The inspection of the parts is the most important and insightful stage. If you are doing a rebuild due to a seize or loss of compression then this could give you the answers to why there was a problem. If rebuilding as part of routine preventative maintenance then now is a good time to just check that everything is as it should be.
Crank
The purpose of inspecting the crank is to confirm that the engine does only need a top end rebuild and doesn’t need a bottom end too. There are a number of things to inspect all can be done just from the cylinder and piston being removed.
- Action and noise (noise is a bit subjective) of the crank. Hold the conrod and spin the crank. It should rotate smoothly with virtually no noise. If it feels notchy OR you can hear a whir from the bearings then this is a good indication that you need to follow the 2 stroke full engine rebuild at phoenixmotos.com
- Any previous overheating of the big end – looking over discolouration from heat on the conrod at the big end
- Check the big end bearing. What we are checking for here is play in the big end bearing. The play we are looking for is up and down play not side to side – there should be side to side clearance between the rod and the crank webs, this allows the big end to be sufficiently lubricated while running. Rotate the crank so that the big end is at the top. Hold the rod by the base and slowly pull up and push down on it to see if there is movement. small movements here is what we are doing to see if there is any up and down movement – sometimes it’s very noticeable and sometimes it’s not
If you discover any of the above then it’s definitely time to follow the 2 stroke full engine rebuild guide at phoenixmotos.com
Piston
We are changing this anyway but we want to see if there is any abnormal wear to the piston.
Is there physical damage to the piston? Piston rings etc.? If there is – identify where on the cylinder that this correlates to and identify any damage on the cylinder. Any damage is going to need work – look at the cylinder section below
Look for excessive scoring (often on the exhaust port) as this is a symptom that something isn’t right and should give you an indication that you should change something. In the case of the exhaust port this can often be that the exhaust port has got too hot which is normally a sign that the jetting is too lean, maybe just slightly at the top end but enough that it is causing physical damage to the piston and ring – if the engine is setup right it will have less wear and will be less likely to experience a problem.
There are a number of other types of piston damage caused be a variety of things. Please review the information on phoenixmotos.com for identification of piston damage similar to your own.
The next thing you can do at this stage is to look at the build up on the top and bottom of the piston. This gives you a really useful insight into how your engine is running in terms of jetting. As a rule of thumb if the top of the piston is completely covered it can mean 1 of 2 things
- Your air filter is not sufficiently filtering out the necessary particles and these are making their way into your engine causing excessive wear! If this is the case, you need to be more careful with your filter care – it’s important!
- Your jetting is lean – maybe not massively lean but still lean. I’d rather be slightly richer than lean as although I am racing I’m not wanting a rebuild after every meeting – me personally I’m not fast enough for that to be worthwhile. I wish I was! It’s sad but true!
If the top of the piston is clean then you are Rich – too rich, you are missing out on throttle response and horse power.
The optimum for the top of the piston is mainly coated but with clean areas which match up to the transfer ports so we are looking for some clean area s, but not too clean similar to this.
So remember completely covered; too lean, large clean areas; too rich.
The other place to look on the piston is underneath (above where the wrist pin would be). If this area has black deposits of carbon your piston is getting too hot and the mixture is lean. The underside should be clean or have a slight oily brown residue.
Now you have deciphered the running condition and if any jetting changes are needed then you really want to find out what the existing piston is so that you can work out what to replace it with – usually there are markings on the top – clean it with scotch brite and collect the markings. You should be able to google the markings and find out what that piston is.
If not you should measure the piston diameter from approx 1” above the bottom of the piston skirt with a vernier caliper / gauge. This is the size of the current piston.
If a routine piston replacement with no loss of compression then you want to order the same piston. Else you need to assess the state of the piston vs the cylinder. Ultimately if the cylinder looks in good condition then the chances are that you need to replace with the same size piston with the exception of a Nikasil bore which you will need to do a piston to wall clearance test on. With Nikasil the bore wears very fractionally and therefore there are A, B and C pistons which go up in size by 0.01mm.
Use the resource guide on phoenixmotos.com to find out what clearance you should have. if it is slightly over the desired clearance spec then consider going up a size. If you are already on the biggest size then you may need a replate, contact me on [email protected] with the information and I can advise on the next steps to take with your cylinder.
To check the clearance select the desired size feeler gauge & place it down the piston so that the bottom of the feeler gauge is 1 inch above the bottom of the piston skirt. Now try to insert the piston into the cylinder. If it goes in then move up a size of feeler gauge or use 2 to get your small increments until it won’t, work up in small increments as the size differences are small. The size that fits just before the one that doesn’t is the clearance.
We should really follow the cylinder checks before making 100% certain that we are ordering the correct piston so please hang fire until we have done the next step.
