Board index Flathead Power-Technical Questions, Answers, and Suggestions Big Twin Flatties stuck


Post Mon Aug 07, 2006 7:14 am

Posts: 9
Location: Knocksville, Tennessee
Here’s is my story,
I’m working on an old 1940 ULH with about 2200 miles, post rebuild. The cycle was properly broken in for the first 1500 miles W/O the sidecar. It was about 95 degrees that day when I was running in traffic at about 65mph. I have a generally stock looking ULH, except for the Velorex side car, and VM 36mm Mikuni carb. I had stopped for fuel, retarded the spark to start the motor, and neglected to re-advance it before pulling away to rejoin traffic. After riding about 12 more miles, I felt a power loss. That nauseous feeling came over me as I pulled to the shoulder. That’s when I noticed the distributor was still retarded. The engine was too tight to restart.
When I pulled the top end, I discovered the rear piston had stuck on the rear (exhaust) side. My plugs were a dark grayish brown in both cylinders. I checked for air leaks prior to disassembly and found none. I had experienced no problems with idle or popping or performance. No damage to the front cylinder. My rear Cyl. is o.80 over. I am aware that there are .090 pistons available. The front is .040 over. I have a spare .020 oversize 13 fin front cylinder, but no extra rear cylinder. My plan is to send my rear cylinder to LA Sleeve, and sleeve back to stock. I want to send along a new piston and specify exact clearance

Does piston clearance need to be different if I use pistons without the strut. What’s best? The stuck piston had the steel strut. I honed the clearance to .0025 with a Sunnen hone. Where is a good source of Pistons? Must I buy them as a set? Steel strut or no?

I wonder if anyone has ever tried using the really late model flat top pistons in a old side valve engine.
I guess things like pin hole offset, would need to be considered too. Do you suppose the piston would reach high enough in the cylinder if it did fit on a rod?

I believe that I’ve relearned a hard lesson about timing. Much too costly. I look forward to your shared experience and recommendations.
Thanks a Bunch,

Post Wed Aug 09, 2006 11:35 am

Posts: 104
Sorry to hear about your problem. I own a '40 ULH myself,IMHO the piston clearance needs to be increased,as we have discussed before I prefer a little piston slap to being too tight. As far as the late model pistons are concerned,you need to check the center to center rod length on EVO's,I don't know what that is. This would determine your deck height and compression ratio. STROKER

Post Wed Aug 09, 2006 7:03 pm

Posts: 919
Location: RENO,NV. U.S.A.
I concur,when I had Venolia make up the pistons for my '41 ULH,I purposely opened up the piston to cylinder clearance. 10,000 miles plus without a hitch including Daytona and back pulling a sidehack plus a ole' lady. BREWSKI

Post Wed Aug 09, 2006 10:33 pm

Posts: 9
Location: Knocksville, Tennessee
Thanks a bunch for the replies. I realize that I am asking questions that are risky to answer here on a forum. The consensus is more clearance. What is "more"? .003 perhaps?

I would like to use really good quality available pistons. I doubt I can afford to have a set "Made-up". I too have great respect for venolia pistons. I used to use them back in the 60's and 70's when I worked on foreign stuff a lot. What is the cost if I'm not too nosey?

How many of you out there advocate using pistons made with the steel expansion strut, over the pistons made W/O the strut?
My bike is on the bench, and there is no emergency here. I don't want to commit at a purchase or repair plan until I have gained some information,but i miss riding my ol U. I am new to this forum and have really enjoyed the helpful tech info. and humor as well. Thanks again.

Post Thu Aug 10, 2006 12:11 am

Posts: 630
Location: belgium
The strutted pistons were a big improvement over the round pistons.
The factory says there should not be more than 3 sizes difference between front&rear pistons.
Imo your rear cyl did more work(more heat) than the front + the retarded ignition and severe use(sidehack)...your lesson.
Good luck Stéph

Post Thu Aug 10, 2006 8:21 am

Posts: 15
Location: Windham, ME 04062
Don't know if this means anything relating to HD, but for R71 flathead BMW's, piston clearance for a solo is .003" and for sidecar duty is .006".

Post Thu Aug 10, 2006 3:33 pm

Why bother.
Last edited by panic on Sat Aug 12, 2006 7:04 am, edited 1 time in total.

Post Fri Aug 11, 2006 4:05 pm

Posts: 919
Location: RENO,NV. U.S.A.
Panic,I have to disagree,theory is all fine and good,but welcome to the REAL WORLD!. I will probably catch some flack on this(WHO CARES).i HAVE NEVER SEEN A SIDEVALVE CYLINDER STAY COMPLETELY ROUND AT OPERATING TEMPERATURES. I've miked several sets of cylinders during a top end rebuild,with them still at operating temperatures and none of them were round,all were out of round by .002-.003". Someone prove me wrong. Venolia will only make 4 pistons( their minimum order) the price ,if I remember right was somewhere around [$edited by Pa] [sorry Brewski] for a set of 4. BREWSKI

Return to Big Twin Flatties