Back to the 2 ring pistons on Towers of Power

[Texasmark]

Just wondering about those pistons. The original Tower (including Direct Charged) had cam turned pistons, 3 square rings, and a crown to help combustion gasses go where they were supposed to go.<br /><br />Loopers came out with 2 wedge shaped rings on a flat topped piston, with wedge shaped ring grooves.<br /><br />My curiousity is what the Tower replacement looks like.<br /><br />So, you guys that have been involved in the mix and match, would you mind elaborating on what the piston looks like and what effect on engine performance the 2 ring has when mixed with 3 ring due to the stated (by someone on here) difference in compression ratio?<br /><br />Sure numerous folks, besides me, would be interested in the results.<br /><br />Thanks,<br /><br />Mark

 

[Texasmark]

Re: Back to the 2 ring pistons on Towers of Power

Just moving this back to the head of the pack to see if I can get any hits.<br /><br />Mark

 

[Clams Canino]

Re: Back to the 2 ring pistons on Towers of Power

The mercury 2 ring replacement pistons look the same as the 3 ring type. <br /><br />The rings are beveled on one side only. The new design is supposed to resist coking around the rings a bit more. Performance is the same.<br /><br />I use the Persan made 3 ring replacement pistons as a rule. Same exact piston as the original Mercury 3 ring - only a bit better alloy.<br /><br />Never use the Wisco replacements in an inline 6, too much wrist pin clearance in the pistons.<br /><br />-W

 

[rodbolt]

Re: Back to the 2 ring pistons on Towers of Power

piston ring design is a function of sealing and heat transfer, over the years the 2cyl,3cyl,4cyl and 6cyl have used 2 and 3 ring square designs, some of the modern 2 ring designs are square some have a keystone or semi keysone top ring.<br /> the keystone type has the advantage of using compression pressure to aid in expanding the ring to the wal and sill allowing the ring to move in and out of the ring groove to aid in keeping the groove clean.<br /> the 3 ring piston has the advantage of better heat transfer from the piston to the cyl wall but suffers a parisitic loss from friction, the one ring design used on a lot of racing pistons has the advantage of low frictional losses and the disadvantage of severly shortened life . the 2 ring design seems to have worked the best at both ends. the keystone type top ring helped as well. the keystone type rings only seal in one direction so the second ring is square on a two stroke for primary compression issues.<br /><br /> <br /> confused yet

 

[MercFan]

Re: Back to the 2 ring pistons on Towers of Power

Yep. Agree totally with Rodbolt. Tapers point to the top top expand the ring under compression stroke. Also, during combustion stroke the rings are kept expanded and firmly pressed against the bottom of the groove. Hence no vertical slap movement on the rings.

 

[Texasmark]

Re: Back to the 2 ring pistons on Towers of Power

Okayeeeeee.<br /><br />Thanks guys for the info. Cept for one more thing. What about the difference in compression? Any noticable vibration of the engine?<br />Mark

 

[Texasmark]

Re: Back to the 2 ring pistons on Towers of Power

Okayeeeeee.<br /><br />Thanks guys for the info. Cept for one more thing. What about the difference in compression? Any noticable vibration of the engine?<br />Mark

 

[rodbolt]

Re: Back to the 2 ring pistons on Towers of Power

nope, from the outside running it there is no difference in either piston design as far as two or three rings go. the only time you may see it is if they are mixed and matched in a motor. not all had the same weight but most were close.

