Vintage Dirt Bike Q & A

I've noticed on several vintage boards, people are asking about CR125 boost port cylinders. Many people think that the 75 cylinder with the rear transfer port is a boost port cylinder. I had to bore my cylinder recently and I thought I would post pictures of the boost port to show the difference. My cylinder is a 74 FMF HAAG & HAAG Boost port cylinder. It has been resleeved once due to a rod failure. It uses a single ring piston with a hole to feed the boost port.
If anyone has any questions feel free to ask. Coll

Thanks very much for this info, and the excellent pictures!

I wanted very much to try a piston-fed boost port like your Haag & Haag in my MR175, but I would need a single-ring piston like yours to leave enough skirt area to fit the window, and with MR175's you're lucky to find any pistons at all. I might still be able to do it using a Dykes-ring Wiseco snowmobile piston, but that unit has other issues (shorter skirts, lower wrist pin placement) which make it kind of a compromise.

I picked up a fairly nasty '78 CR125 cylinder on eBay, just to see if I could bore/plate or resleeve it to get a transfer-fed boost port setup. This is still in the works.

If you ever get a chance to measure them, I would love to know the dimensions and placement of the Haag & Haag boost port and piston window. I assume the port is a simple 'scoop-out' of the cylinder wall, with no pass-through to the intake tract?

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

How is the 74 cylinder with your added boost port superior to the easy to find 75 cylinder with rear transfer port?

I know that adding a rear boost port is a big help for the MR-175 engine since it was never available with a rear transfer.

I have a theory... The piston-fed boost port uses trapped fuel/air mixture inside the piston (that would otherwise be wasted) for added mixture volume and improved piston crown cooling, and feeds the port directly from the crankcase for higher pressure and velocity than the '75 CR boost port, which (in my opinion) simply steals pressure, velocity, and volume from the main transfers and redirects it towards the exhaust port. It seems to me to be the closest to the ideal situation (again, in my opinion) of a boost port fed by a separate passage directly to the crankcase, as found in many rotary-valve engines (the CR125M and MR175 cylinders have no room to cut this passage into the cylinder wall, which I don't think I would want to do anyway).

It's just a theory.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray, I think you nailed it with your reply. As for the MR175, I am running a boost port also. There is room for the piston hole under the stock 2 ring piston due to location of intake ports and longer rod length. I'll post pictures after next tear down. Coll

Coll,

So you've put a piston-fed boost port in your MR175 cylinder? Cool! Did it make a noticeable difference in performance?

I did some quick measurements on my cylinder and piston a while back and figured it wouldn't have resulted in a high-flow boost port; guess I should have taken more time. I think I measured 15mm from the top of the intake port to the piston crown at BDC and 7mm from the crown to the bottom ring land - does that sound about right? I figured that would leave room for a 6mm high piston window as wide as the cylinder boost-port cutout (maybe 4mm narrower than the space between the ring locator pins). I was also thinking the top edge of the boost port would end maybe 1mm or so below the top edges of the main transfers, leaving them with full crankcase pressure for their initial scavenging blast and reducing mixture loss out the exhaust port when "off the pipe".

I'd love to get measurements and pix when you pull off the head next, but anything you can remember in the meantime would be helpful too - thanks!

I haven't gotten the jetting sorted out yet in the MR175 motor with the correctly-ported cylinder and rev pipe, so I still don't know the full performance potential of the porting specs I posted a while back. The improperly-ported bike is still pulling hard, though.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

I have a CH Industries (Clifford Hughes) ported 74 CR125 cylinder with a boost port. The sleeve is cracked, but we may get it duplicated on another cylnder. That was one fast engine in it's day. E.C. Birt used to do some of these also.

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