Livening up my top end

Vintagewannabe · June 09, 2026, 12:54:59 AM

I am doing a full rebuild on my 73 350, and I don't think my local two stroke guy is gonna be able to do my cylinders.
I figured I would try to come on here and gather some wisdom since I am likely going to have to do it all myself. I have no issue with doing the rebuild and getting the bires done, but had performance questions.

For the ports I figured I would just knock out the casting flash and clean it up. I would really like to have a guide to do any more than that. Does anyone have somewhere to point me?

Is it of any benefit to chamfer the oil holes above the mains?

I know balancing pistons can help the motor with balance and vibration. Where is it safe to take material from?

I have seen where for some bikes a small hole is drilled in the piston to help lubrication, but since the 350 has no exhaust bridge is this unnecessary?

On a street bike running pump gas, is the corrected squish safe? I know it takes a lot to get there, and I could cut the cyl down to use the 400 head gasket to get me closer. I tried to measure, and it didnt even touch my .066 solder

m in sc · June 09, 2026, 07:47:57 AM

all of that wont hurt. the pistons, that's minimal gain at best w the weight, and honestly not something id bother with. if you must remove material around the inside of the wristpin boss, but, these pistons now adays are produced so well they will probably be the same weight.

the oiling hole is up to you , and cleaning up the ports etc never a bad idea but if you want more power, would strongly recommend head work or replacing them w more efficient ones. but modifying stock ones to tighten the squish correctly is a good place to start and one of the best bangs for the buck. but cutting the cyl vs cutting the head... the head is where the work needs to happen to get the cc right. a stock rd350 head gasket is around 1mm or under which is a good place to start. however, no -real- downside to getting the cyl cut but they will then both need to be done together obviously, and then become a matched set. IF you are paying to have all this done, it may be worth getting a set of the billet VP heads which are just a bolt on vs modifying the stock stuff my 0.02

Vintagewannabe · June 09, 2026, 08:49:07 AM

Quote from: m in sc on June 09, 2026, 07:47:57 AMall of that wont hurt. the pistons, that's minimal gain at best w the weight, and honestly not something id bother with. if you must remove material around the inside of the wristpin boss, but, these pistons now adays are produced so well they will probably be the same weight.

the oiling hole is up to you , and cleaning up the ports etc never a bad idea but if you want more power, would strongly recommend head work or replacing them w more efficient ones. but modifying stock ones to tighten the squish correctly is a good place to start and one of the best bangs for the buck. but cutting the cyl vs cutting the head... the head is where the work needs to happen to get the cc right. a stock rd350 head gasket is around 1mm or under which is a good place to start. however, no -real- downside to getting the cyl cut but they will then both need to be done together obviously, and then become a matched set. IF you are paying to have all this done, it may be worth getting a set of the billet VP heads which are just a bolt on vs modifying the stock stuff my 0.02

I am hoping to use the same shop that my engine guy likes, so hopefully they know what I am asking for. Worth going to talk to them and feel them out.
So I guess the proper procedure would be to bore and assemble, then send to be cut. But my solder wouldn't even reach.
Also it is starting to sound like a performance snowball

m in sc · June 09, 2026, 10:32:46 AM

use modelling clay and get measurements front and back and then you can have it all done at one shot

Vintagewannabe · June 09, 2026, 10:54:33 PM

Quote from: m in sc on June 09, 2026, 10:32:46 AMuse modelling clay and get measurements front and back and then you can have it all done at one shot

Never thought of modeling clay! But what do you mean front and back? Just of the cylinder?

m in sc · June 10, 2026, 07:31:00 AM

front and back of the dome of the piston as it will rock in the bore if you just do one side

Vintagewannabe · June 10, 2026, 01:02:13 PM

Well sadly I already had the engine apart before making this post. But I am putting in new pistons/bore over, and a vito's crank, so it seems worth it to measure after all of that.
Is there harm in bringing the squish closer even if it is not perfect? Say going to the 400 head gasket and just resurfacing my head?

m in sc · June 10, 2026, 01:26:31 PM

youre going to need to machine the top of the cyl to fit a 400 head gasket. once you do that you cant go back. so... will it probably work? sure. but just guessing is risky

Vintagewannabe · June 12, 2026, 11:21:20 PM

Quote from: m in sc on June 10, 2026, 01:26:31 PMyoure going to need to machine the top of the cyl to fit a 400 head gasket. once you do that you cant go back. so... will it probably work? sure. but just guessing is risky

I think the recess for the headgasket was about .025 to .03 in, so I will take that off and re measure.
My question was more is there a benefit to getting the squish tighter without hitting that .030 that I have read people use for pump gas

m in sc · June 13, 2026, 07:44:40 AM

yes, anything tighter will be better. you may not feel it but the motor will/should run a bit cooler

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Livening up my top end