Willard's vintage Hotsaw Builds

[Bigmac]

Here is the stick on temp gauge https://www.motosport.com/factory-effex-temperature-stickers

 

[Willard]

I would probably set the piston clearance closer to air cooled tolerances.

I checked the service manual and both air cooled and liquid cooled clearances looked to be the same.

Wonder if I could run a 66.50 OEM NOS piston in my 66.75 cylinder bore?
Found a couple of these NOS pistons on Ebay for about $40 each. Might be worth while to give them a try so I can put a degree wheel on to check port #'s and then check the squish.

 

[Bigmac]

You could, that’s pretty cheep to check, but not to run. Is that a TRx piston?

 

[Willard]

You could, that’s pretty cheep to check, but not to run. Is that a TRx piston?

Not TRX they were 1 ring OEM 1984 CR250R pistons.
Trouble is I can't even find a good used piston on ebay.
When I get home I'm going to see my local Honda dealer where I buy my Husqvarna and Stihl saws. One of his partsmen has years of experience with CR250 and ATC/TRX. He will have no problem finding me a piston I want along with CR500 bearings etc.

Nice thing with dealing with him is no paper trail...I can pay cash and my wife doesn't get upset

 

[Billy Currie]

I think it has been covered in this thread, but the CR250R bearings are now the same part number of the CR500R bearings. I checked after I read it. Not that it affected me or anything, just made me curious.

 

[Willard]

I think it has been covered in this thread, but the CR250R bearings are now the same part number of the CR500R bearings. I checked after I read it. Not that it affected me or anything, just made me curious.

Yep, but CR500 bearings sounds better

Not only did these two share these bearings ....Kawasaki, Suzuki and Yamaha shared them also.

 

[Bigmac]

I am probably get you pretty close port timing number if you give me these measurements, top or cylinder to top of exhaust port then top of transfers, then base of exhaust and base of transfers

 

[Billy Currie]

Yep, but CR500 bearings sounds better

Not only did these two share these bearings ....Kawasaki, Suzuki and Yamaha shared them also.
View attachment 215661

Wow, I didn't know that!?!? That's good stuff to know!

 

[Bigmac]

Yep, but CR500 bearings sounds better

Not only did these two share these bearings ....Kawasaki, Suzuki and Yamaha shared them also.
View attachment 215661

Ya I agree, lol

Most of the aftermarket bearings are 7 ball. Just an FYI, even if you order cr500. And as another side note, all the major makes copied the Honda 66.4mm x 72mm bore stroke combo by the 90’s that was Honda’s advantage for years

 

[Willard]

I am probably get you pretty close port timing number if you give me these measurements, top or cylinder to top of exhaust port then top of transfers, then base of exhaust and base of transfers

Hey I appreciate your help here Nathan!
No piston to measure from but a upside down 66+mm Fluid Film can worked good with the caliper.

Exhaust ports measured from top of cylinder:
42mm to top of exhaust port. 74.50mm approx to bottom.

Transfers:
58mm to top
73.50 to bottom

 

[Willard]

One other thing I remember your comment about the little holes in the piston for cooling the bridge.
Well I hope to remove the bridge as what Arlen did pictured below and he said he never had a problem snagging rings as long as the chamfer is correct.
And seeing my engine rotation will be reversed piston side thrust should be opposite. And cylinder being horizontal may help even out the thrust pressure from the intake side also.

 

[Bigmac]

So with the piston, 1.5mm in the hole, your timing would be 182/126 and that would put the transfers even at the bottom of the port, 2mm in the hole 184/128, I am guessing it’s 1.5mm in the hole. If it’s 1.5mm in the hole, I would deck the cylinder 1.5mm lift the transfers 1mm and the exhaust 3mm, that would set you up with 191/130

how thick is your head gasket?

I wouldn’t remove the exhaust bridge, you can and will run fine, rings aren’t an issue, you just can’t make the port as big. No wings, and without sub exhausts, I don’t think it preforms as well.

 

[Willard]

Thanks. I think head gaskets are.040
Adding sub exhausts in my 1983 cylinder?
So piston edge 1.5 mm below top of cylinder sleeve edge?
Mill 1.5 off base of cylinder then raise exhaust and transfer ports on top of that?

 

[Bigmac]

Thanks. I think head gaskets are.040
So piston edge 1.5 mm below top of cylinder sleeve edge?
Mill 1.5 off base of cylinder then raise exhaust and transfer ports on top of that?

