Rings and Bore

[BIGHOSS]

Maybe some of you who have had more experience with re-boring cylinders could help us who have no experience. When you bore an engine from the original 2 5/8 (I think), is there another fixed diameter that you shoot for or do you just mill the bare minimum to clean it up?

The other question, is there a relationship between the new bore size and the correct rings to provide the necessary gap? For instance, if the bore is .060 over, do you simply get a .060 over rings?

There are plenty of good machine shops in our area that do not specilize in rebuilding engines. So, if I take an engine to them, I want to tell them what to bore it to and that is all they do. Then I go to a parts house and get the new rings. valves etc. I would need to know what size rings to get to match the new bore.

[wvpolekat]

You will need new pistons and rings if you bore it, not just rings.

Ideally, you would buy the pistions, bring them with you to the machine shop, let them mic the piston and bore appropriately.

[beaconlight]

Machine shop many times have better parts prices than you can get elsewhere. The idea is to bore the least to fit a standard oversize. 20, 40, 60 thousandths oversize. Then you replace the pistons for those ot that size. Some people expand the skirt on pistons. That is the bottom. You want the piston to go up and down in the cylinder with out banging the bottom of the piston against the cylinder wall. This is called piston slap. Oversize rings can have the ends groud off till you get correct end gap. I forget what is correct for a cub but it may be 7 thousandths.

Bill

[dracer398]

The rule of thumb is usually .004" per 1" of bore on the ring end gap.

Brian

[FuryIII]

since your going to have it bored at a shop, let them use there mic and tell you how much it needs to clean up the cylinders. that way you know what it will be when they finish. after you have the correct calculations from the shop youll be better prepaired for buying parts. my $.02

[Brandon Webb]

Buy your pistons first, measure them, they won't be exactly .60 over for example. Then bore to meet that size, and hone for clearance whatever's stated in the manual. If you just take a block in and say bore it .60 over. The pistons you buy might end up having to much play, or you won't be able to hone it enough to get good ring seat, for fear of making your clearances to big. Brandon.

[denton]

My machine shop owner has always treated me fairly. He has always been willing to tell me if I have to bore or hone cylinders or turn down the crank etc... He is very knowledgable and I just have to believe he has the experience. He does order parts before machining but after measuring so I would say that the advise the others have given is sound.

[BIGHOSS]

Thanks for all your good advice!

I gather from what I have just heard that you need to develop a relationship with a good machine shop with experience in boring and fitting rings, bearings, crank etc. Rather than send it to a "good" machine shop that may have limited or no experience with engines. The latter might be a little cheaper possibly, but you are taking a change with the parts fitting as they should.

Thanks again

BigHoss

[George Willer]

The rule of thumb is usually .004" per 1" of bore on the ring end gap.

Brian

Your rule of thumb would produce an end gap well within specs. On this engine the specs as I recall are .007" to .017".

.007" = ideal. Anything over .017" it's replace time. They should start out at minimum and they will increase to maximum over their service life.

[dracer398]

Thanks George,
I didn't know the specs for the end gaps. I did look after I posted on Rudi's manual server and seen that they could be tighter. Tighter is better to a point.
But I guess when the specs are unknown you could use the .004"/1" rule. The main thing is that you don't want the ring ends to touch and bind when in operation so I wouldn't go any tighter than the sugested minimum clearance.
Most of the time, the rings come with the correct gap as long as the block has been bored correctly.

George, do you know if they make rings for these that you can file the end gaps? I know I used to buy rings for my race engines with the overbore size +.005".


Brian

[cowboy]

Brian the std and +.010 are the same part for a cub. I had to get +.020 rings for my std bore engine and file them down.

Billy

[BigBill]

Are the cub pistons forged pistons or cast pistons? There are two different clearances for forged pistons and cast pistons. The forged pistons require more clearance than cast pistons. If the clearance isn't enough the piston skirts will crack when the swell with the heat. Make sure your piston skirts have that slot in the skirt too if the orginal pistons have it. I had a new set of 4 cylinder jeep pistons some with no skirt slot and someone made a mistake. I sent them back. Just make sure the replacement pistons match the looks of the old pistons to besafe.

[Clark Thompson]

I would have to agree with George 100%. The book gives the spec's and should be followed. However the book doesnt give the specs for the aluminum flat top pistons. for aluminum pistons you need a minimum of .004 clearence for just the piston.any less and your piston will seize up after the engine gets hot. when getting your piston rings you have to get rings for the aluminum pistons. they allow for the clearence needed for the aluminum piston.Most manufacturers stamp the clearence needed on the inside of the piston. The cast iron pistons dont need as much clearence so .003 is sufficient. The rings for the cast iron pistons that are .020,040 and so on will fit with no problem and provide proper end gap.A engine machine shop will know all the proper clearences. There is a place in arizona that will converet your second compression ring to a total sealed ring. I am doing this to a cub engine now and will let everyone know the outcome of this experiment.