61x topend and stroker cranks

Emailed Crankworks 3 weeks ago to see how it was going. Says its ready for assembly but they've been shorthanded. Hoping to get it in my hands soon. I was going to go with extended cylinder studs because of running the spacer. There's some quality control issues being worked out on those so I looked into counterboring the head to gain back enough threads. I determined the head has more than enough meat to sink the nuts down 8mm with a diameter of 21mm. That allows a 14mm socket to fit and still leaves a ridge all the way around for strength. I didn't have a 13/16" endmill to plunge so I did a quick and dirty setup on the cnc to get what I needed. Hopefully the next update will involve some dry assembly.IMG_20220216_183155088.jpgIMG_20220216_182735968.jpg
 
Last edited:
Big day today. I got the crank back from Crankworks and attempted to drop it in the case. In a previous discussion with Crankworks, he told me the hotrods connecting rod big end was a bit bigger than standard. They offer a service to bring the size down and finish with a shotpeen. The rough math at the time said it would be close. Unfortunately it's still a bit big, so I'll have to set the cases back up and take another 20 thousands off. IMG_20220311_181143994.jpgIMG_20220311_201944996_HDR.jpg
 
Lots of good news today. I trenched another 0.035", so about 0.055" total. that gave the bigger conrods just over 0.020" of clearance. For a reference, I checked the thickness of the case at the bottom of the valley and it was 0.118" thick. I dry fit the bottom end with both base gaskets, spacer, and cylinders/pistons. Checked squish with some clay and came in between 0.045" and 0.050". Should be good enough for the real assembly. The piston bottoms out level with the exhaust port and pokes above the cylinder 0.100", as the math suggested. IMG_20220313_213605140_HDR.jpgIMG_20220314_193754528.jpgIMG_20220314_211805447_HDR.jpg
 
Motor is put together and running. Initial cold and oiled compression was 180 stator/190 pto psi. After first start to blow out the excess oil was 175/180. Keep in mind I'm using the same used pistons and rings as before, probably 200+ gallons but still looking good. I want to dial in the tune and hopefully get one more season on it before boring it out. It does have new conrod bearings, pin, and circlips. Will be a few weeks before it hits water. Very happy with how little vibration there is. Using OEM couch mounts.


IMG_20220319_145115727_HDR.jpg
IMG_20220327_182945685_HDR.jpg
 
Last edited:
I was able to get the ski out to the lake this week for the first water test. No other changes to the ski from last year other than adding 15 pounds of reinforcing fiberglass, switching to a kart tank, and a bump in battery size from a btz9 to btz12. Took it easy and checked over for leaks. Engine felt responsive and acted similar to how it did last year. It ran quite rich but didn't foul plugs after burning a couple gallons. For carbs I'm using silver mikuni sbn44s with drilled returns and running a 70 jet. Last year that got me in the 1.5 psi idle, 6psi wot fuel pressure. I figured that should be rechecked and planned on bumping the jet up a bit due to the stroker crank. Turns out there was some crud in the return jet. I should've checked pressure before cleaning it. I tested on the stand with a 70 and 90. the 90 should work fine, it gets me close to what it was last year. Sounds much more crisp now that it isn't clogged. Also found one of the low speed screws was a half turn different from the other. Will update after more testing is done.
 
Top Bottom