Re: xl250s engine qu's (cb250rs really)
FT500 motor goes in relatively easily but is rare as hens teeth and also slightly delicate. Anything else is more difficult and will need work to make fit.
If it's only rattling at lower rpm, are you sure it's not camchain?
If it's not the camchain and is followers then they are in the process of breaking up and are putting metal filings throughout the engine. I've run a rattly RS for 5000 miles without it going bang (I had a spare motor to go in). In the end it sounded like something was chiselling its way out of the top end. Half of a follower was worn away. The cam lobe was pitted and scorched and the cam was eating its way through the head - but it still ran.
XL650VY - TKC80's, Mivv Suono "silencer", Arrow Headers, PAIR system removed, 125 mains, Renthal 677 bars, MRA Vario Screen, Scottoiler, Honda Main Stand, Daytona Heated Grips, Sparkbright Charge Monitor
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