High compression heads/pistons

[sparkroost]

my 1973 is supposed to have high compression heads and pistons. How can I tell if it still does? I am thinking someone had done this before to this motor.. they might have used lower compression pistons/heads.<br /><br />1973 115 115393M<br /><br />someone mentioned the top ring being a certian distance from the top.<br /><br />I took the block in today to be bored and they were skeptical as to that years piston.<br /><br />I will get the rod numbers tomorrow.

 

[Dhadley]

Re: High compression heads/pistons

The 1973 V4 powerhead is unlike any other year. The block, crank, rods and pistons are one-of-a-kind. The easiest way to tell is that the rods use the 70 HP style rod bearing. The 1974 to 1976 135 powerhead will bolt on as a complete unit but the internal components are not interchangeable. <br /><br />The ring location on early 70's V4's was very close to the top of the piston. Set up properly this combination works well. Anything that adds combustion temp is the mortal enemy of any 2 stroke. Especially the high ring piston motors. Pistons produced today for those motors have a lower ring location.<br /><br />That motor can be bored and pistons are available in +.020, +.030, +.044 and +.064 oversize. If the sleeve is cracked it can be resleeved.<br /><br />Good luck!

 

[SCO]

Re: High compression heads/pistons

I also have a 73 135 hp v4. Considering that we all use unleaded and these engines were designed for unleaded, what numbers should these engines be timed for? My service manual specifies 5 degrees advance at idle(I presume idle in the water in gear at 650 rpm) and 4300 to 4600 rpm in gear at 22 degrees advance. Thanks

 

[sparkroost]

Re: High compression heads/pistons

I would not know what to look for as for the style of bearing. I am concerened that someone else may have changed the internals on this one. The rod #317777. I can't find a number on the piston. the top ring groove is .1665 from the top of the piston's flat area. .4755 from top to 2nd groove. The top of the piston looks domed, but on one side there is a half moon protruding where it fits into the head.<br />I cannot find any number on the crank.

 

[sparkroost]

Re: High compression heads/pistons

2nd question.. That was a ******* year.. did they change the ring location to reduce combustion temps in later years for longevity?

 

[Dhadley]

Re: High compression heads/pistons

Sounds like you have the original 1973 powerhead. Order your pistons for 1973 115/135 and all should be well. Save at least one old piston and when you get the new ones hold them up next to each other. The wrist pin holes should be in the same location. If they send you the wrong piston the wrist pin hole will be very different in height.<br /><br />good luck!

 

[sparkroost]

Re: High compression heads/pistons

Thanks DH!<br /><br />I just found that out myself a few minutes ago from the machinist.<br /><br />Trying to get my ducks in a row!

 

[Dhadley]

Re: High compression heads/pistons

Just thinking - post the part numbers for the pistons hes going to order and we can tell if they are the right ones.<br /><br />Quack - quack - quack! They look pretty much in a row to me! Lol!<br /><br />Good luck!

 

[sparkroost]

Re: High compression heads/pistons

ok, I got my quote today. Not too bad! Here is the part numbers I got.<br /><br />Piston .020 3174P2<br />966 head gasket<br /><br />305587 weak spring<br />305591 strong spring<br />is there 2 springs? or I have the option of one or the other?<br /><br />2 pistons, full gasket set, 4 set of rings, 2 lower and 2 upper rod bearings, 235 gsc sealant and gel seal, thermostat, plate gasket, check valve, stat valve. all for just under 500.00 to my door step.<br /> I have inspected the bearings and am only replacing the the lower bearing and housing as well as the 2 small/big end bearings on the rods with new pistons. I did't see any reason to replace anymore bearings. Sound good, or am I half a$$ing this one? BTW the crank journals are pristine. The failure was in the lower bearing/housing. Sat too long and rusted the lower bearing.<br /><br />Am I missing anything?<br />Am I missing anything?

 

[Dhadley]

Re: High compression heads/pistons

Thats the right piston numbers. The rings come with the pistons so you should only be buying the 2 standard ring sets which will be from E/J if the other pistons are original. Im not sure about which springs you are refering to but it sounds as if your supplier is pretty good.<br /><br />Let us know how you make out. Good luck!