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Old 07-19-2010, 07:25 PM
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mapbike mapbike is offline
Motorized Bicycle Elite Member
Join Date: Mar 2010
Location: Central Texas
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Question HELP NEEDED....Piston Issue

I am in the proccess of rebuilding a 66cc slant head engine that thrashed on me a while back, I ordered a new piston for it a few weeks ago and today as I was getting my parts together and looking them over I noticed that the new piston has a lot less dome on the top of it than the old piston.

I will be making a .016" thick head gasket instead of using the stock .027" gasket but even at .011 reduction in gasket thickness my compression ratio will be way to low if I use this piston with the angle plug head.

Question: Do the straight plug engines normally have the flatter top piston?

Question: Is it the 6MM head that is for the Flatter top piston or is the 8MM straight plug head also for the flatter top piston?

Using a caliper I measured from the top edge of the wrist pin hole to the center top of the pistons, the original piston from the angle plug engine measures .965" from this point on the new flatter piston it measures .907" that is a huge .058" difference so I know I need a different head in order to use this piston but I'm not sure which head I need, I would love to have one of the heads that Jim @ Manic Mech. makes but I think they were set up for correct compression with the domed piston and not the flatter one I have.

The pics I have included show the differences in the 2 pistons, I need some solid facts on this so I will not order the wrong head, I'm not sure who all stocks the head I need, but if anyone has the head I need for this build please let me know either by reply to this thread or send me a PM, make me a fair deal and I will just buy the head from one of you guys that know for sure that you have the right one.

Thats DAX sells the straight plug head in 6MM & 8MM but I dont know if either one is right for the piston I have.
Thanks in advance to all and any that can help me on this......!
Attached Images
File Type: jpg 66cc_pistons1.jpg (242.1 KB, 34 views)
File Type: jpg 66cc_pistons2.jpg (248.3 KB, 36 views)

Last edited by mapbike; 07-19-2010 at 07:53 PM.
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