. They do have compression, and it's improving as we "motor" them. It's almost certainly carburation, but I'm kinda out of ideas at present.

03-15-2010, 08:39 PM
That's interesting. I have a kit for the Red Wing Thorobred engine I am building for a friend. It seems like everytime I get ready to work on it something comes up to interrupt me. Now I am wondering if I will have carb problems too.

03-15-2010, 09:13 PM
It's hard to say if you'll have problems. If your model is common, you might be able to find help (if needed) from other builders. Debolt's models aren't that commonplace, as they're relatively expensive and are made in limited numbers. Right now, only the Olds and Star models are available from Debolt.

I did see a Vaughn model at Zolfo Springs a week ago that the owner wanted $1500.00 for, just the price he paid for the mechanic's kit. Sounded good to me, but I already have one. :-)