Cylinder inc power valve
The cylinder bore should look clean and unmarked with a fine cross hatch pattern present – it’s common that the cross hatching has worn away and will either need be re honed or running down with a glaze buster, it is most likely that the cylinder will need at minimum de glazing but lets check the rest of the cylinder out.
Are there any cracks anywhere?
Any marks on the bore need to be identified and you need to assess whether the mark can be felt with a finger nail – if it can then it will need work. With a bit of luck it can simply be honed out. The cyclinder will need to be assessed by an engineer to see if honing will be enough or if you will need a replate or rebore. email pics to [email protected] and we can give you our advice.
Some marks are bigger – gouges for example which will clearly need significant work which we will come back to shortly.
The next items to look at are the ports. Is there any damage, cracks, blockages or sharp edges?
Damages should be assessed and you should evaluate if they can be dressed up (cleaned up) and run as they are or if they could cause any potential issues like snagging a ring etc. If you’re not sure then There are resources on phoenixmotos.com to help you decide whether the damage can be repaired and how, if you aren’t sure then you can send me detailed pictures of the damage to [email protected] so that I can assess what the best course of action would be in your instance and if this is something I can help you with.
Rebore/liner/replate If a simple hone job won’t sort the cylinder then you must then consider if you need a rebore/liner/replate but how do you know?
On phoenixmotos.com we’ve got a database of 2 stroke engine specs covering a wide variety of 2 stroke bike engines we have listed there the cylinder bore type (Nikasil/iron).
Manufacturers started toying with chrome bores in the late 70’s and then most moved back to iron until the mid 80’s when most cylinders started using Nikasil (not chrome), though some have remained on iron throughout.
A cast iron bore is effectively a liner pressed into aluminium cylinder. The theory of the move to Nikasil is beneficial for 2 main reasons
- Nikasil is harder than iron and will wear more slowly meaning the bore will stay in spec for longer and we can therefore use 0.01mm increments of piston size (A, B, C pistons) rather than 0.25 / 0.5mm
- As Nikasil is a coating applied directly to the aluminium cylinder the heat transfer / cooling is much better in a Nikasil bore
To test if your cylinder is iron or Nikasil just put a magnet on the bore, if it sticks it’s iron, if not it’s Nikasil!
If you have a Nikasil bore you need to carefully check if the Nikasil has worn through at any point in the bore. If it has there will be a patch which is a different slightly darker colour. If this is you then the cylinder needs to be replated.
If the cylinder needs to go off for replating we can organise that for you in the UK and are growing our partner network so please contact [email protected] and let us know where you are in the world and we should be able to point you in the right direction of known trusted partners. Likewise if your Nikasil coated cylinder has other damage to it we can point you in the right direction.
If you do need to send your cylinder for replating and other repair work you will need to strip your power valve out of it before sending it off. There are guides in the Phoenixmotos.com resources section which will help you with procedures for removing specific types of power valve. If you can’t find the resources you need please contact us on [email protected] and we will make sure we produce the instructions that you need.
You’ll also want to check your ports before sending your bore for replating! Imagine your piston and rings travelling super fast up and down your cylinder. If there are any sharp edges at the ports they will damage your piston and rings. With a Nikasil bore these sharp edges need to be smoothed out or chamfered before sending your cylinder for replating. Please skip to the “prepping the parts” chapter where the chamfering process is covered.
rebores and Liners – iron bores are a bit more forgiving and less costly (well unless you need a new liner!) generally if you have damaged to the bore you can have it rebored to a larger size and use a bigger piston. When I’m getting a rebore done I give the engineer the barrel and the piston and specify the clearance. Again in the UK I can supply the piston for you and have this work done, in the rest of the world I may have partners in your area who can do this just message me on [email protected] and I can let you know what I can do for you.
What happens with iron bores is that eventually they get rebored so often that there is not enough liner left to bore a bigger size. At this point the old liner has to be pressed out and a new liner pressed in. The liner may be an off the shelf product or one that an engineer needs to make. Usually you are at this stage when a larger sized piston than the one you are currently using is not available and the bore is damaged to a point that you can’t use it. This can also be the case when the bore that you have has been badly damaged by something like a snapped conrod which can cause a large gouge in the liner. Often the gouge will be so large that the engineer boring the cylinder will not be able to take enough material out of the liner to remove the gouge. Obviously this should be looked at by a professional engineer but the chances are that this kind of damage will need a new liner. Get in touch for options at [email protected]
There is a lot that you can work out yourself with cylinders but often you’re best leaving this work to a professional specialist to sort out. I’m happy to help you get sorted with your cylinder if it does need work. After time you will grow your own contacts and understand what is and isn’t possible with your cylinders.