 

[Clams Canino]

Re: Back to the 2 ring pistons on Towers of Power

I try to at least match piston types in each cylinder pair. Never had a problem.<br /><br />-W

 

[gungalunge]

Re: Back to the 2 ring pistons on Towers of Power

1974 1150 115 hp direct charge is my engine.I was Questioning the pistons two or 3 rings And the mix and match thing.I baught a used powerhead on ebay.I embarked on dissassembling the used block and am doing the full rebuild.I did not know the history of what I baught I just took it apart.There it was a mix and match piston engine. It sparked my attention.It had two 2 ring pistons and four 3 ring ones .Right now I am holding the two pistons in my hands. They are both high dome type.The top piston rings on both pistons are square type not keystone shaped.The pistons are placed side by side the top ring lines up equal so they start at the top equally.Both skirts measure 2.866 dia. 2 ring part number 745-5206 3 ring 7429-745.Stamped on them.The piston ring thickness is the same.060 inch.Now the thing that got me going here is that the 2 ring pistons seem to have scrapes or wear on the sides and skirts.The rings are sticky in the grooves both of them. The 2 ring pistons were stressed more.and so are the cylinder walls they ran in.The blow by fuel stains also show how the combustion was getting past the second piston ring. Now looking at the 3 ring pistons. Way less skirt wear "showing some but no scrapes" the 3 piston rings slide easy in all 3 grooves and the blow by stains are minimal.there is some but looks like 1/3 less.

 

[gungalunge]

Re: Back to the 2 ring pistons on Towers of Power

If I may add on more ..On top of the piston heads, I can see a detanation pattern blown into the carbon build up.The 3 ring pistons show a pattern 1.504 inches wide across top the piston head. <br />both 2 rings show At 1.145 inches.wide. Clearly a difference when looking at the pistons. This measurment may not mean anything.I just noticed it. How accurately the pattern measured out between the two.It probally means something. Like the timming may need to be different if you use all 6 of one kind . but in a mixed piston group up like this engine was. To me that shows an imballance in combustion . perhaps a timing setting some where in the middle may ballance that. If you had a mixed piston engine.

 

[gungalunge]

Re: Back to the 2 ring pistons on Towers of Power

Hey what if we put a top keystone Shaped piston ring on a 3 ring piston ??? Good Idea Or Bad Idea. If they even make one .Would it work?? Would be fun to try . To see What happens ..

 

[Texasmark]

Re: Back to the 2 ring pistons on Towers of Power

Hey Gung<br /><br />Your project is what sparked my curiousity in the first place.....back when you bought the block and noticed the difference in the pistons.<br /><br />Mark

 

[Clams Canino]

Re: Back to the 2 ring pistons on Towers of Power

Gung - very BAD idea. <br /><br />The ring lands in the pistons are cut very differently. Just make sure you have ALL Low-Dome or all Hi-Dome and you'll be fine.

 

[Texasmark]

Re: Back to the 2 ring pistons on Towers of Power

Ok Clams,<br /><br />You just hit upon the reason for my question. Would you mind elaborating on low vs high domed pistons in 2 or 3 ring configurations....and related compression?<br /><br />Thanks for those who need to know....and my curiousity.<br /><br />Mark

 

[Clams Canino]

Re: Back to the 2 ring pistons on Towers of Power

Hi and low dome are 1/10 of an inch different in the height of the eyebrow deflector, not the actual height o fthe flat-top of the piston. Dome height has no bearing on what ring style is used.<br /><br />All the ring species come in low-dowm and hi-dome. Compression difference is no more than 5-7 psi, all else being equel. The low-domes are the default replacement (if doing all 6) due to todays gas for less detonation risk. <br /><br />-W

 

[gungalunge]

Re: Back to the 2 ring pistons on Towers of Power

Thanks Clams .. I follow that. Notes i saw about the lower dome versions help extend life a little more I guess.A little less comperssion. The pistons I have here are the high dome versions. Thanks too Tex. I am going to post a note on my progress in another post.Lots to report on how things are going on my rebuild.A few weeks ago I had no Idea what pistons use following the good advise availble has helped me make up my mind .

 

[Texasmark]

Re: Back to the 2 ring pistons on Towers of Power

Welcome. Good luck on your restoration.<br /><br />Mark

 

[Clams Canino]

Re: Back to the 2 ring pistons on Towers of Power

With hi-domes "the rules" become more important. 21 degrees max advance, prop to make at least 5500, midgrade gas.<br /><br />-W