Author Topic: srx barrel rebore minimum / recommended clearance  (Read 1750 times)

Steve Lake

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srx barrel rebore minimum / recommended clearance
« on: March 26, 2026, 11:35:33 AM »
all was going well, took barrel + 50thou oversize piston to the engineers last week.

they rang me up today and asked for the factory recommended clearances.

being a race prep works (mainly 1275 S BMC A series engines & BMW JCW cooper S's) they are understandably a bit fussy about these things.

and i haven't a clue, my workshop manual doesn't help much so i'm reaching out to the club experts.....

iansoady

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Re: srx barrel rebore minimum / recommended clearance
« Reply #1 on: March 26, 2026, 02:31:44 PM »
It's tricky as it depends on piston material, lubrication, block / liner material, air / water cooled etc. Can you contact the piston manufacturer? For my old clunkers, I generally use much bigger clearances than you would, eg .005" for a 79mm piston. I'd expect there to be quite a difference between A series and BMW mini engines.

My philosophy is always to err on the generous side as I'd rather put up with a bit of piston slap and oil consumption rather than risk seizure and spend hundreds of miles running in.
Ian.
1964 Norton Electra
1969 BSA-Suzuki-Steib S501 (the B'Zuki)
1948 BSA C11

Steve Lake

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Re: srx barrel rebore minimum / recommended clearance
« Reply #2 on: March 26, 2026, 10:48:55 PM »
pretty much my feelings Ian ....

but this afternoon whilst rummaging through copious SRX literature that i've amassed over the years i found this....

so have forwarded it to the engineer.

iansoady

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Re: srx barrel rebore minimum / recommended clearance
« Reply #3 on: March 27, 2026, 09:52:34 AM »
Hard to read upside down but those are much tighter than I would use. It is a modern engine however.....
Ian.
1964 Norton Electra
1969 BSA-Suzuki-Steib S501 (the B'Zuki)
1948 BSA C11

themoudie

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Re: srx barrel rebore minimum / recommended clearance
« Reply #4 on: March 29, 2026, 11:25:49 PM »
Aye Steve,

Apologies, late for the update. Lion equinox weather on the Inner Hebrides and in Perthshire today. Gales, snow, marble hail, down to petit pois size, along with warm sun and horizontal rain! Be canny if considering the West Coast/Hebrides tour this year, Calmac only have 3 of their 11 major ferries operating as of this weekend, due to worn out old ships, lack of new ships, breakdowns and planned service! Bl**dy SNP and the quango that are supposed to order new boats couldn't organise a p...up in a brewery! Very sad.

Your piston clearances correspond with those given in the Yamaha SRX600 workshop manual.

Piston OD should be taken at 90° to the gudgeon pin, 5mm above the bottom of the piston skirt.

Top ring end gap (installed): 0.30 - 0.45 mm (0.012" - 0.018")
2nd ring  "      "         "       : 0.30 - 0.45 mm (0.012" - 0.018")
Oil ring    "      "         "       : 0.20 - 0.70 mm (0.008" - 0.028")

Wear limit top ring end gap: 0.60 mm (0.024")
   "       "   2nd   "     "     " : 0.60 mm (0.024")

Cylinder bore should be measured 40 mm (1.57 ") from the top land of the liner.
Cylinder bore "Out of round"/Ovality limit is 0.05 mm (0.002"), between the side to side and the front to back dimensions.

If you require the piston ring Xsection sketches, along with their dimensions, side clearances (installed) and wear limits, ASK!

I also see that the majority of the times that I try to log into SteveH website SRX600.net, I receive a 503 Error, which appears inconsistently! :(  This too has the dimensions that I have given, but nothing of detail for the 400 engine. I would suggest that the XT400 engine dimensions would be the same as your twin shock SRX400 engine, if you can find them.

I suspect that SteveH's site provider is taking the mickey, or else the old programme, with which SteveH created the site is starting to go flaky, so will get in touch, with him.

Good health and "Slainte!", Bill