So, I recently took on a new project, a 1974 CR250. The guy I bought it from said he had just put a new piston in it and afterwards he said the piston was hitting the head. At the time I wasn't concerned as my intention was to fully restore it and go completely through the motor. And I was convinced he probably put the wrong piston in it.
So today I pulled the motor and pulled the top end off of it to see what was going on as you could hear and feel the piston hitting the head when you tried to crank it. The guy had three head gaskets and two base gaskets on it and the piston was still hitting the head. It has a DG head and after looking and measuring the new piston that was put into it, I'm convinced it's the correct Honda Piston (has the ART marking on the inside). I measured the piston heights (From bottom to top, pin center to top, and bottom to crown) based off of what Vintco has listed on their website for the 1974 piston, the Honda and Vintco pistons share identical measurements.
I would also think the rod is correct as it appears it has been in there for a while. The rod and main bearings still seem to be tight.
I then put one base gasket back on and reinstalled the piston and cylinder and I then noticed the piston extends well above the top of the cylinder. I measured it and the outside edge of the piston extends 1.73mm above the above the top of the cylinder sleeve. The cylinder doesn't appear to have been decked or had any machine work done on the base or head surfaces.
So my question is for you guys that have had these apart. Have any of you seen this where the piston extends that far above the top of the cylinder?
Edit: I've had allot of bikes apart over my 50 years of riding and have never seen a piston extend above the cylinder like this thing.
When the piston is at the bottom of the stroke, does the top of the piston line up with the bottom of the transfer ports?
Could it be that the DG head has been milled to boost compression. Any chance you can have or can borrow a stock head to check? Is this a highly modified bike from past or pretty stock?
May have a long rod kit in it.
Let me re-emphasize. Long rod or Wrong rod.
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Here is a picture of the piston at BDC and you can see how far above the transfer ports it sits.
It's had some minor port work done and has a DG head and pipe. One of the reasons I bought it is it spent its life as a flat track bike, so the frame wasn't all beat up.
Since the new piston matches the Vintco pistons, I would tend to agree with Redman. The photo looks like .100-.125 " sticking out. That is way too much to make up with gaskets. To me, does not look like it goes down far enough to fully open the intake/transfer ports I Be interested in final outcome.
The rod's too long, Vintco is showing 121mm for the 1974...
and 125.3mm for the 1975...
I'll split the cases this weekend and measure the rod. One thing I forgot to do is see if the rod had the three little notches on the little end for the selective-fitted bearings, I knew they did this on the CR125's but didn't realize it on the 250 until I looked in the manual. I'm sure it's a Honda rod as it has a printed C on one side and a stamped S1 on the other (the same markings I've found on other Honda rods).
I'll update this once I measure the rod.
Make a cylinder spacer run it as a long rod setup
Excellent suggestion!
if it turns out that rod is indeed speced for 1975- couldn't you find a 1975 cylinder to match if the crank is good? I had a 1974 CR250 back in the day and my group was all pretty disappointed when the 1975 came out with the Red stripe on tank and up pipe with giant silencer on there. A guy I worked withs dad bought 1 they shared and I rode it and it seemed sluggish and heavier and slower than my 1974. We never dug into specs to see what was different mechanically from 1973/4. IIRC, the 1976 was basically the same bike but painted all Red? We were all switching to YZ monoshocks by then so my memory of all the yearly changes to CR250 is spotty, but think I read somewhere they skipped the 1977 Production year and unleashed the all new 1978 instead? Apologies to Honda guys...
I ended up splitting the cases today. The good news is that it had a billet crank in it. The bads news is that it did indeed have a 1975 rod in it that measured 125.3mm. The other bad thing is that the seal journals have some minor pitting. I'm thinking at this point I might just buy a VintCo crank and just put this one aside until I can hunt down an OEM rod. I attached a picture of the crank.
Here is a picture of the numbers I've read that they put on the billet cranks. They are very faint, plus my eyesight isn't what it once was.
Bike is almost complete and is running great with the new Vintco crank.
The long rod works great in these bikes so no need to waste it - just make a 4mm spacer for under the barrel using the base gasket as a template.
Billet cranks are sought after , and the seal area can be addressed with a speedisleeve . Someone will buy it if you aren’t going to
https://www.bearingboys.co.uk/Speedi-Sleeves-Shaft-Repair/CR99098-24942504mm-SKF-Speedi-Sleeve--64909-p
Looks awesome…. My first restore was a 74 CR250…. After a few hard rides the frame started cracking everywhere….Thats where I learned to sand blast, inspect, reweld critical areas on any vintage restore that I planned to ride….
Pit Row
Nice snores !!
I never liked mine much, 2nd gear was made of glass, right footpeg/frame were weak, down pipe, had to find a reed valve, pipe mounting needed work, aluminum tank cracked, airbox was useless, points to PVL ign was needed, to me it was just stone age. I made the switch to a mag hoosk and was never happier.
and yet when they came out gates at races were dominated by CRs- until Yamaha monoshocks came out and took the technology to another level entirely.
In that era here in N Ohio maico hoosk and pentons ruled the snores
really? growing up in SoCal the Euro stuff was less represented in our riding areas unless you were sponsored, getting help from a shop or parents with parts and repairs. I had a 1970 Bultaco Sherpa S125 that was hard to keep running and expensive for parts. Once early YZs showed up and Suzuki TMs started arriving in large quantities there was a switch away from Euro & Spanish makes for the cheaper and more reliable Japanese bikes for the masses. Having Gary Jones winning titles on Yamahas and then Elsinores kinda added to the demand for the CRs. Having DG, FMF and other speed shops popping up offering hi perf parts and packages and even full bikes kitted with their parts made it explode.
Theres this guy named penton in loraIn ohio...
right, real familiar with Pentons. Also Hodakas which sold way more volume. Basically the Euro stuff like CZ, Maico, Husky were the most sought after "real" MX ready competitive bikes right out of the crate and more expensive than plentiful Yamaha MX bikes available. Then Suzuki who already had World Championships decided to water down those championship bikes and offer TMs in mass quantities as an alternative to the pricier and less reliable stuff from Europe. In my area many people switched to TMs from the others and gates at Carlsbad, Saddleback etc. were filled with them and edging out the exotic stuff. Then the Elsinores showed up and changed everything again.
Yea the snores were popular in 125 class here but not in 250 or open of the 74. In hare scrambles it was prolly 99% euro bikes
That’s a beauty…. Great job.😎
Oh man , is that an SL100 in the background? That’s a beautiful bike too , what year is that ? 70,71,72?😎
Yes, it's a 72 SL100 with right at 1900 original miles.
That is a beauty….😎
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