Newbie squish questions

Vintagewannabe · October 11, 2026, 08:11:59 PM

Hello, so I am rebuilding my 73 RD350, and I had the cylinders cut for the 400 headgaskets. I took a pic of the measurements and where they were at. From what I know I want 1mm for the premium pump gas I plan on using.
I used an athena gasket set, so I believe I can bring the cylinders up a hair with OE base gaskets.
I figure I should try this first, but do you guys think I will end up needing to cut/recut the chamber?
The inner circles are the first measurement, then a second measurement on the outer circles.

m in sc · October 11, 2026, 08:58:17 PM

1mm is a good general aim. you can run them tighter but no worries. you'll be fine w the numbers you have

Vintagewannabe · October 11, 2026, 09:02:49 PM

Wow, thats good to hear. I'm kind of shocked though, I thought this was tight.the local 2 stroke guy is saying he likes to stay 1mm or over for pump gas, but I came for more opinions
Also, does it matter that I am running wiseco/forged pistons?

m in sc · October 12, 2026, 05:35:28 AM

shouldn't.

each motor is different, but i run tzr at .8-.85 mm for a data point.

Vintagewannabe · October 12, 2026, 11:31:20 AM

Quote from: m in sc on October 12, 2026, 05:35:28 AMshouldn't.

each motor is different, but i run tzr at .8-.85 mm for a data point.

is that on pump gas as well?

m in sc · October 12, 2026, 11:38:22 AM

i run everything on 91-93 10% ethanol. except the suv, that gets 87

Vintagewannabe · October 12, 2026, 02:17:59 PM

So I guess this head has enough room in stock form to not be a compression/detonation problem?
The only other thing is I was told to torque at half value for the measurement.
For a bit of comfort and playing it safe I may run the thicker oe gaskets.
I got my 400 headgaskets from hvc, and they don't have the crimp/crush lip on them. They measure .5mm so they should be plenty thick

m in sc · October 12, 2026, 02:30:56 PM

just needs to be checked
. ive never heard the half torque thing. i always check as its going to be run, full torque to compress the base gasket fully

Vintagewannabe · October 12, 2026, 10:31:12 PM

So with the stock chamber shape/cc, am I just worrying about contact between piston and head at rpm?

m in sc · October 12, 2026, 11:10:31 PM

it won't contact unless something breaks... as long as the squish is measured ok. id def aim for .9-1.1. check squish with an x cross of solder.

Vintagewannabe · October 13, 2026, 03:49:49 PM

On the outer circles on my pic is where I measured, basically on 4 corners. Or do you mean front back left and right?
I would honestly rather play it safer with my squish, so if the new base gasket doesnt bring it where it needs to be, I'll have to have the squish band milled.
The heads have been milled flat, can you think of another reason that my squish is tighter at the back? It didn't change almost at all when I swapped heads. I read about a head centering tool and pinning it, but that seems like overkill for mine being a street bike.

m in sc · October 13, 2026, 08:36:46 PM

unless it wast cut flat or measured wrong, no. 1 pc of solder for each leg of the x evens the load out on the piston .

sav0r (CL MotoTech) · October 13, 2026, 11:04:52 PM

I saw your FB post and Ed Toomey was saying 1mm.

He's probably saying that because many are a little sloppy and get it wrong once you start dialing it down closer. It's easy to be too tight on one side and that can cause issues.

Probably the best recommendation is to get the doweled so that you get that variance in orientation removed. Thus avoiding a tight spot that's even tighter than what you have.

m in sc · October 13, 2026, 11:35:07 PM

i agree. 1mm is safe but not nec optimal

Vintagewannabe · October 14, 2026, 09:59:55 AM

Quote from: sav0r (CL MotoTech) on October 13, 2026, 11:04:52 PMI saw your FB post and Ed Toomey was saying 1mm.

He's probably saying that because many are a little sloppy and get it wrong once you start dialing it down closer. It's easy to be too tight on one side and that can cause issues.

Probably the best recommendation is to get the doweled so that you get that variance in orientation removed. Thus avoiding a tight spot that's even tighter than what you have.

Do you mean having the holes widened to fit a dowel in them? Or do you mean centering the heads and drilling them for a small dowel that locks them in? I have tried shifting them different directions and swapping heads with virtually no change in measurement.
With the solder I just used about a 2cm piece in all 4 corners. The head was surfaced but shouldn't have been shortened. I think the machinist either took too much off the top or bottom of my cylinder. From head gasket to base surfaces is 4.024 in, and I read 4.030 is ideal

News:

Newbie squish questions