Trying to figure out if i have a 4 mil crank

[Penny's Banshee]

I bought a used bike and the person i bought it from said it has a 4mil crank in it. what is the best way to figure this out without splitting the cases? the rods have an R on one side and a 29 on the other. if that helps at all.

[#1JUANstunna]

Measure the length of the stroke in the cylinder from BTC to TDC.

[Koolguyson]

Only real way to tell is to measure the stroke.

 

If you need help with it, I am in Arroyo.

[dirtydownunder]

does it have a spacer plate or cut domes ? spacer plate you can see under the cylinders. with the domes, you have to remove the head. does it run a cooll head ?

 

this is a 4mm longrod crank from M&M http://a3.sphotos.ak.fbcdn.net/hphotos-ak-snc6/167782_10150095692637002_573727001_6162540_1745238_n.jpg

[SuprSlow50]

There are two ways that I have done before. The first is to pull the gas tank off and meaure the stroke of the crank from TDC to BDC with the butt end of a caliper, or with the tank off you should be able to pull the sparkplug and read the part number on the top of the piston to see if it is for a 4mil crank. It may be hard by eye, as I have never tried it that way, but I have an earscope that I have been using to read the fuel rings on nitrous motors and they work great for reading the part numbers too, has a light and a magnifying glass. Hope this helps.

[coryv4]

There are two ways that I have done before. The first is to pull the gas tank off and meaure the stroke of the crank from TDC to BDC with the butt end of a caliper, or with the tank off you should be able to pull the sparkplug and read the part number on the top of the piston to see if it is for a 4mil crank. It may be hard by eye, as I have never tried it that way, but I have an earscope that I have been using to read the fuel rings on nitrous motors and they work great for reading the part numbers too, has a light and a magnifying glass. Hope this helps.

on my 4mil it has +4 stamped on the inside of the crank right next to the rod pin

[Koolguyson]

There are two ways that I have done before. The first is to pull the gas tank off and meaure the stroke of the crank from TDC to BDC with the butt end of a caliper, or with the tank off you should be able to pull the sparkplug and read the part number on the top of the piston to see if it is for a 4mil crank. It may be hard by eye, as I have never tried it that way, but I have an earscope that I have been using to read the fuel rings on nitrous motors and they work great for reading the part numbers too, has a light and a magnifying glass. Hope this helps.

 

True and not true. It could be a short rod stroker, in which case it will use a standard banshee piston. BUT it will/should have a spacer plate at that point.

 

The only real way to know is to measure the stroke. You have no idea what somebody could have put in there.

[Penny's Banshee]

It does have a cool head and i took the domes out about a year ago in my buddies shop and they were misplaced. I might take you up on that Koolguyson when i get some free time ill pm you and maybe we can get together so you can teach me a thing or two. Thanks for the input from everyone.