GM Vortec 305 Flattop piston ?s

I

int76

Cadet
Joined
Jun 23, 2009
Messages
27
So I picked up my new reman short block to get the boat going again (99 Glastron w/ 5.0 Volvo Penta GL GM 305 block SX outdrive) and I noticed the new motor has flattop pistons rather than dished pistons like the original motor had. Is there going to be any issues with these flattop pistons with the marine cam? Im new to this motor/motor rebuild thing and just want to make sure this isnt going to be an issue before I put everything together and get it in the boat...a little more info, Im using my original heads (had them checked at the machine shop) with the new reman short block assembly and all my marine acc. waterp pump, oil pump etc....
 
J

John_S

Rear Admiral
Joined
Jun 21, 2004
Messages
4,269
Re: GM Vortec 305 Flattop piston ?s

int76 said:
So I picked up my new reman short block to get the boat going again (99 Glastron w/ 5.0 Volvo Penta GL GM 305 block SX outdrive) and I noticed the new motor has flattop pistons rather than dished pistons like the original motor had. Is there going to be any issues with these flattop pistons with the marine cam? Im new to this motor/motor rebuild thing and just want to make sure this isnt going to be an issue before I put everything together and get it in the boat...a little more info, Im using my original heads (had them checked at the machine shop) with the new reman short block assembly and all my marine acc. waterp pump, oil pump etc....
Click to expand...

If you are concerned about valves hitting the top of the pistons, don't think so. I think it would take a very high lift cam, and extreme duration to become an issue. Valves are closed on one cycle, and the next cycle where the piston is at the top, exhaust is closing while intake is opening.

But, your engine does have higher compression than it had. Have you asked the supplier why? The merc 5.7L dished pistons were 10cc, don't know what it is for the 305. It maybe you just have to run higher octane and/or adjust the timing.

PS: What cam did they use? Specs?
 
I

int76

Cadet
Joined
Jun 23, 2009
Messages
27
Re: GM Vortec 305 Flattop piston ?s

imported_John_S said:
If you are concerned about valves hitting the top of the pistons, don't think so. I think it would take a very high lift cam, and extreme duration to become an issue. Valves are closed on one cycle, and the next cycle where the piston is at the top, exhaust is closing while intake is opening.

But, your engine does have higher compression than it had. Have you asked the supplier why? The merc 5.7L dished pistons were 10cc, don't know what it is for the 305. It maybe you just have to run higher octane and/or adjust the timing.

PS: What cam did they use? Specs?
Click to expand...

Yes I was worried about the piston throw being to much with the flattop pistons with the aggressive marine cam. I am using my stock marine cam out of the Volvo 5.0 (only had 65hrs on it) so Im not sure on the specs but it is just the stock one. Im glad I dont have to worry about the piston/valves colliding and I usually run midgrade fuel anyways so I hope that should suffice for the higher compression...
 
I

int76

Cadet
Joined
Jun 23, 2009
Messages
27
Re: GM Vortec 305 Flattop piston ?s

any other insight/opinions?....
 
You must log in or register to reply here.
Top