Two things I’ve not mentioned:
- Measuring your bore
- If your bore is no longer round
Many years ago I remember spending a long time trying to figure out accurately what the bore size of a cylinder was and if it needed a rebore. My friend who was a mechanic with a vast amount of experience had us using bore mics (measuring equipment) to measure the cylinder. We couldn’t seem to get an accurate reading and although we thought the bore was oval we couldn’t get the measuring equipment to prove it 1 way or another. I then spent a long time looking at expensive bore measuring tools and how to use that equipment to measure the bores correctly. I’d got it all together in my head what we needed and how to use it properly. It was going to cost a lot of money to do and I’d only had a need for it a couple of times on the engines I’d been working on before.
When I stopped thinking about the actual process of measuring the bore and what was best to do for this particular engine I came to a number of conclusions:-
- This wouldn’t really be a problem in a Nikasil bore – If the Nikasil wore through you’d have other issues
- If you think it’s out of round (Oval) then it probably is. An engineer will be able to tell you very quickly as they have the measuring tools setup for this
- If there is too much piston to wall clearance then you’re going to need to have a rebore anyway and that will make the bore perfectly round again so long as enough material is taken out in the boring process.
With that in mind, I decided to stop trying to measure the cylinders myself. The equipment wasn’t really worth it for me and I’d be better taking it to an engineer unless I started doing the boring and honing myself. At this stage I haven’t worked it out to be financially viable.
Finally the power valve! They are all different but the function is the same – at low revs it restricts the exhaust port reducing the volume of exhaust gases that can exit the port and then as the revs build it starts to open until full maximum revs when it is fully opened. There can be damage caused to the power valve but most often what happens is that the power valve gets carbon build up preventing it from opening or closing cleanly. There is also power valve seals which ensure that the exhaust gases can’t escape out of the power valve and preventing air being sucked back in. When doing a top end or engine rebuild I find it is very worthwhile to remove and clean up the power valve to ensure it is clean, sealed properly and operates flawlessly!
In the resources section of phoenixmotos.com I am collating instructions for removing model specific power valves. Please check on there for your bike instructions, if they aren’t on there please email me on [email protected] and I’ll endeavor to get your instructions added.
Head
A few things that I want you to think about with your head:-
- Are the threads for the spark plug in good condition OR do they need a repair?
- Is the sealing surface on the head (the surface that seals with the head gasket) flat? You can use a straight edge to check this or a known flat surface like a piece of glass or engineered flat plate. When placed on a known flat surface, does the head rock or can you see gaps beteen it & the surface. If you decide that your head is not flat you can simply “flat it” on the flat surface with some fine emery paper – you put pressure on the head and sand it down using a figure of 8 motion until you have taken off a little material across the entire sealing surface – it does work, you have to be careful to only take a fraction of material off evenly across the head. A more precise method is to have an engineer skim the head back to flat. If you are doing this you are going to alter the piston to head squish clearance so you will need to check and alter this very carefully. Squish clearance is covered in the 2 stroke tuning guide at phoenixmotos.com as skimming & raising compression needs to be performed carefully. If you need your head skimming I can arrange this for you just email [email protected]
- Compression dome damage. It’s really common that at some point a foreign body has made it’s way into the engine, travelled up to the combustion chamber and rattled around for a while damaging the head, piston and cylinder. It’s always better for the head to be perfect but in the real world replacement heads can be expensive or even for older bikes impossible to find. So depending on how bad it is will depend on what you need to do from cleaning up sharp edges and bring the combustion chamber back to being as close as possible to original shape TO having the head welded and reshaped by an engineer.
If you need engineering work done on your head I can help just message me at [email protected] – it is always worth searching for another head so you have an idea if a replacement or engineering work would be the best option!
Reeds and manifold
Not all of the bikes you’ll be rebuilding will have Reeds but if you do they are worth checking. To do this you want to remove the inlet manifold and reed block. The reeds should be snapped shut so that they do actually close. If they are not snapped shut or they look damaged then I would always recommend replacing the reeds – they are a consumable part and perform a critical task so they should be replaced if they aren’t right. V force or Boyesen reed blocks are likely to be available and do offer power gains.
The other critical item here is the inlet manifold. The inlet manifold is an item that I see being in a bad state so often that it is scary. 1 of these reasons is that most bikes I get have come from America where the heat and the UV light have damaged the rubber of the manifold and they crack or tear this is less of an issue in cooler wetter climates like the UK where our rubber and plastic parts stay in good condition but the rest of the bike rots away. The manifold rubber being in a great condition is critical to preventing engine damage from air leaks. If it’s cracked or torn – do not try to seal it with silicone, replace it!