How To Rebuild The Top End On Your 97-01 Honda CR250
I wanted to make this as easy to follow as possible, so I’ve broken the top end service into 15 steps that cover the removal, inspection, and installation of each component in order, followed by the leak down test and break-in.
In this article, we are going to cover what specialty tools and supplies you need, along with precautions and shop setup.
Top End Tools
Besides the necessary hand tools I’ve listed in the individual steps, you will need a few specialty tools.
You will need to build a piston pin puller and a piston holder.
For the pin puller, all you need is a length of all-thread, two nuts, one fender washer, and a large socket.
Thread one nut onto the all thread, followed by the washer and the socket. Run the all thread through the piston pin, and install the nut. Place a shop towel between the socket and the piston, and tighten the socket against the piston.
Hold the all thread in place with one hand, and tighten the socket nut with the other to press the pin out of the piston into the socket.
For the piston holder, find a short 2×4, drill a ¾ inch hole in the middle, then cut an opening from one end to that hole. Make sure to sand your holder to remove anything that could potentially fall into the crankcase.
D.I.Y. Tools Cost
- Piston Pin Puller – $10
- Piston Holder – Free
You will be cleaning carbon build up on the cylinder head, inside the cylinder, and a lot on the exhaust valve components.
For the cylinder head, all you will need is a scouring pad, but for the exhaust valve parts, I had to use a wire brush on a drill, and a Dremel with a sanding wheel. Stock, your cylinder is Nikasil plated, so you will either need a honing brush, or a scouring pad and time to safely de-glaze it.
Considering a honing brush costs $80, I use a scouring pad, and I make the time.
You CANNOT Use A Ball Hone On Nikasil Plating!
If, however, your cylinder has been bored and sleeved, you can use a ball hone to quickly de-glaze and crosshatch. A ball hone goes for around $40 if you need one.
Cylinder & Cylinder Head Inspection Tools
For inspections, you will need a machinist’s straight edge, feeler gauges, telescoping bore gauges, and calipers.
We will use the machinist’s straight edge and feeler gauge together to measure warp limits.
Compress the telescoping arms, Insert the gauge into the opening you need to measure, find the median measurement, then tighten the screw at the end of the handle to keep the arms in place.
Because the ends of the gauge are spring-loaded, all you need to do is wiggle it a few times, and it will find the median measurement on its own.
One important note about bore gauges is that YOU need to make sure it is perpendicular to the surface you are measuring.
After you set your gauge, you will need to measure it with calipers. Ideally, you would use a dial micrometer because the bore gauge will seat on each end. Dial micrometers can be very expensive, so I am using a set of regular calipers.
If you use regular calipers, you will need to take your time to position the bore gauge between the caliper arms just right.
When you find your measurement, DO NOT let the telescoping arms snap back to position.
Hold the arms while you loosen the set screw and let them gently reset.
Inspection Tool Cost
- Machinists Straight Edge – $30+ (eBay)
- Feeler Gauges – $7
- Telescoping Bore Gauges – $20 – (eBay)
- Digital Calipers – $15 – (eBay)
Miscellaneous Tools You May Not Have
You will need to use a plastic scraper to clean the mating surfaces, this is a flat razor, but plastic.
For the cylinder head, you will need a spark plug thread follower to true the spark plug threads. For the cylinder base nuts, you will need the Motion Pro Torque Wrench Adapter, because you can’t get anything else on 3 of the four nuts.
When using the adapter, make sure to keep it at a 90-degree angle to your torque wrench to maintain an accurate measurement.
Miscellaneous Tool Cost
- Plastic Scraper – $10 – (eBay)
- Spark Plug Thread Follower – $5 – (eBay)
- Motion Pro Torque Wrench Adapter – $13
Aside from basic parts cleaners and shop towels, you will need molybdenum grease and lithium grease for installation. You can find molybdenum grease in the tube at any auto store, but I like to use this Yamalube moly grease simply because the squeeze tube is super convenient. As far as assembly lubricant, I like to use Lucas Assembly Lube. It’s not critical, but I already have it, and I like to use it, but you can get away with 2-stroke oil mixed 1 to 1 with transmission fluid.
Top End Rebuild Supplies
Top End Rebuild Parts
As a general rule, I never buy parts before I start, unless it’s a mandatory replacement like gaskets and O-rings.
For example, unless you are dead set on installing an aftermarket piston, hold off of buying one until you get a look at your old one. If it’s undamaged and within spec, you can reuse it with new rings and pin clips.
You will need a gasket set (I am using the Tusk Complete Top End Gasket Kit), cylinder alignment dowels(Honda OEM), and an O-Ring for the exhaust valve cap (also Honda OEM).
Top End Service Parts Cost
- Tusk Gasket Set – $12
- Cylinder Alignment Dowels – $1.34
- Exhaust Valve Cap O-Ring – $2.69
All you need to do is Take Your Time, and stuff a shop towel in every hole you open, and your top end rebuild will go smoothly.
Make yourself a clean workspace with plenty of room for tools and supplies. Ideally, a workspace you can use for a few days if you can’t do everything at once.
I taped a large trash bag to the top of my workbench to protect the wood.
During inspection and assembly, I used an old hand towel. This protects the components I am working on, It ensures my workspace is free of contaminants, and if I drop a small part, it won’t bounce away.
You will also need access to a hot water source. If all you have is a bathtub or kitchen sink, be ready to clean them thoroughly when you’re done.
2-Stroke Top End Rebuild Time & Cost
It’s hard to say how long a full top end rebuild will take you, but set aside a full day to complete it.
This service will cost between $20 and $400 depending on what tools you already have and what parts you need.
And if you need to send parts out to be machined or refinished, you could be looking at $1,000 or more.
For example, a full cylinder re-plating can cost upwards of $400.
If you have any questions before you start your top-end service, please let me know in the comments or on our Facebook page.
Keep Your ’97-’01 CR250 Running Right!