No, deck off the top 1.5mm, and raise the transfers 1mm and 3mm on the exhaust. Or deck 2.5mm of the top leave the transfers alone and ease the exhaust 2mm, but then you might have to stack gaskets or make a small spacer plate to get zero deck, most Honda base gasket are.020 so you would need 3 base gaskets.

 

[Bigmac]

I am against a bridge removal, but your builder may not. If you remove the bridge, the port is basically max out at stock width. So only 2.5 to 3mm bigger, you can widen the port that much on each side with the bridge. With the stock cylinder I leave the bridge, on the aftermarket cylinder I like the no bridge cylinder and sub exhausts

 

[Bigmac]

Here is a fun option...if you 1.5 in the hole...3mm stroker, that make the deck 0, puts the transfers at 130, so you don’t touch them, and lift the exhaust 3mm is 192°

that’s what makes the 4mm stoker on the trx work, the piston is 2mm in the hole stock stroke and port timing is mild, I will be at zero deck and 128° transfers and lifting the exhaust and not have to deck the cylinder.

 

[Willard]

Great advice!
So of course the head needs to be cc'ed and squish re adjusted?

 

[Willard]

I am against a bridge removal, but your builder may not. If you remove the bridge, the port is basically max out at stock width. So only 2.5 to 3mm bigger, you can widen the port that much on each side with the bridge. With the stock cylinder I leave the bridge, on the aftermarket cylinder I like the no bridge cylinder and sub exhausts

Reason why Honda tried the CR250R bridgeless in 1984 and went back to the bridge in 1985 for the long haul.
Ok my mind is made up I'm sticking with the bridge. Now I'll need that Wiseco 526 ATC/TRX piston with the narrower cutout so I can widen the exhaust ports.
I'll have to check if the pistons can be returned as what I want to do is order a 1983 446MO and the newer 526MO ATC/TRX piston. Compare them both in size etc. If the 526 will work then I'll send the older 446 back. My honda partsmen has total Wiseco ordering access.

I think I'll stick with the stock 72mm stroke seeing I want to go high rpm with the one ring piston and with those high port #'s.
Shouldn't be a problem myself raising the ports.
I have to recheck my 66.75 bore for clearances or just order the larger piston and get a fresh rebore (bored with torque plate).

 

[Willard]

Nathan, I'm showing 2 pistons here.
446MO 1983 Cr250r top and below a 526MO 1985-1986 ATC/TRX.
They both look the same to me besides a bit more lightening groove above the wrist pin.
1983-2001 Honda con rod kits are the same.
So I'm hoping I can run the ATC/TRX piston in my 1983 cr250r and have all the benefits like 69mm plus big bore, single ring and teflon coating they have.
View attachment 215122 View attachment 215123

I may not be able to run the newer 526MO ATC/TRX piston in my engine with the reversed engine rotation.
As the above pics show my 1983 446 piston pin is dead center neutral between the skirts.
The 526 piston pin is offset forward towards the exhaust side which may produce some weird side thrust pressures with the engine reversal. I wonder what the reasoning was of the offset change?
I didn't check if the newer after 1986 562MO pistons had pin offset.

 

[Bigmac]

I would once your zero deck, take the bead and have a machinist match the dome of the piston, and and take the dome out to 69.75, then have them take the the lip off . That would let you run up to a 69.5 piston, it you can only go 68, then go 68.5 on the dome, to get max power you can match the dome up to each bore, but that’s not practical.

do some more research on the piston offset, I am guessing they did it because the piston is heavier on the exhaust side, and skirt length is 3mm longer then intake side vs engine rotation, but could be wrong.

This is a pic of how I set up the head, this is the minimum cut, I will install it, then make an adjustment after that if needed, with the oem head, your a little limited, thankfully the 85-86 atc and 86 head are good, the 87-89 trx head is completely junk!! I don’t know what Honda was doing there!

Honda has been pretty good of get it close once you machine everything, they left the engine vary moldable to a guy that understands setup, you could make a stout runner with manly machine work and no porting, machining 2mm off the top of the cylinder and doing what it would take to get zero deck, base gasket wise along with doing what I mentioned to the head would be on of the best overall motors with no port work, you would also be looking at a 5-7 hp gain, and no port work,