+4mm stroke crank, a good idea ? and how to fit

[ozzysbikes]

I am considering a +4mm crank for my F2 YPVS as I am going to strip the engine for a rebuild and my thinking is that rather than fit a recon or new standard crank I might as well get the stroked version (don't want to trench cases so 4mm is enough), 

Question(s) is what do I need apart from the crank ? I know the crank comes with 115mm rods but where does this place the pistons ? what other mods or parts would I need ? I would prefer not to have spacers under the cylinders so the pistons that accommodate the setup would be preferable and then would the porting need any alteration ? I guess that it would as the extra stroke is going to see the pistons way above or way below the standard ports,

I know there is some info on the forum but when I did a search I couldn't find all the info I need before deciding whether or not to fit the long stroke crank 

Thanks for any help 

[Tobyjugs]

Personally i think it's a good way to extract some more oomph out of your bike.
Off the top of my head you need to reprofile the cylinder head to make sure the top of the piston fits in the head, Use another type of piston where the gudgeon pin is higher up into the crown effectively cancelling out the longer conrod. These are made by Wiseco and i think also Wossenor.
Hope fully Midlifecrisis, Yogi or Lrcarl can explain more.
One thing i forgot you need to sew some velcro to your trouser's and bike seat for when you give it WOT

[dusty350]

Steve - Midlife - has a 385 for sure. I had one for a time but didn't build it. I know it had the +4 crank, forged Wiseco pistons with the offset pin, reprofiled head, which was also O ringed if I remember correctly. It had billet intakes, carbs , pipes, zeel etc to add to the mix. No case mods though.

Dusty

[midlifecrisisrd]

My build info here

rdlccrazy.proboards.com/thread/15507/385-ypvs-2014-mods

Basically +4mm 115mm rod crank

Wiseco 795 pistons to take up the extra 5mm rod length

Either recut the head for the extra stroke or a cool head probably easier with recut domes.

By the time you consider the large squish it's less that 2mm

A decent tuner will sort the post timing.

Makes a few more top end bhp but more midrange

Steve

[reedpete]

Think LC RD400 vs RD350.

Just be aware that as per above, although you may not ever get to or want to use it, the ultimate rev limit of the stroked crank is proportionally lower than the stock stroke.

[ozzysbikes]

Thanks for the replies, slightly confusing me is that only Steve (midlife crisis) mentions porting, does it need reworking or not ? if so any recommendation as to who would have the experience of doing the porting

[midlifecrisisrd]

It will run without porting but really not altering porting is a waste of the potential

Mick Abbey did my porting, yogi will know who did his

The main thing is lowering the port floors

As the piston travels 2mm further downward the port windows can be brought down 2mm so the bottoms are level with the piston crown at bdc

This gives a bigger area to the transfers and exhaust so helps flow

Also the extra travel in both directions affects the port closing times as they are altered a few degrees.

With Porting, pipes, ignition and carbs you can expect 75-80 reliable rear wheel bhp.

At 7k rpm mine makes the same as a standard one at 9500

Short shifting between 5 and 7 makes fun riding without thrashing it's tits off

Steve

[lrcarl]

Yes ports need sorting
Any good tuner should be able to do this
Carl

[paulincayman]

Contact Martin at Mad Biker designs , ( MBD) he will give you all the answers and options re porting. I'm doing the 385 with the +4mm crank , he did the porting inc the pvs, rebore ,supplied the pistons and profiled the head. To be fair I'm sure others can do the same .. got good recommends on this forum plus super nice guy to deal with.